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72nd EAGE Conference and Exhibition incorporating SPE EUROPEC 2010
- Conference date: 14 Jun 2010 - 17 Jun 2010
- Location: Barcelona, Spain
- ISBN: 978-90-73781-86-3
- Published: 14 June 2010
201 - 300 of 797 results
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Overpressure in the Kutai Basin – A Radical Reappraisal
Authors N. R. Goulty and A. M. RamdhanThe Kutai Basin is the second largest hydrocarbon province in Indonesia and the largest for gas production. All hydrocarbons produced to date have been reservoired in Neogene strata. Overpressure is encountered in Upper Miocene strata at relatively large depths of 3–4 km in the shelfal area, where the main mechanism of overpressure generation has long been thought to be disequilibrium compaction. This interpretation necessitated an additional hypothesis that there are pore pressure discrepancies between sands and mudrocks. We have re-examined pressure and wireline log data from the shelfal area to arrive at a very different conclusion. We have found no convincing evidence of pressure discrepancies but clear evidence of unloading processes, in the form of trend reversals in well logs, and of substantial chemical compaction with mudrock densities exceeding 2.6 g/cc at the top of overpressure. Our preferred interpretation is that the mudrocks in this area are not undercompacted, but overcompacted.
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Pore Pressure Prediction from Seismic Attributes Based on Factor Analysis
Authors D. Grana, P. Cibin and J. DvorkinPore pressure estimation from seismic data is an important issue in exploration studies for prospect detection and drilling planning. The target of this paper is to present a strategy based on statistical techniques to predict pore pressure from seismic attributes. The proposed statistical approach to pore pressure prediction is based on factor analysis method and it is compared to classical deterministic models based on physical equations which link velocities to pressure. We tested the methodology on laboratory measurements and applied it to a real case study in North Africa.
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Tensor Character of Pore Pressure–stress Coupling in Reservoir Depletion and Injection
Authors T. M. Mueller, B. Mueller, J. B. Altmann, A. Weisshardt, S. A. Shapiro, F. Schilling and O. HeidbachThe state of stress within and around a reservoir is a key parameter for fluid flow, fracture stimulation, design of wellbore arrays and wellbore stability. Therefore pore pressure induced stress changes (pore pressure/stress coupling) have immediate implications for the reservoir management. We analyze the effects of pore pressure changes on the individual components of the principal stress tensor and not only the minimum horizontal stress component. The results show that pore pressure stress coupling has a tensor character and can cause significant changes in the stress field within the reservoir as a function of distance to the injection point and tectonic regime. The tensor character of pore pressure/stress coupling leads to changes in the differential stress of the system which is essential for fault reactivation. It is demonstrated that injection as well as depletion can lead to fault reactivation, and that the rock stability depends on the tectonic regime and on the location with respect to the injection (depletion) point. Furthermore, the tectonic regime can be locally modified within the reservoir. Thus an improved understanding of pore pressure/stress coupling effects contributes to fault seal prediction, optimized placement of in-fill wells, stimulation operations and wellbore stability assessment.
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Carbonate Seismic Geopressure Prediction Using Neural Networks
Authors N. M. AlBinHassan and Y. H. WangThe prediction of high-pressure zones in carbonate reservoirs presents a major challenge in reservoir geophysics, as we cannot explore the assumption as in clastic reservoir prediction that the pressure mechanism is due to mechanical compaction. In this paper we use unsupervised and supervised neural network methods to predict geopressure in carbonate reservoirs. We test two approaches. The first approach uses multilayer perceptrons method, a supervised neural network, to predict porosity and in turn uses self-organizing map (SOM), an unsupervised neural network method, to predict geopressure from porosity. The second approach also uses SOM method but use both seismic impedance and estimated porosity together to predict pore pressure. We use a real data example to demonstrate that these approaches are robust in clustering the seismic attributes from a carbonate reservoir.
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Seismically Derived Pore Pressure Prediction Model for Pressure Profiling, Offshore Mozambique – A Case History
Authors C. E. Otor, D. Barkwith and L. LuratiPore pressure model was derived for the estimation of pressure cubes in order to implement drilling programs and mitigate geohazards at the Offshore Mozambique. The model was purely based on the seismic data because there were no offset well logs on the offshore wells. The compressional slowness was extracted from the seismic veloicty cube and the bulk density was created using the modified Gadner velocity-density relations. This enabled the calculation of the effective stress and overburden pressure from which the pore pressure was derived. This also enabled the implementation of the seismic inversion workflows. The derived pressure data have been compared with measured formation pressure and a good agreement was observed.
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Overpressures in the South Caspian Basin – Occurrence and Nature
More LessTwo basic factors are identified in the South Caspian Basin (SCB) from well logs, downhole measurements up to depths of approximately 7 km and 2D basin modeling: tectonic stress (rapid subsiding and sedimentation) and temperature (hydrocarbon generation). Accordingly to this, two main overpressure zones are established in this basin: upper zone (depth interval 600-1200 m), conditioned by a disequilibrium rock compaction (undercompaction) of rocks and lower zone (zone of decompaction of rocks) resulted from hydrocarbon generation (depth below 5 km). The lower overpressure zone is the most intense and depends on the thickness of the shale sequence, content and type of organic matter and the temperature conditions of its transformation into hydrocarbons. In this zone the greatest risk may be associated with the gas generation at depths greater than 9 km, due to both more intense thermal breakdown of kerogen and the cracking of liquid hydrocarbons generated earlier. Overpressures are a major cause of diapirism and mud volcanism in SCB.
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Randomized Dimensionality Reduction for Full–waveform Inversion
More LessFull-waveform inversion relies on the collection of large multi-experiment data volumes in combination with a sophisticated back-end to create high-fidelity inversion results. While improvements in acquisition and inversion have been extremely successful, the current trend of incessantly pushing for higher quality models in increasingly complicated regions of the Earth reveals fundamental shortcomings in our ability to handle increasing problem size numerically. Two main culprits can be identified. First, there is the so-called ``curse of dimensionality'' exemplified by Nyquist's sampling criterion, which puts disproportionate strain on current acquisition and processing systems as the size and desired resolution increases. Secondly, there is the recent ``departure from Moore's law'' that forces us to lower our expectations to compute ourselves out of this. In this paper, we address this situation by randomized dimensionality reduction, which we adapt from the field of compressive sensing. In this approach, we combine deliberate randomized subsampling with structure-exploiting transform-domain sparsity promotion. Our approach is successful because it reduces the size of seismic data volumes without loss of information. With this reduction, we compute Newton-like updates at the cost of roughly one gradient update for the fully-sampled wavefield.
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Acoustic Multi-source Waveform Inversion with Deblurring
Authors G. T. Schuster, G. Zhan, W. Dai and C. BoonyasiriwatThe theory of preconditioned multi-source waveform inversion is presented where many shot gathers are simultaneously back-propagated to form the multi-source gradient of the misfit function. Synthetic tests on 2D Marmousi model data show that multi-source waveform inversion using an encoded multi-source deblurring filter as a preconditioner can provide an accurate velocity model at 1/100 the computational cost of conventional waveform inversion.
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2D Elastic Frequency-domain Full–waveform Seismic Inversion Using the Varying PML Approach
Authors A. Abubakar, T. M. Habashy, Y. Lin and J. LiuIn this work, we present a two-dimensional (2D) FWI method for elastic data inversion. The method is based on the multiplicative regularized Gauss-Newton inversion approach. The forward problem is a finite-difference frequency-domain (FDFD) method using a perfectly matched layer (PML) absorbing boundary condition. The PML is used to truncate both the forward and inversion domains. The PML is a medium with special material properties that are determined by the material properties of the physical domain. In the inversion process, the material properties of the physical domain are changing after each iteration; hence, the material properties of the PML medium will be modified accordingly. Otherwise, an artificial reflection caused by the PML medium will contaminate the simulated data corrupting the inversion result. This varying PML approach will affect the computations of the sensitivity matrix calculation (adjoint fields). The use of the varying PML approach has been introduced before for the acoustic problem. In this work we will discuss its extension for the elastic approximation and for multiparameter inversion (the Lam'e parameters and mass density).
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Robust Frequency-domain Multi-parameter Elastic Full Waveform Inversion
Authors R. Brossier, S. Operto and J. VirieuxElastic full-waveform inversion is an ill-posed and highly non-linear data-fitting procedure that is sensitive to noise, inaccuracies of the starting model and the definition of multi-parameter classes. In this study, we investigate the performances of different minimisation functionals, such as the least-square norm (L2), the least-absolute-values norm (L1), and some combinations of both (the Huber and the so-called Hybrid criteria), with an application to a noisy offshore synthetic data set. The four functionals are implemented in a massively parallel, 2D elastic frequency-domain full-waveform inversion algorithm. Results show that, unlike the L2 norm, the L1 norm, the Huber and the Hybrid criteria allow for successful imaging of VP and VS models from noisy data in soft-seabed environment, where the P-to-S waves have a small footprint in the data. The Huber and the Hybrid criteria appear however to be sensitive to a threshold criterion, which requires tedious trial-and-error investigations for reliable estimation. The L1 norm provides a robust alternative to the L2 norm in the framework of efficient frequency-domain full-waveform inversion where a limited number of frequencies are involved in the inversion.
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Extension of the Finite-difference Contrast-source Inversion Method to 3D Geometries
Authors A. Abubakar, T. M. Habashy, M. Li, G. Pan, P. M. van den Berg and L. ZhangWe extend the finite-difference contrast-source inversion (FD-CSI) method to 3D geometries for solving seismic full-waveform inversion (FWI). The FD-CSI algorithm uses a finite-difference frequency-domain formulation. The most promising advantage of the FD-CSI method is that it does not require the explicit solution of the full forward problem in the inversion process, while some other popular inversion techniques require us to solve at least three forward problems in each iteration. Further, to enhance the quality of the profile reconstruction, the inversion is regularized employing a weighted L2-norm regularization term, which is effective for imaging structures with sharp boundaries, as illustrated in the numerical results. The weighting parameter of this extra regularization term in the cost function is determined automatically by employing the multiplicative regularization technique. This technique is robust in terms of noise suppression and in handling limited measurement data. We present some inversion results on the 3D SEG/EAGE salt model.
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A Helmholtz Iterative Solver without of Finite–difference Approximations
Authors D. A. Neklyudov and V. A. TcheverdaWe propose a preconditioned iterative method for solving the Helmholtz equation in heterogeneous media. Our method is based on Krylov type linear solvers, similarly to several other iterative solver approaches. The distinctive feature of our method is the use of a right preconditioner, obtained as the solution of the complex dumped Helmholtz equation in a 1D medium, where velocities vary only with depth. The actual Helmholtz operator is represented as a perturbation of the preconditioner. As a result, a matrix-by-vector multiplication of the preconditioned system may be effectively evaluated via 2D FFT in x and y directions followed by solution of a number of ordinary differential equations in z directions. To solve ODE’s we treat 1D background velocity as piecewise constant and search for exact solution as a superposition of upgoing and downgoing waves. We do not use explicit finite-difference approximations of derivatives. The method has excellent dispersion properties in both lateral and vertical directions. We illustrate the properties of our method using realistic 2D velocity model, and demonstrate in particular, propagation of signals without dispersion, and, a fast convergence rate for a wide band of temporal frequencies (from 2 Hz to 80 Hz).
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Seismic Modelling with the Discontinuous Galerkin Finite–element Method – Application to Complex 3D Elastic Media
Authors V. Etienne, J. Virieux and S. OpertoWe present a discontinuous Galerkin finite-element method (DG-FEM) suitable to seismic modelling in large scale 3D elastic media. The method makes use of unstructured tetrahedral meshes locally refined to the medium properties (h-adaptivity) and of interpolation orders that can change from one element to another according to an adequate criterion (p-adaptivity). These two features allow us to reduce significantly the numerical cost of the simulations. While the efficiency of DG-FEM has been largely demonstrated with high interpolation orders, we favour the use of low orders more appropriate to the applications we are interested in. In particular, we address the issues of seismic modelling or seismic imaging in case of complex geological structures requiring a fine medium discretisation.
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A Comparison of Finite-difference Grids for Anisotropic Elastic Modelling
Authors H. Bernth and C. ChapmanWe analyze and compare the computational requirements, and dispersion relationships, for the Lebedev and rotated staggered grids for anisotropic, elastic finite-difference calculations. Comparing the computational costs of these two methods for equivalent dispersion errors, we conclude that the Lebedev grid is preferred. It has the added advantage that for models with common material symmetries, it can be decomposed into uncoupled subgrids, and only one of these grids must be stored. This has important implications for applications such as elastic reverse-time migration and full-waveform inversion.
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Scale and Direction-guided Interpolation of Aliased Seismic Data in the Curvelet Domain
Authors M. Naghizadeh and M. SacchiWe propose a robust interpolation scheme for aliased regularly sampled seismic data that uses the curvelet transform. In a first pass, the curvelet transform is used to compute the curvelet coefficients of the aliased seismic data. The aforementioned coefficients are divided into two groups of scales: alias-free and alias-contaminated scales. The alias-free curvelet coefficients are upscaled to estimate a mask function that is used to constrain the inversion of the alias-contaminated scale coefficients. The mask function is incorporated into the inversion via a minimum norm least squares algorithm that determines the curvelet coefficients of the desired alias free data. Once the alias-free coefficients are determined, the curvelet synthesis operator is used to reconstruct seismograms at new spatial positions. A real data example is used to illustrate the performance of the proposed curvelet interpolation method.
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Four-dimensional f-k Interpolation of Wide Azimuth Towed Streamer Data
Authors M. Naghizadeh, O. K. Leirfall and M. D. SacchiWe extend the frequency-wavenumber (f-k) interpolation method introduced by Gulunay (2003) into a Four-Dimensional (4D) application. The 4D f-k interpolation is used to interpolate between streamers and shots in Wide Azimuth Towed Streamer (WATS) data. The f-k interpolation method utilizes the low frequency non-aliased portion of data to interpolate aliased high frequencies. Therefore, it can handle highly aliased spatial domains. In addition, the multidimensional nature of the 4D f-k interpolator extracts information from non-aliased spatial axes to interpolate highly aliased axes correctly. Synthetic and real data examples are provided to examine the performance of the proposed interpolation scheme.
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5D Interpolation of Seismic Data With a Least-squares Fourier Inversion
By S. JinI developed a new robust 5D interpolation method for seismic data regularization with a preconditioned least-squares inversion. Based on the assumption of linearity, the preconditioner is derived from the amplitude spectra automatically predicted from the regularized low frequency data. It plays a key role in the stabilization of the inversion and de-alsasing of the data. The method is efficient because it uses Non-uniform fast Fourier transform. A field data example demonstrates how it performs when applied to highly irregular data.
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Passive Interferometry of Deepwater Ocean Bottom Seismic Data
Authors P. J. Hatchell and K. J. MehtaPassive energy recorded by ocean bottom seismic (OBS) sensors in 1000 m of water is analyzed using seismic interferometry. A low-frequency dispersive arrival is observed that is interpreted using the normal mode theory of water waves. The frequency of the passive energy is below the bandwidth of the airgun sources used in this survey allowing this analysis to take place during times of active shooting. The passive wave arrival times are useful for QC of the sensor timing and positions.
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Event-driven Seismic Interferometry with Ambient Seismic Noise
Authors D. Draganov, X. Campman, J. Thorbecke, A. Verdel and K. WapenaarBy cross-correlating recordings of ambient seismic noise, one can retrieve the subsurface reflection response. The quality of the retrieved reflections would depend on the qualities of the ambient noise. In a previous study, we cross-correlated ambient-noise data recorded in a desert area in North Africa and showed that we retrieved reflections. This was done assuming that body-wave noise continuously illuminates the recording array. But this is not necessarily true - noise which carries body-wave information can be present only at certain times. We now use only parts of the recorded noise during the correlation process. These parts contain identifiable body-wave events. We show that the results, retrieved only from the noise containing the events, exhibit clearer reflection arrivals.
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Kirchhoff–Helmholtz Representation for Interferometry and Virtual Reflectors in Acoustic Media
Authors F. Poletto, K. Wapenaar and C. BellezzaIn this work we analyze the theoretical aspects in the joint representation of the VR and SI wavefields with the same source and receiver configurations. We present the formulation of the Kirchhoff-Helmholtz representation integrals used in the combination of the SI and VR wavefields in arbitrary acoustic media. The analysis calculates the VR and SI combination terms and coefficients for wavefield representations by receivers surrounding two sources in 2D and 3D (thus, without loss of generality for reciprocity, performing the synthesis of a virtual receiver at one of the sources).
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3D Seismic Imaging over Two Structurally Complex Surveys in TAL, Pakistan
Authors V. Dolgov, R. W. Vestrum, G. Wittmann and J. M. GittinsIn a tectonically complex area of northwest Pakistan, MOL Pakistan and its partners acquired two 3D seismic surveys. Extreme topography led to irregular shooting geometry. Varied surface access over this terrain required a mix of vibroseis and dynamite source types to maximize subsurface coverage. With the combination of difficult surface conditions over a structurally complex subsurface, the data processing and merging of these two surveys required close attention to detail throughout the processing sequence, and we had to rely on the most robust algorithms in the data-processing toolkit. There was no single technology that stood out in the processing, but careful attention to details in the application of the fundamental processes created a readily interpretable seismic volume.
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Improved Sub-basalt Structural Imaging in the Faroe-Shetland Basin Using Full Sequence Migration Multi-velocity Analysis
Authors A. J. Hardwick, T. C. Travis, S. V. Stokes and B. BaumbachIt is widely accepted that the key to successful sub-basalt imaging in the Atlantic Margin region is to generate as much low frequency energy as possible. Whilst recent acquisition in the Faroe-Shetland Basin (FSB) has seen the towing of sources and streamers at increasing depth to concentrate more energy into the low end of the amplitude spectrum, a large volume of data exists with a more conventional and shallower towed configuration such as TGS’s FSB 1999 and 2000 surveys. Through the reprocessing of these datasets we demonstrate that a significant improvement can be made to image potentially prospective Mesozoic and Palaeozoic sub-basalt structures. This is achieved without compromising the relatively high resolution of the overlying Tertiary section. We describe three key processing approaches which in combination are seen to provide a dramatic uplift over the original time processing. These are the enhancement of available low frequencies through a propriety spectral manipulation technique, multi-domain noise attenuation and a method we term ‘Full Sequence Migration Multivel Analysis’. The latter adopts strategies analogous to those used in pre-stack depth imaging and is seen to greatly enhance the interpretability of previously concealed structures at depth.
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Resolving the Ambiguity in Attenuation Imaging
Authors W. A. Mulder and B. HakSeismic data enable imaging of the Earth, not only of velocity and density but also of attenuation contrasts. Unfortunately, the Born approximation of the constant-density visco-acoustic wave equation, which can serve as a forward modelling operator related to seismic migration, exhibits an ambiguity when attenuation is included. Different scattering models involving velocity and attenuation perturbations may provide nearly identical data. This result was obtained earlier for scatterers that did not contain a correction term for causality. Such a term will lead to dispersion when considering a range of frequencies. We demonstrate that with this term, iterative migration or linearised inversion will almost but not fully remove the ambiguity. Because the initial update in a gradient-based optimization scheme that minimizes the difference between modelled and observed data is still affected by the ambiguity, the reconstructed model starts to approximate the true model only after a very large number of iterations.
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Inverse Q Filtering for High Resolution with High Fidelity and High S/N Ratio
By Y. H. WangA stable inverse Q filter should be able to produce high resolution with high fidelity and high S/N. This paper presents a number of case studies on the application of inverse-Q filtering to improve the resolution of seismic data. Stabilized inverse-Q filtering is able to simultaneously compensate the amplitude and correct the phase of seismic waveforms. After processing, the frequency bandwidth has been increased and the width of the wavelet has been narrowed, so that we are able to identify reflections of thin sand layers clearly. Because of the high signal-to-noise ratio with the stabilization scheme, low-amplitude zones of interest corresponding to high-fracture areas can be easily identified, and the detail within the anomalies can also be observed. Finally, spatial variations of tight-sand layers of gas reservoirs are depicted in the inversion profile with high resolution.
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3D Q Tomography for Compensating Frequency Dependent Attenuation and Dispersion
More LessComplete Q compensation involves the estimation of Q and then using the resultant Q to correct the amplitude and phase effects. In this paper, we present a ray based tomographic method that is derived from our previous work on amplitude tomography for estimating Q. The estimated Q can then be applied directly to migrated traces and uses raypaths information within a 3D Q model to restore amplitude and phase by integrating the Q effect along the rays. We demonstrate, through examples, how we include the estimated Q volume in the Q compensation process to fully account for attenuation and dispersion.
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Azimuthal Anisotropy Characterization Using 3D Multi-azimuth Towed Streamer Dataset
Authors L. C. Moffat, R. A. Clark, M. van der Baan and T. ManningUnderstanding the spatial orientation of fractured reservoirs and the stress field around them is essential for the development of an oil reservoir. Fractures and stress create anisotropy effects that can be observed in seismic data. The preferred method for analysing anisotropy from data is the amplitude variation with offset and azimuth (AVOA). Because of the ambiguity in the directions obtained using this method, attenuation is increasingly being used to fully characterise these environments with confidence. This abstract demonstrates through a case study the potential of using attenuation variations with offset and azimuth (QVOA) from a multi-azimuth towed streamer dataset. The results show a dominant direction of maximum attenuation that could be related to the presence of the local stress field.
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Refined Approximations for Polarizations, Velocities and Attenuations in Arbitrarily Anisotropic Viscoelastic Media
Authors P. N. J. Rasolofosaon and V. FarraSeismic attenuation and anisotropy are well accepted characteristics of geological media. Modelling such media as arbitrary anisotropic viscoelastic media, without being common, is not rare now. In the context of moderate attenuation and anisotropy, perturbation theories have been proposed to deduce the best parametrization of such media and to help to understand which parameters of the media, and in which combinations, affect individual wave attributes. Here we focus on high-order perturbations. In the case of homogeneous waves, and using the so-called "correspondence principle" (i.e., viscoelastic equations are formally identical to the corresponding purely elastic equations provided that all the corresponding quantities be complex), we straightforwardly adapt existing very accurate formula initially for pure elasticity to the viscoelastic case. The great accuracy of these higher-order approximations is preserved not only for the velocities, but also for the attenuations and for the polarizations of the three bulk waves (qP, qS1 and qS2). This is successfully checked on experimental data in a water-saturated marble sample, considered as an arbitrarily anisotropic attenuating medium, exhibiting moderate attenuation (Q>40), moderate velocity anisotropy (<20%) but huge attenuation anisotropy (>500% for the qS-waves).
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Q Estimation on Conventional and Dual-sensor Sytems
Authors A. K. Vigner, M. Deighton and C. SwiftSeismic absorption along the propagation path of a seismic wave has a considerable impact on amplitude and phase of recorded signal wavelets. It has been demonstrated that the knowledge of the absorption parameter (known as Q) is highly desirable when extracting lithological information. In this paper, we discuss the estimation of Q using different spectral ratio methodologies. We show that the choice of the reference wavelet is determinant in the accurate estimation of Q. Finally it is demonstrated that Q estimation takes great advantage from the broader bandwidth provided by dual-sensor (pressure and velocity) acquisition system compared to the conventional (pressure only) system.
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Some Formulas for AVF-AVA Inversion of Reflections from Absorptive Targets
More LessRecent reports of frequency-dependent seismic data anomalies, detected near low-Q hydrocarbon zones, have led several researchers to suggest that the spectral imprint of an absorptive reflection coefficient is being observed. By expanding various mathematical forms for an absorptive reflection coefficient about small parameter contrasts and incidence angles, and examining them frequency by frequency (AVF), or angle by angle (AVA), simultaneous variations in wavespeed and Q, may be separately estimated.
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2.5-D Poro-viscoseismic Wave Modeling in a Double Porosity Medium
Authors X. Liu, S. Greenhalgh and Y. H. WangFor the double porosity Biot model, the local flow energy dissipation mechanism can be approximated by a single Zener visco-acoustic element. This replaces the convolution integrals of the governing equations with the memory equations for the memory variables. From the 3-D governing equations for poro-viscoseismic wave propagation, the 2.5-D governing equations are obtained by taking a Fourier transform in the medium-invariant (strike) y- direction and trasnforming to the wavenumber domain. For a heterogeneous, double porosity 2.5 D medium, we obtain numerical transient solutions for a point source by solving the poro-viscoseismic modelling using a time splitting method for the non-stiff parts and an explicit 4th-order Runge-Kutta method for the time integration and a Fourier pseudospectral staggered-grid for handling the spatial derivative terms. Since the 2.5-D scheme can be used to calculate the 3-D wavefields, it is clearly more realistic than 2-D (line source) modelling. By this method, the stress, particle velocity and pore pressure can be calculated simultaneously. Comparison with the analytical solution for a homogeneous model shows the correctness of this approach.
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The Fault Reactivation Potential As Critical Factor in Reservoir Utilization
Authors I. Moeck, G. Zimmermann and G. KwiatekReservoir utilization is frequently linked with treatments that shall enhance the reservoir productivity by hydraulic stimulation. The aim of hydraulic stimulation is to increase the formation pressure by fluid injection to create artificial fractures that act as additional fluid pathways. Large scale fluid injection used in gas and recently also on geothermal reservoirs can induce seismicity and fault reactivation depending on reservoir geomechanics and stress regime. Recent case studies in stimulation of geothermal reservoirs have shown induced seismicity as undesired side effects which need to be understood in advance of massive fluid injection. Slip tendency analysis has been successfully used to characterize fault slip likelihood and fault slip directions in any stress regime. In our study we apply the slip tendency analysis to assess the reactivation potential of shear and dilational fractures in a deep geothermal reservoir in the Northeast German Basin, based on the notion that slip on faults is controlled by the ratio of shear to normal stress acting on the plane of weakness in the in situ stress field. The results from slip tendency analysis are supported by the spatial distribution of recorded microseismicity indicating slip rather than extension along a presumed NE striking failure plane.
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Borehole Breakouts in St. Meinrad Sandstone under Hydrostatic to Highly Anisotropic Far-field Stresses
Authors B. Haimson, S. Stanchits and G. DresenWellbore instability often leads to failure in the form of breakouts, which are a major concern in the petroleum industry because they interfere with production and facilitate unwanted sanding. We report on two sets of experiments in which a high-porosity quartz sandstone was subjected to conditions inducing breakouts around boreholes. As in other quartz-rich sandstones, breakouts are not “dog eared” but rather tabular, long and very narrow. They resemble compaction bands that have been partially emptied. The apparent creation of compaction bands initiated by the borehole stress concentration constitutes an added concern, since such bands have diminished porosity and can impede the free flow of fluids through the sandstone.
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Modelling of Damage Evolution by a Two-Scale Homogenisation Approach – Application to Geomechanical Modelling of Reservoir Exploitation
Authors A. M. Dobrovat, C. Dascalu and S. A. HallExploitation of both seismic anisotropy and micro-seismic data has been shown to be very successful for monitoring geomechanical evolution in and around subsurface reservoirs during production of hydrocarbons or sequestration of CO2. However, to fully exploit such observations requires simulation models that can be used to predict, and match, the changes to enable better management of the reservoir and avoid mechanical changes/failures that may be costly. Unfortunately, commonly used geomechanical constitutive models do not account for rock property evolution and thus cannot be used to predict any changes in the measurable elastic properties. Here a two-scale damage model with an explicit description of the micro-scale mechanisms of crack-damage initiation and propagation is presented. The model provides directly the evolution of (anisotropic) elastic properties as a result of the damage evolution. In addition acoustic emissions due to micro-seismic events could be simulated, as the crack evolution is determined through the solution of an energy dissipation problem. Results from the simulation of the overburden response to a reservoir production scenario from an initial macro-scale implementation of the model are also presented.
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Temperature Dependent Rock Fracturing in Boreholes
Authors B. Shen, M. Rinne, T. K. Kim, J. M. Lee, S. C. Lee, J. Y. Kim, H. M. Kim, E. S. Park, M. W. Wuttke, T. Backers and O. StephanssonThe paper describes a recent numerical code development and laboratory investigations on coupled thermal-mechanical processes of rock fracture propagation. A series of laboratory tests were conducted on rock strength and fracture toughness within a temperature range from -80C to 400C, and key temperature dependent parameters were obtained on granite specimens. The numerical development is based on a fracture mechanics code FRACOD that has previously been developed by some of the authors of this paper. The code simulates complex fracture propagation in rocks governed by both tensile and shear mechanisms. For the latest development an indirect boundary element method, namely the fictitious heat source method, is implemented in FRACOD to simulate the temperature change and thermal stresses in rocks. This method is particularly suitable for the thermal-mechanical coupling in FRACOD where the displacement discontinuity method is used for mechanical simulation. An example case is presented where a borehole drilled into the rock formation. Depending on the initial reservoir rock temperature, cooling fractures may or may not occur in the borehole wall from the same differential temperature between the borehole wall and the reservoir rock.
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In-Situ Primary Stresses and Hydro-Mechanical Properties of Saline Aquifer Formations in the Altmark Area in North Germany
More LessThis paper presents in-situ primary stresses and hydro-mechanical properties of the several selected strata in the Altmark area in North Germany. The in-situ stress measurements were done by the specialized core-based experiment called RACOS®. As results this paper presents the magnitude of the in-situ maximum and minimum horizontal stresses and spatial distribution of their orientations in the Altmark area. Results from this study were compared to the spatial distribution of maximum horizontal stress orientations in the North East German basin from the World Stress Map. Additionally, hydro-mechanical properties including strength, deformation parameters and Biot’s coefficients of the reservoir formations that comprise the Altmark area within the depth ranging from -2,700 m to -3,700 m were determined by the geomechanical correlations from the literatures and by the laboratory experiments on the tight gas formation in the North East German basin. These properties are later used in the hydro-mechanical coupled numerical modelling of sequestration of CO2 and the resulting long-term stability of caprock formation.
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Coupled Strata and Gas Behaviour in Co-extraction of Coal and Methane
Authors H. Guo, D. P. Adhikary, N. Ishihara and M. FujiokaAs shallow coal deposits amenable to conventional extraction methods are being exhausted, coal mining is increasingly carried out in a deeper and more challenging environment with, for example, increased coal seam gas content, and reduced coal and strata permeability. These conditions increase risks in mine gas drainage performance and mining operations and, if not managed properly, can have significant negative impact on coal mining productivity and recovery, mine health and safety, mine gas emission and its environmental impact. Coal mine gas (methane in particular) has been a cause of major coal mine disasters and significant greenhouse gas emissions, and a key limiting factor to coal productivity improvement. However, when captured and utilised, coal mine methane becomes a source of abundant clean fuel or energy. An integrated mine gas simulation system has been developed to predict strata conditions and methane gas emission during longwall mining. The system can be used to assist selection and planning of suitable mining, gas management and utilisation strategies for new and existing mines. Several key mine gas assessment processes, such as mine site gas and geotechnical characterisation, and fully coupled mechanical deformation – fluid flow computer simulation, have been integrated in the simulation system. The integrated simulation approach is described in this paper and an example of mine site application is presented.
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Full-field Laboratory Imaging of Localised Deformation in Sandstone
Authors S. A. Hall, E. M. Charalampidou, E. Tudisco, P. Roux and G. ViggianiRocks are neither truly homogeneous nor are laboratory test boundary conditions perfect plus at some stage of loading deformation in a specimen generally localises into shear/compaction bands, tensile/shear cracks or fractures. In the presence of such localised strains and heterogeneity point-wise measurements at a boundary do not well characterise the mechanics of the system. Therefore “Full-field” measurements are needed to better study the behaviour. In this paper full-field measurements, including x-ray tomography, 3D-volumetric digital image correlation and ultrasonic tomography, are used to characterise localised deformation phenomena in a sandstone deformed under triaxial compression. X-ray tomography allows 3D characterisation of the internal structure of laboratory specimens and the consequences of deformation are visible where there has been sufficient change in material density. Complementing x-ray tomography by 3D-volumetric digital image correlation (DIC) techniques allows quantification of 3D displacement and strain fields between different stages of loading by comparison of the corresponding x-ray tomography images. In addition ultrasonic tomography provides full-field measurement of ultrasonic velocities, and thus elastic properties, in laboratory specimens. DIC and x-ray tomography can provide information on structural changes due to deformation and the associated kinematics and strains, whilst ultrasonic tomography gives insight into damage (degradation of elastic properties), which are to be expected, e.g., due to compaction and related porosity reduction or grain crushing.
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Borehole Instabilities Due to Pore Fluid Pressure Fluctuations in Shales
Authors M. Witthaus and C. LemppThis study shows that the pore fluid pressure is an important parameter to characterize borehole instabilities in pelitic rock formations. Due to its dynamical fluctuations and its interactions with drilling mud the pore fluid pressure within the rock formation has a fundamental influence on the risk for unstable boreholes. The presented laboratory tests indicate, that an increasing pore fluid pressure progressively accelerates the softening process in mudstones as well as in claystones, shales and argillite formations. In such mudstone formations the maximum load capacity of about 20 – 40 MPa will be reduced to about 50%, if the pore pressure level increases due to drill mud pressure changes and at last reaches values near to the lithostatic pressure in this water-saturated formations. An additional effect of increasing pore fluid pressure is a stress fluctuation that will be induced in the microstructure, so that in some parts crack propagation (hydraulic fracturing) can be detected. Nevertheless, ductile behaviour and weakening are predominant in those rocks. Consequentely, crack growth result in borehole breakouts and the ductile deformation result in borehole convergency. Both types of borehole instabilities interact at increasing pore pressure conditions. Therefore, the risk of unstable drillholes mainly depends on the pore pressure level in the pelitic rock formations. Hydraulic fracturing as well as the struture-softening effects due to high pore pressure conditions of about 0.8 lithostatic pressure are also obtained in shales and argillites with stiffness of about 50 – 100 MPa. In this rocks the maximum load capacity and the Young`s modulus decreases to about 1/3, if pore pressure increases with 50 %. Accordingly, the deformation behaviour tends from brittle to ductile. These results represent an important reason to develop new drilling strategies in order to improve the stability of deep boreholes for example in oil exploration or in enhanced geothermal systems.
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Diffractions – A Historical Perspective
Authors H. C. Hoeber, K. Klem-Musatov, T. J. Moser and M. PelissierDiffraction is a phenomenon that is caused by medium features that are small compared to the wavelength of the wave. Sommerfeld defines diffraction as “any deviation of light rays from rectilinear paths which cannot be interpreted as reflection or refraction”. Landau and Lifshitz state that diffraction refers to phenomena which are the “consequence of deviations from geometrical optics”. In this paper we present a brief historical sketch of diffraction theory. We present original material from the key works of Grimaldi, Huygens, Young, Fresnel, Kirchhoff and others representing milestones in the development of the theory. The examples range from the early experiments by Grimaldi in the 1600’s to the seismic diffraction modelling of edge and tip waves.
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Progress in Seismic Diffraction Theory – From Edge and Tip Waves to Multiple Reflections-transmissions with Diffractions
Authors A. M. Aizenberg and K. D. Klem-MusatovSeismic waves traveling in the subsurface can be described by two physically different processes: propagation inside layers with smoothly varying properties and reflection and transmission at internal reflectors, which are formed by parameter discontinuities. Evaluation of the multiple reflections-transmissions for complex subsurface geometries is not straightforward. The presence of diffracting edges and vertices limits the applicability of geometrical seismics. Reflections-transmissions in the Fresnel zones near the shadow boundaries of the ray tubes can be described as the sum of geometrical seismic waves and diffracted waves in the boundary-layer approximation. To describe the reflection-transmission in the caustic zones caused by interface curvatures, we introduced the tip-wave superposition method. This method explicitly represents the scattered wavefield as the superposition of events multiply reflected and transmitted in accordance with the wavecodes, which allows modeling of selected events independently. Each event is formed by the sequential action of the propagator matrices inside heterogeneous layers with effective reflection and transmission coefficients at curved interfaces. Each element of the matrix contains the tip waves diverging from a small interface element. Effective coefficients generalize plane-wave coefficients used in the conventional seismic modeling for curved reflectors, non-planar wavefronts and finite frequencies.
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Review of Ray-born Forward Modeling for Migration
By T. J. MoserThe ray-Born approximation is a very useful tool for forward modeling of scattered waves. The fact that ray-Born modeling underlies most seismic migration techniques, and therefore shares their assumptions, is a justification in itself to consider it for forward modeling. The ray-Born approximation does not make an explicit distinction between specular reflections and non-specular diffractions. It therefore allows to model diffractions from structural discontinuities such as edges and tips, as well as caustic diffractions. In its simpest implementation ray-Born seismograms are multiple-free. It can be orders of magnitude faster than finite-difference modeling, both in two- and three dimensions.
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Sea Bed Diffraction and Impact on 4D Seismic Data – Observations from Synthetic Modelling and Field Data
More LessThe water layer represents a major source for non-repeatability in 4D seismic processing. In this study we have analyzed the effect of seabed 4D timeshifts due to changes in water velocity and 4D seismic acquisition mis-position for the case when diffractions are positioned at seabed. Real 4D seismic data from the Norne Field and synthetic data using a single point diffractor at seabed model are used in this study. We find that, in addition to tidal correction, timeshift correction should include water velocity correction and relative strength and position of the seabed diffraction observed. A main result from the analysis shows that 4D seismic acquisition mis-position adds non-negligible timeshifts close to the seabed diffraction apex point.
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Orient VSP Receivers Using Diffracted Energy
By M. HumphriesDiffracted arrivals from geological discontinuities in the vicinity of a borehole are often seen in Vertical Seismic Profile (VSP) datasets. A method of locating VSP diffracted arrivals using a micro-seismic location technique has proved successful, but one source of error in this location method is the orientation bearing of the receivers’ horizontal components. A method of orienting VSP receivers using diffracted energy from a known diffraction source is investigated and is shown to improve the orientation of VSP horizontal components.
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Unified Approach to Velocity Estimation and Imaging
More LessSeismic prospecting is based on the theory of the echo. Essentially it suggests only one (time-distance) equation that always contains not less than two unknown. It creates problems that we learned to overcome by using redundant fold and multikilometer offsets. The former raises cost, the later increases departure of reflections from a hyperbolic moveout. Together they tend to decrease resolution of seismic images. A fundamental solution to the problem is attraction of one more equation. In 3D seismic imaging it can be borrowed from the theory of diffraction by an aperture. Diffraction by a limited aperture accompanies migration of each reflected wave. It may play a constructive role in seismic imaging the same way it takes place in optics. Besides, it makes possible velocity determination at any point of an areal geometry common source gather.
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Parameterization of Seismic Measurements by FPRs – Combining Physics With Mathematics
Authors A. J. Berkhout and D. J. VerschuurThe data of a seismic represents a temporally and spatially sampled version of the continuous elastic wavefields. Therefore, seismic data traces can be conveniently arranged in a so-called data matrix. After removing waves that have travelled along the surface, the data matrix contains signals that can be expressed in terms of wavefield operators describing propagation and reflection in the subsurface. The result is the so-called WRW model. This model shows how to decompose seismic measurements in terms of directional responses of individual subsurface gridpoints. These Focal Point Responses (FPRs) can be used in full wavefield seismic processing algorithms.
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Separation and Imaging of Seismic Diffractions in Dip Angle Domain
More LessDiffraction events containing in seismic data characterize small size geological objects. This information can supplement conventional reflection method. Typically diffracted energy is much weaker than reflection one. Therefore diffractions have to be extracted from the full wavefield before diffraction imaging. We present a method for reflection-diffraction events separation using the hybrid Radon transform.
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Diffractions – Yesterday, Today, and Tomorrow
By E. LandaSeismic diffractions are the response of important small scale elements in the subsurface such as faults, karsts, fractures. We distinguish between reflective and diffractive contributions to the wavefield and recognize the diffractive component as a key ingredient in establishing resolution. We present past, present and future of using the diffractions for seismic imaging and illustrate it with many interesting and exiting examples.
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Imaging Random Media Effects for Imaging
Authors L. Borcea, F. Gonzales del Cueto, G. Papanicolaou and C. TsogkaSensor array imaging in heterogeneous, strongly backscattering media is complicated by a serious issue: The coherent echoes from the scatterers that we wish to image are weak and overwhelmed by reflections from numerous inhomogeneities (the clutter). Coherent imaging in such media cannot work unless we filter the unwanted clutter reflections. We present two filtering approaches: The first uses arrival times and is specialized to finely layered media. The second approach is more general, and it is based on the spectral analysis of the local cosine transform of the scattering matrix recorded at the array.
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Diffraction Imaging of Vertical Fractures and Karst With Full-resolution 3D GPR, Cassis Quarry, France
Authors M. Grasmueck, M. Coll, G. P. Eberli and K. PomarVertical fractures with openings of less than a centimeter and irregular karst cause abundant diffractions in Ground Penetrating Radar (GPR) records. Conventional GPR data acquired with half wavelength trace spacing are uninterpretable as they are dominated by spatially undersampled scattered energy. To evaluate the potential of high-density 3D GPR diffraction imaging a 200 MHz survey with less than quarter wave length grid spacing (5 cm x 10 cm) was acquired at a fractured and karstified limestone quarry in southern France. After 3D migration processing diffraction apices line up in vertical fracture planes and cluster in locations of karstic dissolution features. Such high resolution 3D GPR imaging offers an unprecedented internal view of a complex fractured carbonate reservoir model analogue. As the nature of reflection seismic data is very similar to GPR, similar improvements in imaging of fractures and features at the resolution limit can also be expected from high density seismic diffraction imaging.
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Fracture Detection by Diffraction Imaging
Authors C. Tsingas, B. El Marhfoul and A. DajaniThe main objective of imaging diffracted arrivals is to produce high resolution seismic sections in time or depth, which in turn will enhance the interpretation of fault edges, pinchouts, reef edges, fracture zones and other geologic discontinuities. In naturally fractured reservoirs a detailed understanding and mapping of the subsurface fracture network is often necessary to optimize field development plans. There is a variety of seismic driven technologies that attempt to map and detect fracture zones. For example, by analyzing seismic anisotropy one can obtain preferential fracture orientation, generating volume and/or horizon based extracted attributes potential fracture corridors can be identified and mapped.In the following we illustrate the methodology for imaging discontinuities, which enables us to obtain high resolution fracture maps. We then present, using synthetic and real data examples the ability of the methodology to map fracture corridors by focusing the associated scattering energy in the prestack domain and conclude by highlighting the benefits of the application on land seismic data (i.e., sparse acquisition and low signal to noise ratio). Complementing one another, the diffraction image sections should be interpreted in conjunction with the conventional reflection time migrated results.
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Specular–diffraction Imaging by Directional Angle Decomposition
More LessThis work presents a new seismic imaging system for generating amplitude preserved, three dimensional directional gathers. The proposed directional angle decomposition enables the implementation of both specular and diffraction imaging in real 3D isotropic/anisotropic geological models, leading to simultaneous emphasis on both continuous structural surfaces and discontinuous objects, such as faults and small-scale fractures. Structural attributes at each subsurface point, e.g., dip, azimuth and continuity, can be derived directly from the directional angle gathers. The proposed system is most effective for imaging and analysis below complex structures, such as sub-salt and sub-basalt, high-velocity carbonate rocks, shallow velocity anomalies, and others.
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3D Poststack Interval Velocity Analysis using Diffractions
More LessA practical method for applying 3D interval velocity analysis in the depth migrated domain using poststack data as input is presented. The method takes advantage of the unique appearance of migrated diffractions in the Local Angle Domain. Diffraction energy is used in an iterative manner to update the three-dimensional subsurface interval velocity model.
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Velocity Estimation by Image Focusing Analysis
By B. L. BiondiMigration velocity can be estimated from seismic data by analyzing focusing and defocusing of residual-migrated images. The accuracy of these velocity estimates is limited by the inherent ambiguity between velocity and reflector curvature. However, velocity resolution improves when reflectors with different curvatures are present. Image focusing is measured by evaluating coherency across structural dips, in addition to coherency across aperture/azimuth angles. The inherent ambiguity between velocity and reflector curvature is directly tackled by introducing a curvature correction into the computation of the semblance functional that estimates image coherency. The resulting velocity estimator provides velocity estimates that are: 1) unbiased by reflector curvature, and 2) consistent with the velocity information that we routinely obtain by measuring coherency over aperture/azimuth angles. Applying the proposed method to zero-offset field data recorded in New York Harbor yields a velocity function that is consistent with available geologic information and clearly improves the focusing of the reflectors.
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Characterization and Exploration Potential of Palaeogene Mass-transport Deposits in the Espirito Santo Basin, Brazil
Authors D. Gamboa, T. Alves, J. Cartwright and P. TerrinhaThe work presented characterises the distribution of continuous strata intercalated with Mass-Transport Deposits (MTDs) and their internal heterogeneities within a Late Palaeogene unit in the Espírito Santo Basin, Brazil. MTDs have low permeability and are considered as barriers to fluid flow, but sand-prone strata within the interpreted MTDs can constitute viable reservoirs as long as permeable pathways are present. Our work shows that the presence of continuous strata varies along the stratigraphic interval. They are of limited expression in the lower sections of the Palaeogene unit where thick MTDs occur. In contrast, the upper part of the latter unit shows continuous strata through a large portion of the interpreted survey, and relatively thinner MTD successions. Within the basal MTD numerous remnant blocks were identified, being related to pre-MTD faulting. Three block deformation types were established, namely low, moderate and high, reflecting the internal block tilting. We infer that the presence of pre-MTD faults and remnant blocks establishes permeable conduits across the studied MTD successions, eventually promoting fluid flow from pre-MTD successions to the Late Palaeogene reservoir-prone (continuous) strata. Proof of that is the distribution of basal remnant blocks, which partly follows pre-MTD faults.
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Evolution of Seismic Imaging in Colombia
By H. AlfonsoOver many decades ECOPETROL (Oil state company of Colombia)has done efforts in improving the quality of seismic in Colombia, through different seismic campaigns and applying different technologies. Imaging complex environments, or specific structural features are often difficult, so you have to use the right technology to achieve the best result, regarding also cost benefit relationship. The primary goal of this paper is to show seismic lines that illustrate the evolution of seismic imaging in several Colombian basins, highlighting the key aspects in improving the image and the use of geophysical technology.
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The Role of the Evaporite in the Peruvian Foreland Structural Style (North Ucayali)
Authors I. Moretti, M. Principaud, P. Baby and J. P. CallotIn the North Ucayali basin, HC reserves have been already found but the structural style is rather complex due to the variability of the sedimentary covers and the presence of inherited structures. The risk of undercharge of the prospects due to a timing problem exists. A calibration of the erosion at the top of the various structures published by Bertolotti and Moretti (2009) indicated that the tertiary structures have different ages. Although the importance of the Paleozoic depot centres and faults in localizing has been largely published, a major role of the early Mesozoic evaporitic pillows was only recently proposed although seismic data evidences these pillows that influence the thrust geometry. Such behaviour of the evaporites is similar with the one observed in other compressive fronts. However, in the classical example of compressive front above salt layer, the evaporite forms either the main decollement level (as in the Zagros). In the north Ucayali basin, the salt pillows are isolated rather flat bodies, there are also deeper decollement levels. We have designed analogue models to discuss the influence of the depth and continuity of the salt pillows on the structure style.
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Applying Fluid Inclusions Mass Spectrometry to Support Exploratory Plays, Marañon Basin, Peru
By A. ChalcoThe interpretation of fluid inclusions mass spectrometry analysis from core plug samples of the Ungumayo-1X exploratory well and to core plug samples from a neighbouring oilfield in the Marañon Basin (Peru), suggest the potential presence of an hydrocarbon accumulation, either updip of the Ungumayo-1X well or at underlying Early Cretaceous undrilled reservoir, opening the opportunity to test a new play in the area.
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Pre-salt Depth Imaging of the Deepwater Santos Basin, Brazil
Authors Y. Huang, D. Lin, B. Bai, S. Roby and C. RicardezSeveral discoveries, such as Tupi, Bem-Te-Vi, Parati, and Guara, have been announced in the Santos Basin off the coast of Brazil. Most of these recent discoveries have been in pre-salt layers. These layers were well imaged by a salt flood volume produced in 2003, but distortions in the base of salt (BOS) and pre-salt layers were still obvious. Therefore, a depth migration with a complete salt model is necessary for correctly positioning the reservoir structures. In order to obtain high-quality subsurface images, building an accurate velocity model and using the optimal migration algorithm for the geology is paramount. For pre-salt imaging, not only the pre-salt velocity, but also the overburden velocities, i.e. sediment velocities, salt velocities and salt geometry, are critical. The unique aspects of the local geology in the Santos Basin make building the velocity model challenging. The three major issues are: 1) the presence of a thin Albian layer above the salt, 2) the sensitivity of the top of salt (TOS) picking to pre-salt imaging, and 3) the existence of evaporite layers within the salt. The following techniques are introduced to improve the accuracy of the velocity model and the pre-salt image: Albian layer tomography, iterative TOS interpretation, and intra-salt tomography.
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New Oil Exploration Concepts in the Southern Area of the Middle Magdalena Valley, Colombia, SA
More LessThe prolific Magdalena basin is an elongated and narrow depression located between the Eastern and Central Colombian Mountain Ranges. This basin has been divided in three subbasins called Upper, Middle and Lower Magdalena basins. Recent studies carried out by Cepsa in the southern part of the Middle Magdalena Valley Basin (MMVB) strongly suggest that this area have undergone a very distinctive tectonostratigraphic evolution. The current tectonic configuration in the southern MMVB has been generated by the structural convergence between the Ibague transpressive fault system and the late Miocene-Pliocene inversion and uplifting of the Eastern Cordillera (Figure 1). The technical evaluation indicates that faults related to the Ibague transpressive system, related to the Caribeean plate motion to the NE, controlled the sedimentation during the late Cretaceous and Tertiary times. Afterwards, the Andean Orogeny exhumed the Cretaceous organic facies along Bituima and Alto del Trigo fault planes. On the other hand, the geochemical modeling suggests that significant hydrocarbon generation processes took place during late Eocene-late Oligocene times. In addition, upper Cretaceous organic facies are currently near the early stages of the oil window.
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Integrated Methodology of the Discretization of the Porosity in Naturally Fractured Reservoir
Authors P. Y. F. Chaves, E. S. Mercado, C. U. P. González, E. R. Solorzáno and W. O. V. BarbozaDue to complexity associated to petrophysical characterization of the porosity in naturally fractured reservoirs, a methodology to discretize it in four main systems was developed: matrix, microfractures-microvugs, connected vugs and fractures. This methodology integrates three independent but related procedures, based upon the response of conventional logs and the comparison with core date and special logs. The first procedure is based upon the sonic log response to determine matrix porosity. The second determines the fracture porosity by means of the variable cementation coefficient (m) method. The third is known as the Probability of Occurrence of Flow (POF), which separates the vugs connected to fractures from the microvugs connected to microfractures and the matrix porosity.
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Identification of Permeable Carbonates with Well Logs Applying the Differential Effective Medium Theory
Authors R. Gullco and M. AndersonFor carbonates, the porosity alone, as calculated with the combination of the Neutron and Density logs, is not enough to predict whether an interval will be permeable or not. A rock is permeable if the voids in it (matrix porosity, fractures, vugs) are connected. The well known Differential Effective Medium theory (DEM) allows the modeling of rocks perceived to be permeable or impermeable. Spherical pores in relatively low porosity rocks are assumed to be isolated and therefore resulting in very impervious rocks. Very flat ellipsoidal pores (penny cracks) with a random orientation are postulated to form an interconnected network of pores, resulting in a very pervious rock. These two end points can be identified in a Velocity (from the sonic log) vs. porosity (from the radioactive logs) diagram corresponding to the interval of interest, where the curves modeled with the DEM and the Wyllie curve have also been plotted. Impervious points should fall high above the Wyllie curve in such a diagram, whilst the points representing permeable rocks should fall well below the Wyllie curve. Production tests have been used to check the theory.
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Porous Medium Acoustics in the Presence of Viscous Boundary Layers
Authors T. M. Mueller and P. N. SahayThe importance of the viscous boundary layer flow for porous medium acoustics has been recognized by Biot who analyzed this effect for cylindrical tubes and subsequently extended his low-frequency theory to the full frequency range. In this paper we develop an alternative approach to model attenuation and dispersion associated with the transition from the viscosity- to inertia-dominated regimes. Instead of analyzing the oscillatory Stokes flow in elastically-rigid tubes, we base our analysis on the visocosity-exdended Biot framework. We show that the conversion scattering process from a P-wave into the diffusive slow shear wave is a descriptor of the vorticity diffusion process occurring within the viscous boundary layer. We make use of previous results for conversion scattering from P- to slow S-wave and derive a dynamic-equivalent P-wavenumber from which we deduce attenuation and velocity dispersion. Comparison with the predictions of other models shows that the conversion scattering approach can model attenuation and dispersion associated with the transition from the viscosity- to inertia-dominated regimes.
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Weakly Anisotropic Elastic Compliance
Authors B. Ursin and L. A. ThomsenThe elastic stiffness tensor is used to describe the anisotropic variation of velocity; the assumption of weak anisotropic parameters makes this feasible in the seismic context. Here, these same familiar parameters are used to describe the anisotropic variation of the elastic compliance tensor, thus making it feasible to use this tensor in several geophysical contexts. Among these are the effect of fluid substitution in anisotropic rocks.
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The Role of Shale Anisotropy in Reservoir Characterisation of Heterogeneous Sands
Authors P. A. Avseth and R. BachrachIn this paper we study the effect of shale anisotropy in a deviated well, and we are able to infer the anisotropic elastic moduli and Thomsen parameters by comparing the measured phase velocities in a deviated well with the velocities in an adjacent vertical well. After deriving shale anisotropy parameters, we apply Backus average combined with Gassmann theory to estimate 3D rock physics template models of apparent AI and Vp/Vs values at a given deviation angle (72 degrees), for varying reservoir heterogeneity and saturation.
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Quartz Cementation During Mudstone Burial – Implications for Rock Property Development
Authors B. Thyberg and J. JahrenPetropgrahic evidences of two different quartz cement morphologies have been obtained from Late Cretaceous mudstones, offshore Norway. 1) Fine-grained micro-pore-filling quartz found as discrete grains or short chains or clusters of inter-connected crystals interpreted to be part of larger micro-quartz crystal networks typically interlocking with aggregates of micro-quartz and clay crystals found close to and below 2500 m burial (~80-85 ºC). 2) Thin sheet-like quartz platelets mainly oriented normal to the overburden. The quartz platelets originate as flakes (~90-100 ºC) that evolve into well-developed patchy continuous quartz cement identified at 4300 m/150 ºC. The quartz cement is likely sourced by silica released from the dissolution of smectite and precipitation of illite-smectite/illite (and kaolinite to illite) due to elevated silica super-saturation in the system during burial. The observed inter-connected micro-quartz crystals are associated with a sudden Vp-velocity increase reflecting an increase in the mudrock stiffening interpreted to be related to a critical amount of micro-quartz networks and aggregate formation. The quartz platelets will during increased burial act as stiffening agent reinforcing and further cementing together the earlier formed micro-quartz networks and aggregates.
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Estimation of Carbonate Elastic Properties Using Nanoindentation and Digital Images
Authors O. de Paula, M. Pervukhina, B. Gurevich, M. Lebedev, M. Martyniuk and C. Delle PianePetrophysical properties of carbonate reservoirs are less predictable than the properties of silisiclastic reservoirs. The main reason for that is chemical interaction of carbonate rocks with percolating fluids, ion exchange and recrystallization in geological time. Quantification of the elastic variability of carbonate grains on the microscale is the first step to constrain models and to obtain more realistic predictions of practically important rock properties of carbonate reservoirs. In this study we present elastic moduli of an oolite sample from the Pleistocene Dampier Formation of Southern Carnarvon Basin, Western Australia obtained by the nanoindentation technique. Young moduli of this highly heterogeneous sample are measured at 49 points regularly distributed in a 70x70micrometer rectangular grid on the surface. The frequency diagram shows bimodal distribution of the Young moduli that correspond to dense calcite phase and rare (dissolved) calcite phase. These two solid phases are apparent in the high resolution scanning electron microscope images. We used the obtained moduli of the dense and rare phases for numerical modelling of elastic properties of the carbonate sample from micro-CT images. The results of the numerical modelling using finite element code are compared with the elastic moduli obtained from acoustic velocities measured by ultrasonic technique.
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Bounds for Viscoelastic Properties of Heavy-oil Rocks
Authors B. Gurevich and D. MakarynskaHeavy oils exhibit viscoelastic behaviour which is strongly frequency- and temperature- dependent. Due to the viscoelastic rheology of heavy oils the common elastic bounding methods such as Hashin-Shtrikman bounds are not rigorous for heavy-oil rocks. In this paper, we demonstrate that viscoelastic bounds of Milton and Berryman for the effective shear modulus of a two phase three-dimensional isotropic composite can be used as rigorous bounds for heavy-oil rocks. The viscoelastic bounds provide an effective tool for testing laboratory measurements and theoretical predictions for heavy-oil rocks.
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Joint Velocity–resistivity Effective Medium Models
Authors T. Han, A. I. Best, L. MacGregor, J. Sothcott and T. A. MinshullWe develop a 3-phase (quartz grain and brine pore fluid with clay dispersed in the pores) combined self-consistent approximation and differential effective medium (SCA/DEM) model for joint elastic velocity and electrical resistivity of reservoir sandstones. We firstly compare this joint velocity-resistivity model to experimental data obtained from 67 sandstone samples with a wide range of petrophysical properties to see how well it performs, and then we vary the key parameters (i.e., clay content and grain aspect ratio) to investigate how they might affect the joint properties.
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Detection of Reservoir Sorting Characteristics from Seismic Data
Authors M. E. Glinsky, J. Gunning, S. L. Bryant and D. C. DeMartiniIt is usually the case that rock permeabilities do not correlate especially well with elastic properties, so seismic based methods for estimating permeability are not conventionally viewed with much optimism. However, there are certain kinds of permeability variation that are driven chiefly by variations in sorting, and these sorting or grain-size distribution variations have a much more direct impact on elastic properties. We present an overview of modeling work applied to some deepwater field data where the turbidite reservoir facies has substantial grain size polydispersity, and this is shown to have significant seismic and permeability expressions. The effect of poor sorting on elastic properties is well captured by a simple model that takes into account the effect of non-load bearing (or 'floating') grains in the rock fabric. This model has ample corroboration from well logs, core permeability measurements, experimental laser-grain studies and numerical rock-assembly models. It is implemented as part of an open source stochastic seismic inversion. The statistical outputs of the inversion show that the sorting parameter is detectable in the field data.
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3D Microanalysis of Geological Samples with High–resolution Computed Tomography
Authors G. Zacher, J. Luebbehuesen, O. Brunke and T. MayerComputed Tomography for geological purposes can lead to a new dimension of understanding of the distribution of rock properties. Especially spatial distribution of pores and pore-connections as well as cementation properties are of utmost importance in the evaluation of reservoir properties. The possibility to visualize a whole plug volume in a non-destructive way and to use the same plug for further analysis is undoubtedly currently the most valuable feature of this new type of rock analysis and will be a new area for routine application of X-ray Computed Tomography in the near future. The paper outlines the hard- and software requirements for high resolution CT. It showcases several geological applications which were performed with the nanotom, the first 180 kV nanofocus CT system tailored specifically for highest-resolution scans of samples up to 120 mm in diameter and weighing up to 1 kg with voxel-resolutions down to <500 nm (<0.5 microns).
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The Importance of Clay Behaviour for Geomechanical Modelling of Gas Production in Unconsolidated Reservoir Sands
Authors F. D. E. Cuisiat, L. Grande and H. P. JostadHighly unconsolidated reservoirs may give rise to important compaction and subsequent sea bed subsidence are expected during reservoir pore pressure reduction which may have an impact on sub-sea infrastructures, borehole stability and completion integrity of wells. In this paper, 1D and 2D geomechanical analyses were performed to infer the most important parameters for reservoir compaction and assess the impact of parameter uncertainty on compaction and subsidence prognosis. Owing to the complexity of the reservoir architecture, geomechanical modelling is challenging due to; lateral and vertical heterogeneous distribution of sand / clay facies at large scale (reservoir grid cell) and smaller scale, and scaled geomechanical properties of the grid cell; large proportion of clay within the reservoir intervals and potential drainage of the clay during reservoir production.
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The Effect of Intra-reservoir Shales on Effective Stress Sensitivity
Authors Y. HajNasser and C. MacBethIn most clastic reservoirs experiencing pressure depletion, the sands in the reservoir naturally compact. As a consequence, the much lower permeability reservoir shales may experience extension. This extension is counteracted to some degree by pressure equilibration of the shale. The effective seismic response of the reservoir interval may thus be a mix of both hardening and softening reservoir components, depending on the balance of these phenomena. This effect is predicted to alter the overall stress sensitivity of the seismic properties from that anticipated for a homogeneous, fully connected reservoir interval. However, the final resultant response depends on the time period over which this effect is observed. Numerical computation using simplified geological models indicates shales of 1m to 10m thickness should be taken into account when quantitatively assessing the 4D seismic signature from frequently shot time-lapse surveys with a periodicity of 3 to 12 months, whilst 5 to 10m thick shales could impact conventional 4D seismic surveys shot over 5 to 10 years. These conclusions are strongly affected by the mechanical and transport properties of the intra-reservoir shales, their thickness and distribution, and are hence also a function of the depositional environment.
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Frequency and Fluid Effects in a Porous–cracked Rock – Experimental Investigations and Theoretical Predictions
Authors M. A. Adelinet, J. F. Fortin, Y. G. Guéguen and A. S. SchubnelWe conducted hydrostatic experiments on a basalt specimen in both dry and saturated conditions. The investigated icelandic basalt is characterized by a bimodal porosity, i.e., cracks and equant pores. The elastic properties -bulk moduli in our case- were investigated under high pressure through two experimental methods: (1) a classical one using ultrasonic P- and S-waves velocities (frequency 106 Hz), (2) and a new one, using oscillation tests (frequency 10-2 Hz). In dry condition, experimental data show no significant difference between high (HF) and low (LF) frequency bulk moduli. However, in saturated conditions, two effects are highlighted : a physico-chemical effect evidenced by a difference between drained and dry moduli, and a squirt-flow effect evidenced by a difference between HF and LF undrained moduli. We try to predict such a frequency dispersion from a theoretical point of view. An effective medium with cracks and pores is used for calculating the HF bulk and shear moduli. The LF moduli are obtained by taking the HF dry moduli and substituting into Gassman's equations.
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Exploring Trends in Microcrack Properties of Shales Using Stress Dependencies of Ultrasonic Velocities
Authors M. Pervukhina, B. Gurevich and P. GolodoniucStress dependency of full elastic tensor of shales is very important for seismic interpretation, overpressure prediction, 4D monitoring, etc. Using Sayers-Kachanov formalism, we develop a new stress sensitivity model for transversely isotropic (TI) media which predicts stress sensitivity behaviour of all five elastic constants. The model is used to parameterize elastic properties of about 20 shales obtained by laboratory measurements and from literature survey. The four fitting parameters (namely, tangential and normal compliances of a single crack, characteristic pressure, and pore orientation anisotropy parameter) show obvious correlations with the depth from which the shale was extracted. With increase of the depth, the tangential compliance decreases exponentially and the ratio of normal to tangential compliance increases linearly. The crack orientation anisotropy parameter exponentially increases with the depth for the most of the shales indicating that cracks are getting more aligned in the bedding plane. The characteristic pressure shows no visible correlation with the depth. The suggested model allows predicting of stress dependency of all five elastic constants if only two of them are known what can be useful, for instance, for the reconstruction of all five elastic constants of shale from log data.
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Microcracks Induced During Dilatancy and Compaction in a Porous Oolithic Carbonate Rock
Authors J. Fortin, S. Stanchits, G. Dresen and Y. GueguenReservoir rocks can undergo irreversible deformation (dilatancy or compaction) as a result of a change in effective stress during production of hydrocarbon or during CO2 storage. In this study, we investigated the mechanical behavior of the Chauvigny limestone. It is one of the rocks, which constitutes the Dogger, a deep saline aquifer, one of the favorable geological reservoirs for CO2 storage in France. To investigate the brittle-ductile transition in this limestone, we performed triaxial experiments, at confining pressure in the range of 5-100 MPa. In addition, the evolutions of elastic wave velocities were measured periodically with loading. Our results show that diltatant (nucleation and propagation of cracks) and compaction micro-mechanisms (plastic pore collapse) compete. Two limit cases can be distinguished. During hydrostatic compression, the inelastic volumetric strain seems to be mainly associated with plastic pore collapse, whereas for the triaxial experiments at confining pressure < 30 MPa, the inelastic volumetric strain seems to be mainly associated with the development of shear-induced cracks. For the triaxial experiments at confining pressure > 30 MPa, we are able to distinguish a first critical stress state where plastic pore collapse occurs, and a second stress state where shear-induced cracks are initiated.
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Frequency and Pressure Effects on Fluid Substitution in High Porosity Reservoir Sandstones
Authors A. I. Best and J. SothcottCaptain sandstone shows very high levels of ultrasonic attenuation in the laboratory suggesting significant velocity dispersion between seismic/sonic and ultrasonic frequency ranges. Some agreement was seen between squirt flow model predictions and the P-wave observations which suggests that P-wave velocity fluid substitution effects are consistent in the laboratory and in situ. However, uncertainly in model parameterisation (e.g., the true values of the unrelaxed dry frame moduli) makes it difficult to predict actual velocities and attenuations at seismic and sonic frequencies, needed for detailed interpretation of field data. More effort is needed to validate appropriate squirt flow and other elastic wave attenuation models over the whole frequency spectrum of interest.
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Vp-Vs Relationship of Glauconitic Greensand
Authors Z. Hossain, T. Mukerji and I. L. FabriciusThe relationship between Vp and Vs is used to predict Vs where only Vp is known. Thus theoretical, physical, as well as statistical empirical Vp-Vs relationships have been proposed for reservoir characterization. In published work, focus is primarily on the Vp-Vs relationship of quartzitic sandstone. Vp-Vs relationship of glauconitic greensand is not well defined yet. We present Vp-Vs relationships of glauconitic greensand by using laboratory and logging data from the Paleocene greensand Nini oil field in the North Sea. We also investigated Vp-Vs relationships of glauconitic greensand by an effective medium model. Both simple empirical Vp-Vs regression of greensand and Vp-Vs relationship from effective medium modelling provide good prediction of Vs from the measured Vp. Published Vp-Vs relationship for sandstone matched data less well. Keywords: Greensand, glauconite, Vp, Vs
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3D Reservoir Model of the Maiella Anticline (Central Apennines, Italy) – Validation of Different Modeling Approaches
Authors R. Di Cuia, A. Riva and C. TurriniThe main objective of this study is to verify the impact of different geological variables on the reservoir model (in particular on the final hydrocarbon distribution, accumulation and dynamics) and explore different ways to reduce the uncertainties by building a 3D reservoir model of the Maiella Mountain, a field scale structure made of Lower Cretaceous to Miocene carbonates. The Maiella Mountains offers, due to its geology and excellent outcrop exposure, the possibility to validate the impact of different: i) geostatistical methods to propagate the properties of the carbonate sequence, ii) data input and data distribution, into the reservoir model.
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A Comparison of Facies Modelling Strategies in Fan-delta Reservoir Analogues, Sant Llorenç del Munt, Eocene, NE Spain
Authors P. Cabello, O. Falivene, M. López-Blanco, J. A. Howell, P. Arbués and E. RamosThe Eocene aged, Sant Llorenç del Munt fan-delta complex is a well exposed, transgressive–regressive fan-delta, which crops out in NE Spain. Outcrop data were used to test a variety of modelling strategies in which well conditioning, stratigraphic subdivision, modelling algorithm and trends were all varied. The results of these modelling exercises were compared against themselves and against a close-to-deterministically built Base Case reconstruction using a series of static measures including the distribution of fan-delta front reservoir facies, directional connectivity and reservoir-to-well connectivity. The results highlight how well conditioning improves the reproduction of the stratigraphic architecture in the different modelling approaches. That stratigraphic subdivisions including the maximum flooding surface to separate independent grids for modelling the transgressive and regressive sequence sets can actually be detrimental unless additional constrains are included. That an algorithm combining a linear trend and a Gaussian field is the most suitable algorithm for reproducing this type of architecture, but requires defining a three-dimensional trend. And finally, the need for using trends to properly reproduce the architecture when well data are sparse. These results provide guidelines for modelling analogue fan-delta reservoirs in the subsurface.
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3D Analysis of Fracture Patterns Affecting an Outcropping Anticline – Inferences for Similar Buried Reservoirs
Authors G. Bertotti, H. Boro, J. van der Vaart and S. M. LuthiAnticlines are common structural hydrocarbon traps and are often fractured. Outcrops help in improving the characterization of fracture networks. New methods developed for the acquisition of fracture data from pavements and vertical outcrops allow for a full characterization of the spatial organization of fractures in the Tata anticline (central Morocco). Three sets of fractures are observed perpendicular, parallel and oblique to the fold axis. The first two groups have few tens of metres long joints; none of them can be followed across the entire anticline. Spacing distances are <0.8 m for the fold-axis parallel set and between 0.5 and 2 meters for the set perpendicular to the fold axis. The oblique set is composed of shorter features arranged in corridors. In the vertical dimension, fractures are always less high than the thickness of the multilayer. Highest densities are found at the top and the bottom of the succession and changes are gradual suggesting that the single layers are mechanically coupled and that it is the entire multilayer which acts as a mechanical unit.
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On– and Offshore Deep Weathered Basement Rocks, Norway
Authors M. Broenner, D. Bering, E. Dalsegg, K. Fabian, O. Fredin, B. Hendriks, C. Magnus, T. Møller, O. Olesen, J. S. Rønning, T. Solbakk and J. A. ØverlandThere has been an increasing awareness that surface processes next to tectonic processes have a large impact on the topography in Norway. Especially in the northern part in the Nordland county with its steep and high mountain chains, tectonic processes and erosion of the sedimentary succession seem not sufficient to explain the relief. Remnants of deeply weathered basement rocks indicate a deep alteration and later erosion of the bedrock and must be considered as another factor in the development of the relief. Weathered bedrock is also preserved offshore Norway, underlying large parts of the Mesozoic sedimentary rocks. A better understanding of the weathering processes and the distribution of deeply weathered zones onshore Norway is an important factor for society and economy in Norway. Offshore, weathered rocks can act as both reservoirs and migration routes for hydrocarbons. During a joint project of NGU and NPD comprehensive mapping of deep-weathering in Norway and resistivity measurements were carried out, investigating the processes and to estimate the magnitude of the weathering. We present results of the study from the Lofoten-Vesterålen and the Hamarøya area where rather thick layers of deep-weathered bedrock of up to 150 m thickness were observed.
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Mechanical Modelling of Fault Propagation Through Mechanically Layered Sequences – Implications for Fault Zone Complexity
Authors M. J. Welch, R. K. Davies, R. J. Knipe, C. Tueckmantel, V. O‘Connor and H. LickorishThis paper uses a new mechanical model (FaultProp) to study fault nucleation and propagation in mechanically layered sequences. Field mapping of two outcrops in Sinai and Utah show complex fault patterns formed when a horizontal extensional strain is applied to interbedded strata with contrasting mechanical properties: closely spaced low throw layer-bound faults form in certain layers, but the outcrops also contain several larger faults cutting through the entire section. Mechanical modelling with FaultProp shows that the key factors controlling the distribution and density of layer-bound and through-cutting faults are the contrast in friction coefficient and ductility between the layers. If these results can be upscaled then it is hoped that this modelling approach will be able to predict complexity in larger scale fault zones in the subsurface.
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Digital Characterization of an Aptian Carbonate Platform Margin from the Western Maestrat Basin (E Iberia)
Authors T. Bover-Arnal, O. Gratacós, D. García-Sellés, O. Falivene, R. Salas, J. A. Munoz and P. CabelloA digital outcrop model of a flat-topped non-rimmed carbonate platform margin of Aptian age was acquired using LIDAR technology. This was the basis for the reconstruction and modelling of the outcropping carbonate platform system. The resulting three-dimensional model displays the depositional geometries and facies heterogeneity. It also demonstrates the value of digital outcrop models to characterize features at different scales that are difficult to observe in the outcrops, as well as the potential of this methodology to refine outcrop-based sequence stratigraphic analyses.
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Geological Mapping and Structural Interpretation Using a Virtual Outcrop Digitiser, Examples from the Zagros (Iran)
Authors E. Casciello, J. B. Thurmond, N. Fernandez, E. Saura, I. Romaire, J. Vergés and D. W. HuntA vast region of the Lurestan Province of the Zagron ountain range (Iran) was studied integrating conventional geological surveys with the use of a virtual digitiser (VOD) employing high resolution satellite images. Examples are shown of the potentialities of this tool.
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Messinian Facies and Oligo-Miocene Slumping in Offshore Sirt Basin Libya and Sub-Messinian Seismic Imaging
Authors T. Sabato Ceraldi, H. M. Ali, T. Green, K. Benjamin, M. Bourne, M. Kamel, J. Alexander, K. Coterill, A. Poole and A. FraserDuring the Messinian (6.3-5.6 Ma) the Gibraltar Straits temporarily closed as a result of the continued collision of Africa and Eurasia. The relict Mediterranean ocean basin quickly evaporated and the sea level fell dramatically. As a result an erosional and karst surface was formed which can be identified in both outcrops and seismic data in the Sirt basin. At the base of the slope, the erosional surface becomes a bright seismic event which marks the Messinian evaporate basin. Well data indicate that anhydrite and gypsum were deposited in a sabkha environment. The central part of the Sirt Basin became isolated between the karstified slope and the outer basin high. The interval is characterized by a seismic facies of laterally continuous bright events onlapping the erosional surface. This facies is interpreted as clastic deposits within a Late Messinian enclosed lake (Lake Sirt). There is a strong correlation between the seismic facies and seismic data quality. The areas with best data quality correspond to the mapped extent of Lake Sirt characterized by clastic sedimentation. The areas with the poorest data quality correspond to the parts of the basin with sabkha depositional environments and anhydrite, largely surrounding the lacustrine environments.
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Meso– to Cenozoic Sequence Stratigraphy of the Tarfaya Basin, Morocco
More LessAlthough sufficiently mature source rocks (Early Jurassic, Late Cretaceous) and suitable reservoir facies (e.g. Late Jurassic, Early-Middle Tertiary) exist in the Tarfaya Basin (TB), past exploration has yielded few oil/gas shows but no commercially producing wells. In order to better understand the HC system of the TB, an integrated basin analysis approach has been applied including numerical modeling of the source and sink areas. Key methodologies include sequence stratigraphy, thermochronology, biostratigraphy, source-to-sink analysis, geochemistry and basin modeling.. The current sequence stratigraphic interpretation is based on i) well correlation, ii) seismo-/sequence stratigraphic interpretation; iii) data from outcrop analogue. The Mesozoic to Early Cenozoic basin development includes 5 major basin stages i) Permian to Pliensbachian rift- and sag, ii) Toarcian to Cenomanian drift, iii) Turonian to Early Eocene drift with initial Atlasian deformation, iv) Middle-Late Eocene drift with major Atlasian compression; v) Late Eocene to Early Miocene drift with major Atlasian uplift and inversion. This contribution focuses on sequence stratigraphy and numerical modeling in the sink area. At least 51 3rd order sequences have been identified along a key transect in the Jurassic-recent basin fill.
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Integrating 3D Seismic Data and 2D Geomechanically Balanced Restoration, Teak Field, Trinidad and Tobago
By X. LegrandIn the context of field appraisal in Colombus Basin (Trinidad and Tobago), it has been carried out a new structural geology interpretation with regard on Teak field. Detailed seismic analysis, which incorporates the principles of mechanic deformation, reveals evidence of fault segmentation and at reservoir scale. Seismic attribute analysis, core samples observations and 2D geomechanically balanced cross sections were integrated to increase the use of 3D seismic data for structural interpretation. Then X-Faults together with lateral ramps were indentified and lead to a better structural understanding. As consequences, reservoir compartmentalization is greater at the level of prevalent segment linkages.
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Structural Geometry of Late Jurassic Syn-rift Basins
Authors R. Z. Saddique and R. CollierSediment distribution and deposition in rift basins are controlled by source area catchment evolution and by accommodation space generation, as exemplified in the northern North Sea. The analysis and characterization of the structural geometries, fault linkages and the consequent distribution of Late Jurassic strata in the northern North Sea rift basin is demonstrated the mapping and interpretation of 3D subsurface data from the recent survey MC3D-Blocks 35/7 and 35/8 of PGS Exploration and well data of the wells 35/8-2 and 35/8-4 which is publically available via the Norwegian Petroleum Directorate. The detailed mapping of normal fault geometries and key horizons of the Brent Group and of the Heather and Kimmeridge Formations are used to demonstrate hangingwall subsidence patterns, and to delineate zones of footwall uplift and erosion. By integrating the stratigraphic age of strata with sub-crop mapping at unconformities, the study investigates sediment source areas for each basin and the likelihood of sand-rich material with useful reservoir potential being located within each syn-rift wedge. The study presents a structural model for the Middle Jurassic pre-rift and for the Late Jurassic syn-rift and implications for reservoirs plays. The approach to mapping of play fairways within the syn-rift megasequence is discussed.
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Outcrops with Seismic-scale Geometries – An Aptian Example from the Western Maestrat Basin, E Iberia
Authors T. Bover-Arnal and R. SalasThe analysis of seismic-scale geometries and facies heterogeneity, together with the identification and correlation of five surfaces with sequence stratigraphic significance in the platform to basin transition area of Aptian age exposed in the western Maestrat Basin (E Iberian Chain), permitted a successful characterization of a ‘four-systems-tract’ sequence. A subaerial unconformity, a basal surface of forced regression, a correlative conformity, a transgressive surface and a maximum flooding surface bound five differentiated systems tracts within two depositional sequences: the Highstand Systems Tract and Forced Regressive Systems Tract of Depositional Sequence A; and the Lowstand Prograding Wedge Systems Tract, Transgressive Systems Tract and subsequent highstand of relative sea level of Depositional Sequence B. This case study has the potential to become a reference, for both academia and industry, of the applicability of the four systems tract-based sequence stratigraphic method to carbonate systems.
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Late Cretaceous to Present Protracted Convergence between Arabia and Iran
Authors J. Vergés, E. Saura, E. Casciello, M. Fernàndez, I. Jiménez-Munt, D. García-Castellanos, S. Homke, D. Hunt and I. SharpThe Zagros orogeny took place during a protracted period of time, and its complete evolution is difficult to ascertain due to the multiple stages starting with oceanic obduction related processes and culminating with arc-continent and continent-continent collision. We document the geometries and ages of the Zagros foreland basin with a large number of dated samples along 16 stratigraphic successions, and hinterland denudation history with AFT ages on both bedrock and detrital samples. We integrate these data together with a crustal cross-section across the Lurestan region to constrain the total shortening amounts using extrapolations of rates of shortening as well as crustal reconstructions based on area balance from the Late Cretaceous times to present. The orogenic history of the Zagros is much longer and shortening probably much higher. The crustal area balance assuming shallow marine deposition at the onset of flexural basin formation could form with crustal thicknesses of about 27 km in the NW Arabian margin. Comparing to present day thickness of the crust underneath the internal Zagros Mountain presupposes a crustal shortening of about 150 km. This amount fits well will independent extrapolations of rates of shortening calculated in the Late Cretaceous-Eocene foreland basin in Lurestan.
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The Role of Spatial Integration in Play Analysis
By G. SmithWhat are the key decisions in the exploration process, and how do you make them? Peter Rose (2001) suggested that “The most difficult and critical decision in petroleum exploration is not which prospect to drill, but instead, which new play to enter”. We would argue that this is even more so the case in today’s climate of decreasing reserves replacement rates, renewed focus on exploration and increased competition for quality assets. We would also propose that the next most critical decision is what acreage to pursue after the play has been identified and analysed.
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An Integrated Study Resulting in (New) Prospects Generated from Old Data in an Explored Part of the Norwegian Shelf
Authors S. Sperrevik, K. Berry, D. Cumming, S. Eaton, C. Ekeli, J. Hesthammer, G. Maxwell, J. I. Rykkelid and T. WedbergThis presentation focuses on how new prospectivity has been identified and developed in a quite mature area on the Norwegian Shelf. The area of interest was recently awarded to PL 559 in the APA 2009 licensing round. It is located at the Nordland Ridge in blocks 6608/10 and 6608/11 just east of several discoveries and fields. The area has previously been part of license PL 128 since 1986, but was stepwise relinquished over the last few years due to disappointing well results where many of the traditional prospects have been proved dry. In the current study the entire area has been interpreted on 3D seismic, which has been tied to nearby wells. Using a new play concept and integrating data we have identified multiple attractive prospects of high commercial value at several different target intervals which have all been proven to contain oil in discovery wells immediately to the west. Studies including pressure evaluation, fault seal studies and migration modeling have been performed to mature and de-risk the prospects. CSEM modeling confirmed that the prospectivity was feasible for de-risking using CSEM technology, and both proprietary CSEM data as well as published material has been used in order to de-risk the prospectivity further.
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Using of Recent Geological and Geophysical Integrated Data for Evaluation of Residual Oil and Gas Potential of Thrust Be
By S. DhimaThe Cretaceous-Paleogene limestone sequence sealed by the Oligocene flysch is the main oil and gas condensate target, which has been and is still in focus of exploration along Albanides thrust belts. The later represent a segment of Alpine folding chain, situated between Dinarides and Hellenides. Enormous geological-geophysical data and integrated syntheses performed so far have brought about discovering of some oil and gas condensate fields, as well as depiction of many prospects. On the other hand, many wells drilled on some surface flysch folds or only on poor seismic events have resulted dry. What’s happened? The best response comes out from reconsideration of the exploration philosophy, as well as techniques in the course of acquisition, processing and interpretation of the all gained complex data.
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The Impact of Conjugate Margins Analysis on Play Fairway Evaluation – An Analysis of the Hydrocrabon Potential of Nova Scotia
Authors M. N. Luheshi, D. G. Roberts and H. WilsonAn understanding of the rift history of rifting margins is fundamental to developing prospectivity. A new analysis of the rifting history of the North Atlantic with particular focus on Nova Scotia and Morocco shows the possibility of new play systems potentially charged from an Early Jurassic syn-rift source system. The revised rift model is based on acquisition of new refraction data, reprocessing of existing refraction and multi-channel seismic as well as integration of potential fields data from the Nova Scotia/Morocco conjugate margins. The analysis of the rift history is integrated with a sequence stratigraphic based play based exploration evaluation, which has identifed some interesting new play types.
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Potential New Reservoir Targets Discovered in Channel and Canyon Features – Offshore Gabon
Authors J. Greenhalgh, R. Borsato, R. Findlay, S. Raussen, H. Zhu, M. Martin, M. R. Moussavou and F. F. MoukoumbiThis paper outlines examples of potential new reservoir channel and canyon features observed in the interpretation for the Gabon MegaSurvey. The canyons occur at the Base Miocene Unconformity and the channels are imaged at the Ozouri formation and offer exciting new exploration opportunities. Their relevance and origins are discussed with respect to the regional geology. The interpretation describes the various methods used to image these canyons and channels. Methods, that can be incorporated into a workflow that will define a potential prospect to drill. The well developed canyon and channel systems identified in the Gabon offshore area is a potential new reservoir target similar to prospects targeted in the Niger and Congo deltas.
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Prospect Identification Using AVO Inversion and Lithology Prediction
Authors F. Barclay, B. Bailey and A. PaxtonIn 2008, WesternGeco acquired the 5000 sq/km Keystone multi-client survey located offshore between the giant Io/Jansz and Scarborough gas fields on Australia’s NW shelf, in order to image potential gas reservoir’s at multiple stratigraphic levels within complex geology. Searching for leads and prospects within such a large volume using seismic data alone is traditionally a time-consuming process. An efficient prospect screening tool has been produced in the form of a well log-calibrated gas sand probability cube derived from amplitude variation with offset (AVO) inversion outputs. Using advanced visualisation techniques the data can then be rapidly interrogated highlighting a number of direct hydrocarbon indicators (DHI's) and potential prospects. AVO and quantitative interpretation have played a key role in a number of discoveries in this area, particularly within the Triassic.
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The Hydrocarbon Prospectivity of the Walton Basin offshore Jamaica – High-graded Through New Geological Interpretations
Authors S. Westlake, D. Jablonski, C. Gumley and J. LutroThe hydrocarbon prospectivity of the Walton Basin offshore Jamaica has been high-graded through new geological interpretations and advances in seismic imaging. This basin has seen little hydrocarbon exploration in the last 25 years but recent exploration efforts has identified a potential new oil and gas province. Only 11 hydrocarbon exploration wells have been drilled in Jamaica to date and 10 of these have had oil or gas shows, yet surprisingly there has been limited modern exploration interest in Jamaica. Historically the Walton Basin has been plagued by poor seismic imaging from the effects of the thick Tertiary carbonate cover and present day carbonate banks and consequently the Basin has been downgraded. Simply put “if you can’t see it, you can’t explore for it”. Recent advances with the acquisition of new long offset 2D seismic data and reprocessing the data using Beam pre-stack depth migration now provide a step change in the seismic imaging of the sedimentary section. This advance, coupled with the increased geological understanding of the basin, means that the new and re-rated plays within this overlooked basin can be imaged and industry can now focus towards exploration drilling to unlock its hydrocarbon potential.
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Calibration of Spectral GR Log with NMR Log and SEM Analysis for the Evaluation of Reservoir Shale Volume
Authors M. H. Mohammadlou, M. B. Mork and H. LangelandPresence of mixed carbonate and siliciclastic sediments in reservoir formations complicates the use of appropriate petrophysical tools for shale volume estimation. In this case study conventional uranium corrected gamma-ray log (HCGR) together with thorium and potassium spectral logs were initially used for estimation of shale volume. Although this traditional method is straightforward, the results are not supported from comparative studies by NMR T2 spectra. Significant discrepancy resulted in an interval of the well where a considerable shale volume was detected from the logs. The source of the discrepancy was investigated by scanning electron microscope (SEM) analysis. The result verifies a source of potassium jointly from a mixture of clay minerals and potassium feldspar. Knowing the proportion of minerals from SEM mapping allows calibrating the HCGR log for the clay contributed potassium content in selected samples. The new corrected spectral gamma-ray values are used in NMR T2 clay bound cutoff selection and thereby shale volume calculation. The results show a significant reduction of the clay volume in some intervals.
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A Novel Approach for Automated Lithology Extraction from Core Photographs
Authors A. Thomas, M. Rider, A. Curtis and A. MacarthurAn automated lithology extraction methodology from core photographs using object based image analysis technology is developed. A knowledge based scheme (human interaction) is used for defining possible classes present in the image and for selection of appropriate samples (objects) that belong to each lithology classes. This can be saved separately and can be called using the program. Later a classifier (for automated classification) is trained to assign the unknown objects into appropriate classes based on the knowledge base. So the methodology can be quickly adapted to images from various geologic areas with adjustments in the knowledge base used to train the classifier. Hence this new methodology offers a fully automated lithology extraction methodology for any field. The automated classification calibrated to geologist’s interpretation ensures the accuracy of the new methodology and the good match between the two gives the confidence to extend the use of this new methodology to the entire field. In addition, the methodology is applicable to several kinds of borehole images, for example wireline electrical borehole images and the more specialized LWD images.
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Burial Diagenesis of Eocene Chalk of the Atlantic Ocean
Authors A. Awadalkarim and I. L. FabriciusIn this paper, the burial diagenesis as reflected on porosity of a time-slice of Eocene chalk of the deep Atlantic Ocean is addressed. Previous authors combined chalk data from different geological ages in porosity trends. The total overburden and effective stresses were calculated and plotted versus porosity data. The porosity decreases by increasing burial depth either via both mechanical compaction and pore filling cementation. It seems that mechanical compaction is a major cause of porosity reduction in the studied Eocene chalk. It reduces porosity from 70% to about 30%. Our data indicates that mechanical compaction is the principal agent of porosity reduction in the chalk, at least down to about 5 MPa effective stress corresponding to a porosity of about 35%.
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