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77th EAGE Conference and Exhibition 2015
- Conference date: June 1-4, 2015
- Location: Madrid, Spain
- Published: 01 June 2015
41 - 60 of 980 results
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3D Facies Model in Large-scale Fan Delta Complexes (Montserrat and Sant Llorenç del Munt, Eocene, Ebro Basin, NE Spain)
Authors M. Diviu, P. Cabello and M. López-BlancoAn exploration scale facies model (375 km2 and 325 m thick) of two outcropping fan-delta complexes developed during the Eocene in the SE Ebro basin margin was produced. Input data for the model comprises detailed geological maps. This model satisfactorily captures the heterogeneity linked to the T-R cyclicity at two scales. The transgressive and regressive organization of coastal facies belts (at scale of sequence sets and composite sequences) was achieved by the reconstruction of key surfaces and modelling facies belt boundaries in between by using the linear expectation trend of TTG algorithm. The position of these facies boundaries was fixed on the basis of paleogeographic maps extracted from detailed field mapping. Due to its small scale expression, high frequency cyclicity (fundamental sequences) was addressed by the stochastic part of the TTG algorithm. The model reproduces a continuous delta front facies belt connected along the different sequences. However, its complex geometry also capture the existence of potential stratigraphic traps related to the endings of fan-delta front reservoir analogue facies wedges into prodelta and distal alluvial mudstones resulting from the T-R cycles at composite sequence scale.
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Deciphering Turbidite Triggers by Core Facies Analyses. Implications for Geohazards and Reservoir Characterization
Authors J.P. Corella, B.L. Valero Garcés and J. GerardSediment cores retrieved in turbidite sediments from modern lacustrine systems enable to understand the sedimentological processes and depositional patterns of turbiditic flows as well as to evaluate their triggering mechanisms. Lacustrine turbidites were analysed in Lake Montcortès situated in NE Iberian Peninsula (42º 19´N / 0º 59´E). Multiproxy core facies analyses were applied to investigate the turbidite record of this karstic lake system. Integration of geochemical, geophysical and sedimentological analyses and micro-computed tomography (micro-CT) imaging led to the identification of two types of turbidites and distinct triggers have been proposed to generate them: i) low density turbidity currents resulting from sediment-laden hyperpycnal currents related to extreme floods in the watershed and ii) seismic-induced subaquatic slope failures. Geohazard assesments can be implemented by understanding the variability of these extreme events (earthquakes, tsunamis and large floods) preserved in the sedimentary record at centennial to millennial scales. In addition, deciphering triggering mechanims in core facies analyses may have significant economic implications as both processes eventually lead to different turbidites geometries and textures that may affect reservoir quality although lacustrine turbidites are not the most common hydrocarbon reservoirs.
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Least-Squares Reverse-Time Migration of Simultaneous-Source Data
Authors W. Dai, W. Lewis and R.T. CoatesSimultaneous-source recording, where multiple shots are fired at very similar times and recorded by the same receiver array, has been proposed as a method of both improving source sampling and reducing survey acquisition costs. However, this acquisition method complicates the subsequent processing considerably, requiring either data separation or an imaging method capable of accepting simultaneous-source data as input without generating artefacts or crosstalk. In this paper we demonstrate that least-squares reverse-time migration (LSRTM) is a promising candidate algorithm for this application. Using the 3D SEG Advanced Modeling Program (SEAM) salt model we show that LSRTM can effectively image simultaneous-source data suppressing crosstalk while at the same time improving bandwidth.
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Simultaneous Shooting for Sparse OBN 4D Surveys and Deblending Using Modified Radon Operators
Authors R.R. Haacke, G. Hampson and B. GolebiowskiSignificant gains in productivity (and savings on survey time and cost) have recently been achieved using simultaneous sources with high-density Ocean Bottom Cable geometries. However, where the acquisition is receiver-bound, using sparse Ocean Bottom Node (OBN) arrays for example, the argument for use of simultaneous sources is less compelling. Time and motion analysis for a rolling array of sparse OBN, spaced by 390 m with a 30x30 m shot carpet, shows that time savings of the order of 10 % of the equivalent single-source survey duration should be expected. Although small this may be significant in tightly constrained acquisition seasons. Since time-lapse surveying is the main motivation for many sparse OBN acquisitions, the saving in survey time must be balanced against the risk of additional 4D noise created by simultaneous-source crosstalk. Attenuating crosstalk using a new form of Radon operator implemented with cascaded fx prediction and interpolation as part of a kill-fill process, the level of 4D noise created by simultaneous sources is reduced to an ambient level of 6 %, with a highly randomized character, after imaging.
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Free-surface Multiples and Full-waveform Inversion Spectral Resolution
Authors V.V. Kazei, B.M. Kashtan, V.N. Troyan and W.A. MulderLow frequencies play a crucial role in the convergence of full-waveform inversion to the correct model in most of its current implementations. However, the lower the frequencies, the bigger are the amplitudes of the surface waves, causing the inversion to be driven by the latter. If they are not blanked out or removed, this may lead to convergence problems. To analyze this situation, we consider the simplest case where surface waves are present: an acoustic layer over a halfspace. We earlier analyzed the contributions of various wave types to the wavenumber spectrum of a velocity perturbation above a reflecting halfspace, without a free surface. Here, we extend this spectral sensitivity analysis to the case with a free surface, which generates multiples and ghosts. In this setting, the surface guided P-waves can be considered as a superposition of free-surface multiples. Our analysis shows that the conditioning of the linearized inverse problem, which is solved at each iteration of full-waveform inversion, becomes worse when multiples are taken into account. At the same time the inclusion of multiples increases the sensitivity to some low wavenumbers in the model spectrum, which should be beneficial for full-waveform inversion once a suitable preconditioner has been found.
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Well Constraints in Full Waveform Inversion and its Application in Time-lapse Seismic
More LessFull waveform inversion (FWI) has become an integral part of the velocity model building flow due to its resolution, especially in the shallow regimes. Most of the times the wide angle acquisition is not available because of its expense and we still would like to use the power of FWI by utilizing the vintage data. In these older prospects since there is a lot more geological information available besides just seismic data, we can use information from wells for example, to constrain the FWI iterations. Even though well information is at our disposal it is difficult to incorporate it into the traditional FWI which is represented by data misfit objective function. We incorporate wells in the model is to build an apriori model from them and take this into account in the objective function. The objective function can be extended by a model misfit term alongside the data misfit one. We demonstrate how the new objective function works on real data. Furthermore we introduce two workflows to recover model difference between the base-line and the monitor or monitor surveys by FWI in the time-lapse seismic world.
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Elastic Waveform Inversion with Modified Surface Boundary Conditions for Land Seismic Data
Authors C.A. Pérez Solano and R.E. PlessixWe present an elastic waveform inversion for low-frequency wide-aperture land seismic data. The presence of wavelength-sized contrasted layers limits the applicability of acoustic inversion. This happens above about 3 Hz when a few hundred meters thick carbonate layers interleave with shale/sand layers, as seen in the Middle East. Elastic inversion is challenging because of computational cost and, in land, because of ground roll. To retrieve the long-to-intermediate wavelengths of the P-velocity, we invert the weak diving waves and remove ground roll in the pre-processed data. We modify the surface boundary conditions and obtain an elastic modelling without surface waves by zeroing the normal derivatives of the shear stresses at the surface. An analysis of this modification shows that the reflection coefficients become independent of the velocity ratio and the PP-reflection coefficient is similar to the acoustic one. This modification allows us to develop approximate elastic modelling of diving/transmitted waves without implementing ground-roll suppression for the modelled data. Moreover, it allows us to use large velocity ratios to speed-up the inversion without degrading the resulting P-velocity model. This approach can be seen as intermediate between acoustic and elastic inversion with free-surface conditions. We illustrate it with synthetic and real data examples.
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VSP Measurements Used as a Tool for Sub Salt Near Field Development
Authors T. Bartels, M. Gelhaus and M. HumphriesMany of the Rotliegend low permeability dry gas fields in the Southern Permian Basin in northern Germany are covered by salt dome structures that lead to imaging problems in subsalt reservoir between 4000 m to 5000 m depth. As compartmentalization is the key challenge in field development high resolution seismic is essential for well planning. Only a vintage 3D seismic from 1993/94 with limited quality exists. Acquisition of new 3D seismic is not possible due to permitting restrictions and public resistance. Therefore the focus is on the acquisition of 2D VSPs in every new drilled well. The VSP measurements give local insights. With a general structural understanding and production history in mind those local insights can be merged into the overall picture and generate immense value for the understanding of the field.
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Incorporating Seismic Velocity Data in AVO/AVA Low Frequency Models by Honoring Local Geology
More LessLow frequency information is required for quantitative reservoir characterization. Because borehole measurements are laterally sparse and preferential towards reservoir locations, there is much uncertainty on the low frequency models away from well control. Methods to improve the reliability of the low frequency data include the use of low frequency update schemes or seismic attribute maps. The use of seismic velocity data for trend modeling is well recognized, but the methodology for incorporating the velocity is not always clearly described. Especially in case of an AVO/AVA study, where low frequency information for several elastic properties is required, a rigorous workflow is desired. Here, we propose a method to include seismic velocity data. The methodology uses local geological knowledge through rock physics relations. We validate the method by comparing results of a more common method with our proposed workflow at blind wells. This shows that a low frequency model that does not use the velocity data misses significant lateral variations that are representative of the local geology.
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Velocity-porosity Evolution of Mechanically Compacted Brine-saturated Sand-clay Mixtures
Authors M. Koochak Zadeh, N.H. Mondol and J. JahrenIn this study, four brine-saturated specimens of unconsolidated sands and sand-clay mixtures are tested to infer velocity-porosity trends of sand-dominated facies when a full control on mineralogy is applied on the tested samples. The current results show consistency with the published experimentally derived compaction trends of natural sands. The iso-stress curves connecting the data points with the same effective stress in the velocity-porosity plot are comparable to the friable sand model curves proposed to estimate the rock properties of sands within the mechanical compaction domain. As expected, the achieved iso-stress curves for mechanically compacted samples are too far away from the empirical lines suggested estimating rock properties of sandstones within the chemical compaction domain. To check creditability of the rock physics models in both mechanical and chemical compaction domains, the log-derived petrophysical properties acquired from the North Sea are compared to the derived experimental compaction trends. The shallow unconsolidated sands follow closely the experimentally derived iso-stress curves and friable sand model but, the consolidated sandstones show a wide range of physical properties and may sometimes follow different trends compared to suggested rock physics models.
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Stiff- and Compliant-porosity Based Model of Permeability - Theory and Experiments
Authors S.A. Shapiro, G.P. Khizhniak, V.V. Plotnikov, R. Niemann, P.Y. Ilyushin and S.V. GalkinRelation between porosity and permeability is not unique. We propose that comparison of functional dependencies of both, porosity and permeability, on stress provides useful constraints for their relation. Such a comparison shows which part of the void space in rocks controls the permeability, the compliant porosity or the stiff one. The compliant porosity (including very thin cracks and grain-contact vicinities) usually controls load dependencies of elastic moduli of rocks. This leads to exponentially-saturating functional dependencies of elastic properties on effective stress. Stiff pores (high-aspect-ratio pores) are less significant by loads from low to moderate ones (several tens of MPa). However, stiff pores can be of importance for the stress dependency of permeability. We propose a rather general model of permeability as a function of the stiff and compliant porosity. The model includes possibility, that in different rocks permeability can be controlled by stiff pores or, alternatively, by compliant pores or, finally by a combination of them. This model provides a powerlaw or an exponential differential-pressure dependency or a mixed behaviour of permeability in these situations, respectively. We show experimental results, on samples from a broad data basis of sedimentary rocks of Russian Perm region, indicating these types of behaviour.
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Shifted Hyperbola Revisited - The Two Faces of NMO
Authors B. Schwarz, C. Vanelle and D. GajewskiIn many present-day applications in seismic processing, the assumption of a homogeneous model leads to simple yet powerful approximations, which also work well when heterogeneity is not negligible. While the classical NMO hyperbola assumes an effective constant velocity medium, de Bazelaire, based on optical projections, introduced an alternative way to account for heterogeneity by shifting the reference time rather than the velocity. In this work, we provide new insights into the auxiliary medium concept and introduce a generalized osculating equation, which allows for the forward and inverse transformation between the effective and the optical domain, thereby providing a unified view on currently used stacking approximations. Supported by synthetic tests, we reveal that all higher order operators can be described in and transformed between both domains, which, through combined use, suggests interesting new applications, like, i. e., in diffraction separation or surface-related multiple elimination.
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Inversion of Nonhyperbolic P-wave Traveltimes for Interval Transverse Isotropy Parameters
Authors M. Asgharzadeh, D. Nadri and A. BonaIn this study, we analyse the accuracy of layer-stripping method to use P-wave reflection traveltimes in transverse isotropic media with vertical axis of symmetry (VTI) to estimate interval anisotropic parameters that are required for seismic processing. We also present a synthetic Walkaway VSP example in which a comparison can be made between the results of layer-stripping method and conventional method of using P-wave slownesses measured in a borehole. In the error analysis part, we quantify the errors of estimating interval Thomsen parameters, Epsilon and Delta for a horizontal VTI layer underneath a VTI overburden using noise free traveltimes computed by anisotropic ray tracing. Our analysis shows that both the parameters can be well constrained using traveltimes corresponding to maximum offset to depth ratio equal to 1 and overburden anisotropy plays no significant role in the accuracy of the inversions. On the other hand, parameter estimation using synthetic VSP data shows that an accurate and stable inversion requires traveltimes taken from spreads with maximum offset to depth ratio equal to 1.4 for Delta estimation and 4 for Epsilon estimation. The presented layer stripping-method also compares well with slowness based inversion using VSP data.
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Exact Elastic Impedance Tensor for Isotropic Media
More LessConventional elastic/ray impedance approximations are derived as a scalar to heuristically present seismic reflectivity based on the assumptions of weak impedance, isotropic media, or weak anisotropic media. In this paper, considering the concept of impedance in general physics, we define impedance tensors according to the relation between stress and velocity fields. The impedance tensors have explicit physical meaning and thus each of the derived elastic impedance components represents a unique mechanical property of the medium. Because no assumptions are made in the derivation of the exact expressions of elastic impedances, the expressions have higher accuracy than the approximations of conventional elastic impedance, and can thus potentially be applied in the characterization of unconventional formations having strong impedance contrast. The properties of each of the elastic impedance components are discussed through theoretical analysis. Logs of a turbidite fan reservoir are used to study the interpretation capability of the derived EIs in lithology/facies discrimination. We also discuss a practical workflow of seismic inversion, and thus provide a new method of reservoir characterization.
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Reflection-response Retrieval with Seismic Interferometry by Multidimensional Deconvolution from Surface Reflection Data
Authors B. Boullenger, J. Hunziker and D. DraganovSeismic interferometry (SI) allows retrieval of virtual-source responses at positions of receivers, where no actual source is shot, by cross-correlating (CC) the seismic responses between receivers. The theory requires a boundary of subsurface sources to retrieve the surface reflection response. With reflection data acquired with both sources and receivers at the Earth’s surface, the retrieved virtual-source reflection responses suffer from non-physical arrivals and amplitude errors that may be significant. Instead of using the common CC method, we propose an approximate method to apply SI by multidimensional deconvolution (MDD). The method is data-driven and does not require a priori information about the subsurface. Numerical results show that, although its effect on the non-physical arrivals is limited, the MDD method clearly improves the retrieved amplitudes by flattening the spectrum and balancing the illumination of the virtual-source responses. Therefore, the virtual-source response retrieved by MDD is a better estimate of the reflection response than the response retrieved by CC. The additional reflection data retrieved by MDD have higher potential of filling in possibly missing source data in the original dataset.
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Using the SAR Mode from CryoSat-2 to Improve Satellite Derived Gravity near the Coast
Authors G.J. Dawson, C.M. Green and K.M.U. FletcherGravity derived from satellite radar altimeter data has been used extensively in hydrocarbon exploration and geological mapping. In this presentation we investigate if we can improve coverage in coastal areas, by using the synthetic aperture mode (SAR) from CryoSat-2. We directly compare two co-linear satellite tracks off the coast of Central Sulawesi, where the satellite was in low-resolution (LRM) mode one year and SAR mode another year. We identify peaky waveforms in the LRM mode that have been contaminated by land based reflections. These land based reflections adversely affect the re-tracking of the waveform, and we were only able to reliably obtain sea-surface heights approximately 5 km from the coast. When in the SAR mode, contaminations from land based reflections only appear when the nadir point is very close to the coast, and we were able to reliably obtain sea-surface heights up to 1 km from the coast. In areas where the CryoSat-2 satellite is in SAR mode, this improved coverage should lead to better resolution in satellite derived gravity near the coast.
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Combined Surface-wave and Reflection Data Inversion for High-resolution Near-surface Characterization
Authors G. Rizzuti and A. GisolfIn this paper we present a full wave-field inversion approach to address the near-surface problem. Knowing this part of the subsurface will effectively remove its imprint on the target data. We are looking for an elastic, high-resolution model of the near-surface by including iteratively mode conversions, transmission effects and all the internal multiple scattering. An interesting aspect of our work is the natural treatment of surface-waves.
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A High-order Fast Sweeping Method for Calculating Quasi-P Traveltimes in 2D TTI Media
More LessThe fast sweeping method is a potential robust approach for performing seismic anisotropic raytracing. It propagates a wavefront by sweeping different directions to cover all possible wave directions. There is no need to maintain a sorted wavefront in fast sweeping method, thus the computational cost approximates to O(N) ops instead of O(NlogN) ops, where N is the total number of grids. The fast sweeping method has been widely used for isotropic raytracing. Although some efforts have been made to apply the method for anisotropic raytracing by solving static Hamilton-Jacobi equations, they are not straightforward for TTI media. In this study, we propose a high-order fast sweeping method to calculate quasi-P traveltimes in TTI media. We derive an analytical solution of the fourth-order polynomial equation in terms of slowness in the local solver. Our method does not assume weak anisotropy and is an accurate solution of the quasi-P traveltime in a general TTI medium.
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QP Wave Forward Modeling and Illumination Analysis in Viscoelastic VTI Media by One-way Wave Equation
More LessSeismic forward modeling and illumination analysis can help people understand the laws of seismic wave propagation in subsurface media which can guide the design of recording geometry and provide insufficient foundations for the processing and interpretation of seismic data. Experiments and theoretical analysis demonstrate that subsurface media exhibit not only anisotropic properties but also the anelastic properties, which are commonly denoted by viscoelastic model. In this abstract, we calculated the qP wavefields and the illumination of viscoelastic VTI media by one-way wave equation method and studied the propagating laws of qP wave in this kind of media.
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Diffraction Imaging of the Zhao Dong Field, Bohai Bay, China
Authors M.A. Pelissier Company, T.J. Moser, L. Jing, P. de Groot, A. Sirazhiev, I. Sturzu and A.M. PopoviciWe provide an overview of integrated pre-stack depth migration and diffraction imaging for the Zhao Dong field, Bohai Bay, China. This field is highly compartmentalized by a complex faulting. The objective of the diffraction imaging is to better define these faults. Tools to facilitate interpretation include displays with pre-stack depth migration and diffraction images overlain in different colour scales, the combination of pre-stack depth migration and diffraction images into a single volume and a diffraction image obtained with a spatially varying taper calibrated to local reflectivity.
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