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75th EAGE Conference & Exhibition incorporating SPE EUROPEC 2013
- Conference date: 10 Jun 2013 - 13 Jun 2013
- Location: London, UK
- ISBN: 978-90-73834-48-4
- Published: 10 June 2013
101 - 120 of 1113 results
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Multiple Phases of Hydrocarbon Generation and Migration in the Ordovician Carbonate Reservoirs in the Tazhong Uplift
Authors J.Q. Chen, H. Pang, X.Q. Pang and Z.X. JiangThe Ordovician carbonate reservoirs in the Tazhong area, tarim basin, were formed in complex geological conditions, however, rich in hydrocarbon resources. It is of important significance to study the multi-stages of hydrocarbon generation and migration. This study integrates geological and geochemical methods to analyze the issues. The results indicated that the hydrocarbons in the Ordovician carbonate rocks of the Tazhong area were primarily derived from the Mid-Lower Cambrian and Mid-Upper Ordovician source rocks. The oil mainly came from the mix of the two source rocks and the gas from the oil cracking gas of the Mid-Lower Cambrian source rock. Hydrocarbons migrated into the Tazhong Uplift from the northern part of the Majiaer Depression through a series of injection points over four periods. There were a total of six injection points identified at the intersections of the NE trending faults and the NW trending flower strike faults. The hydrocarbons then migrated from the injection points into traps along regional trends from NW to SE and from NE to SW. There were four periods of hydrocarbon accumulation in the area, i.e. the Cambrian period-Ordovician period, the Silurian period-Devonian period, the Carboniferous period-Jurassic period and the Cretaceous period-Present.
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Oil Ocurence and Migration Pathways in Central and Meridional Atlas of Tunisia
Authors A. Belhaj Mohamed and M. SaidiThe analysis source rock of the central and meridional Atlas of Tunisia indicates that the Albian Fahdene Fm has a substantial oil-generation potential. The Cenomanian-Turonian Bahloul Fm exhibits fair to high organic content and petroleum potential immature to mature source rock. The Jurassic source rock exhibits fair to good organic content and petroleum potential located in the south of this area. Molecular characterization indicates that most of the produced oils appear to be sourced from a predominantly shaly marine and mature source rock deposited in suboxic to oxic depositional environment. Oil stains located in the northern part of the area seems to be sourced by a marly or argillaceous limestone. Oil-oil correlations and molecular characteristics of selected source rock samples and maturation models suggests that oils were generated from similar source rock and/or organic facies. The Fahdene Fm is the best source candidate to generate these fluid samples. The integration of benzocarbazoles as geochemical molecules tracer has permitted a relative estimation of the lateral migration distance for the different accumulations of the area. Basin modelling outlined the extent of the different petroleum systems in the area and permitted the elimination of the risky prospects for the next exploration phase.
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Controls on Distribution of Chlorite Coats and Reservoir Quality in Cretaceous Tuscaloosa Sandstones, Gulf of Mexico
Authors S.P. Dutton, W.A. Ambrose and R.G. LoucksSome of the Upper Cretaceous Tuscaloosa sandstones from onshore central Louisiana, Gulf of Mexico basin, retain anomalously good reservoir quality (porosity >25% and permeability >100 md) at depths >6 km. High porosity and permeability occur in sandstones with thick, continuous chlorite coats around detrital grains that inhibited quartz cementation. Other Tuscaloosa sandstones at the same depth are tightly cemented by quartz or carbonate. We examined variable distribution of reservoir quality in Tuscaloosa sandstones in an area of uniform provenance and burial history. Sandstone composition was determined by point counts of 141 thin sections from 15 wells. Core descriptions and facies interpretations provided a sequence-stratigraphic and depositional-systems framework. Volcanic rock fragment content and grain size have weak but statistically significant correlations with chlorite volume. Higher permeability samples also contain fewer ductile grains and have higher intergranular volume. Chlorite is slightly more abundant in lowstand fluvial-channel deposits (5.9%) than in highstand deltaic deposits (4.7%) or in transgressive deposits (4.9%). Understanding the controls on porosity and permeability in onshore fluvial and deltaic Tuscaloosa sandstones will aid in predicting reservoir quality in deepwater Tuscaloosa sandstones deposited on the basin floor, which have been penetrated in deep Gulf of Mexico exploration wells.
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Microcrystalline Quartz and the Preservation of Anomalously Good Reservoir Quality in Sandstones - New Insights into Why Quartz Overgrowths are Inhibi
Authors R. Worden, M.W. French and M. BukarMicrocrystalline Quartz and the Preservation of Anomalously Good Reservoir Quality in Sandstones - New Insights into Why Quartz Overgrowths are Inhibited
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Is Grain-to-grain Pressure Solution Contributing to Quartz Cementation in Sandstones?
Authors J. Jahren and T.E. MaastThe role of grain to grain pressure dissolution versus dissolution at stylolite or clay induced dissolution (CID) of silica in quartz rich sandstones is evaluated from careful examination of samples from the Precambrian Hanglecćrro Formation located on the Varanger Peninsula in northernmost Norway. The Hanglecćrro Formation is an orthoquartzite where all porosity is gone leaving an end product suitable for studying compaction processes in detail. The study show that the intragranular volume (IGV) in all samples is above what would be expected from grain to grain dissolution confirming that this mechanism is not important in siliciclastic sandstones. The variability in IGV was found to be a function of textural parameters where sorting was found to be the main factor reducing IGV. Textural parameters are important only during mechanical compaction. During chemical compaction IGV will remain constant. Empirical equations that predict the IGV as a function of effective stress and textural parameters should be incorporated into state-of-the-art reservoir quality predictive tools in order to more accurately predict mechanical compaction, and the expected porosity range in any given sandstone.
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Reservoir Quality Quantitative Prediction - Methodology and Case-Study
Authors A. Ortenzi, A. Consonni, M. Ciuffardi, F. Golfetto and M. GorzaThe efficiency of a sandstone reservoir is a function of the initial depositional parameters (grain-size, sorting and composition) and of the post-depositional evolution. The depositional parameters are related to the sedimentary environment and the post-depositional evolution depends on the paleo-dynamic geological setting. The reservoir quality of sandstone is currently appraised in Eni with two methodologies. The first is based on the Reservoir Efficiency Index© (REI), a quantitative indicator of the quality of a clastic reservoir calculated on the petrographic data as obtained from thin sections. The values of REI at calibration wells are mapped through a dedicated geostatistical tool (Softkrig). The second methodology is based on a reservoir quality modelling software (Touchstone™), able to predict the mechanical compaction, the quartz cement precipitation and the illite formation. Both methodologies have been applied to an African reservoir. The reservoir is of Cretaceous age and the main reservoir quality reducing factor is the cementation by quartz, linked to the burial/thermal histories in the different zones of the basin. The case-study shows the pros and cons of the methodologies. The REI mapping is recommended in the play fairway analysis, while Touchstone is suitable for the determination of porosity and permeability on prospect/leads.
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Diagenetic and Sedimentary Controls on Porosity and Permeability in Upper Triassic Tight Gas Sandstone, Sichuan Basin
Authors H.H. Zhu and D.K. ZhongThe reservoir quality prediction of tight gas sandstone is difficult due to the heterogeneity of porosity and permeability. Thus developing a better understanding of porosity and permeability in tight sands is critical for future gas exploration. In this paper study of the sedimentary and diagenetic controls on tight gas sandstone quality was conducted using the core data. the result shows: 1) Hydrodynamic condition is a controlling factor of Xujiahe tight sands quality in Southern Sichuan basin. Sandstone deposited with high energy tends to have higher porosity than low energy sandstones. 2) Within single channel sandstone, the porosity of middle and lower part are better than that of upper and bottom due to the energy change in channel deposition. Xujiahe tight sands quality is also controlled by burial diagenetic processes. The major factors of porosity loss are mechanical compaction in shallow burial depth (<2000m) and quart overgrowth in deeper burial period. Authigenic chlorite and illite mineral have greater influence on permeability than on porosity.
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Influence of Shale and Mudstone Heterogeneity on Permeability-driven Fluid Flow - Sample to Geophysical Well Log Scale
More LessThe flow properties of shales and mudstones are important for petroleum systems, hydrogeology and CO2 storage sites. However, these flow properties have been mainly studied on laboratory sample scale in the past, assuming homogeneity on larger scales, although shale and mudstones can be highly heterogeneous, depending on their depositional environment. In this study, we investigate the influence of heterogeneities of shales and mudstones on fluid flow on the next larger scale compared to the laboratory sample scale: the geophysical well log scale. To do so, we have quantitatively investigated more than 500 m of mud-rich core and borehole images from six wells of an offshore gas field in the Nile Delta, Egypt. The quantitative approach allowed for subsequent modeling of the observed heterogeneities, which serve as geometry input for 2D numerical fluid flow simulation to calculate effective permeabilities. The fluid flow models were integrated with grain size measurements and published stress-porosity-permeability relationships. As a final result, stress-dependent effective porosity-permeability relationships for a range of shale and mudstone heterogeneities and associated lithological variability can be provided along with probability distributions of heterogeneity occurrence for most of the depositional environments encountered in the investigated Nile Delta wells.
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Evolution of Porosity and Pore Systems in Organic-Rich Posidonia and Wealden Shales
Authors E. Mathia, T. Rexer, L. Bowen and A.C. AplinFree gas in shales occurs mainly in larger mesopores (width >3 nm) and macropores (width >50 nm) and is likely to be the first or even main contributor to gas production. To determine the maximum free gas porosities shale samples from the Lower Jurassic Posidonia and Lower Cretaceous Wealden formations covering a wide range of maturities have been subjected to a set of porosity determination techniques. A decrease of the original total porosity by a factor of two from the immature state (10%) towards the peak oil window (4%) have been documented and ascribed to a combination of factors: compaction, bitumen saturation and swelling. The subsequent regaining of the original porosities in the gas window samples has been proved as a mutual effect of bitumen conversion and gas generation. In contrast to their immature equivalents, in the gas window shales this porosity is associated with the organic phase with over 60% of organic particles visibly porous. Utilizing the integrated approach the connectivity not only of the whole pore system, but also of its organic component has been suggested. An organic matter related gas transport is expected to omit flow barrier problems encountered in water saturated mineral matrix.
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Handheld XRF - An Application to Shale Gas Plays
Authors D.A. Riley, A. Kalasinska, R. Klischies and T. PearceThis study demonstrates the application of handheld XRF as a chemostratigraphic analyser to provide viable data for up to 30 elements, providing the instrument is well calibrated and its performance is monitored. The closely spaced, high resolution HHXRF analysis at a centimetre scale, highlights the potential of the instrument to model fine scale mineralogical variations associated with clay content and zones of carbonate and siliceous cementation. In addition, the accurate elemental data from HHXRF have also been used to calaculate mineralogy, which relates to rock mechanics, and therefore can provide an estimate of brittleness, which inturn can be used to predict the” fraccability” of shales. Critically, a fully calibrated HHXRF with stability monitoring protocols can provide high resolution, non-destructive chemostratigraphic analysis technique of both core and cuttings, which in the case of cored material can be analysed at a centimetre spacing, in the laboratory, at wellsite and in the field.
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Q Estimation Through Waveform Inversion
More LessThis paper presents an approach to estimate the quality factor Q through waveform inversion. In a viscoacoustic medium consisting of one standard linear solid, stress and strain relaxation times govern the dissipation mechanism. Their difference, normalized to be a unitless variable τ, determines the magnitude of Q. In this paper we iteratively optimize a τ model by minimizing an objective function that measures the residuals between recorded and synthetic seismic data. The τ model is then converted to its corresponding Q model. A viscoacoustic Marmousi model demonstrates the accuracy of the approach. For a field data from the Gulf of Mexico we present a workflow to estimate its Q model and then optimize its velocity model through waveform inversions with the attenuation compensation. The workflow shows some promise to get the final seismic products with the attenuation compensation for physical materials.
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High-resolution Elastic FWI for Reservoir Characterization
Authors R. Lu, S. Lazaratos, K. Wang, Y.H. Cha, I. Chikichev and R. ProsserThe Vp/Vs ratio can often be a good lithology indicator for a variety of depositional environments. The Vp/Vs ratio for sands tends to be lower than Vp/Vs for shales, irrespective of fluid. Integration of Vp/Vs estimates with other seismic volumes can often reduce the risk associated with predicting reservoir presence. In several instances, Vp/Vs volumes can also be used as input for populating seismically constrained geologic models. In this study we demonstrate the feasibility of applying elastic Full Wavefield Inversion (FWI) to generate high-resolution estimates of the Vp/Vs ratio, using a 3D marine streamer data set. The field data results (not shown in the abstract; will be discussed in the presentation) are encouraging and demonstrate the robustness and efficiency of our elastic FWI workflow. The inversion results match the seismic and well data very well, and are in good agreement with geologic interpretation. The results also illustrate a couple of key benefits of the FWI approach: the ability to perform true-amplitude imaging and to test the fidelity of the estimated model parameters by comparing simulated and real data. These may prove significant for reducing interpretation risk for cases of variable illumination, or complex overburden.
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Elastic Full-waveform Inversion Using 4c Data Acquisition
More LessRecent computer computational improvements have allowed us to simulate elastic data in a 3D manner and undertake the challenge to execute elastic full waveform inversion (EFWI). For the field data experiment to demonstrate the technologies, we elected to use a Gulf of Mexico (GOM) ocean bottom cable (OBC) data-set which allowed us to take advantage of relatively large offsets along with the four component acquisition. The velocity fields were validated through imaging algorithm of the elastic reverse time migration and the imaging shows clear structural improvements when inputting the inverted velocities in conjunction with the measurements.
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The Application of Full Waveform Seismic Inversion to a Narrow-azimuth Marine Dataset
Authors T. Nangoo, M. Warner, G.S. O‘Brien, A. Umpleby, N. Shah, M. Igoe and J. MorganWe apply 3D anisotropic acoustic full-waveform inversion to a North Sea narrow-azimuth, marine-streamer dataset. We use a windowed strategy, with 3 stages, first focusing on mainly refracted arrivals with offsets up to (a) 1 km, (b) 2 km and then (c) 3 km with increasing iterations. We demonstrate that our recovered velocity model is realistic.
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Subsurface Focusing Measurement of Diving Waves and its Application to Reflection Tomography
By P.S. ShenThe classical full waveform inversion measures the traveltime residual of diving waves on the acquisition surface. This paper shows, based on the focusing principle, this residual can be also measured in the subsurface via reverse time migration. The objective function for the velocity inversion is posed entirely in the subsurface domain by penalizing the defocusing induced from both reflections and refractions. The optimization algorithm on the reverse time migration platform is described. The robustness of the method is demonstrated with a series of synthetic examples.
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Refraction Traveltime Tomography with Irregular Topography Using the Unwrapped Phase Inversion
Authors Y. Choi and T. AlkhalifahTraveltime tomography has long served as a stable and efficient tool for velocity estimation, especially for the near surface. It, however, suffers from some of limitations associated with ray tracing and high-frequency traveltime in velocity inversion zones and ray shadow regions. We develop a tomographic approach based on traveltime solutions obtained by tracking the phase (instantaneous traveltime) of the wavefield solution of the Helmholtz wave equation. Since the instantaneous-traveltime does not suffer from phase wrapping, the inversion algorithm using the instantaneous-traveltime has the potential to generate robust inversion results. With a high damping factor, the instantaneous-traveltime inversion provides refraction tomography similar results, but from a single frequency. Despite the Helmholtz-based solver implementation, the tomographic inversion handles irrgular topography. The numerical examples show that our inversion algorithm generates a convergent smooth velocity model, which looks very much like a tomographic result. Next, we plan to apply the instantaneous-traveltime inversion algorithm to real seismic data acquired from the near surface with irregular topography.
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2D Surface Wave Inversion in the F-K Domain
Authors C.A. Pérez Solano, D. Donno and H. ChaurisFull waveform inversion offers a robust and automated way to estimate high resolution velocity models at the expense of a good initial model. This technique is classically applied on wide angle data, but more rarely on surface waves because of their dispersive nature. However, surface wave information is significantly useful for near surface imaging. We propose to use a modified approach, here named windowed amplitude waveform inversion, as an alternative to the classical full waveform inversion. We first window the input data, then apply a 2D Fourier transform and minimize the misfit between the absolute values. The windowed amplitude waveform inversion is less restrictive with respect to the initial model as shown in a synthetic example. Inversion results confirm that the proposed approach may converge when using initial models for which the classical formulation does not converge to the exact velocity model.
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Anisotropic Elastic Full-waveform Inversion for a Real Walkaway VSP Data from The Arabian Gulf - Towards High Frequencies
Authors O. Podgornova, J.C. Owusu, M. Charara, S. Leaney, A. Campbell, S. Ali, I. Borodin, L. Nutt and H. MenkitiBorehole vertical seismic profiling (VSP) data address the need for higher resolution images and information about shear wave velocities. Elastic full-waveform inversion (FWI) of VSP data provides promising capabilities to recover quantitative information about compressional and shear wave velocities for reservoir characterization. We apply elastic FWI to a walkaway data set from the Arabian Gulf and report on the results for medium frequencies (up to 40 Hz), which is at a resolution level of surface seismic data, and beyond (up to 60 Hz). The recovered model has a plausible layered structure and agrees fairly well with the sonic log data for both compressional and shear waves velocities in the vicinity of the well at the depth of the receivers antenna. Our FWI algorithm assumes axial symmetry for the VTI medium properties and can be applied for media with mild structural properties variations. The algorithm has a modest computational cost, comparable to the cost of 2D methods, yet it models true amplitude 3D point source solutions.
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Automatic DEXP Imaging of Potential Fields Independent of the Structural Index
Authors M.A. Abbas and M. FediA new imaging method is developed to estimate the depth to the sources of potential fields, independent of the structural index. The method consists of applying the Depth from Extreme Point (DEXP) method to the ratio between two different-order partial derivatives of the field. We show that the scaling function of this ratio does in fact not depend on the structural index but only on the difference between the two orders of differentiation used. The scaling exponent, needed to perform the DEXP transformation, is then automatically determined. The depth to the sources is simply estimated from the extreme points of the DEXP image, which is obtained in a automatic way. The structural index can also be determined. The method is tested with synthetic examples and shows a good agreement with true values of the source parameters. The method was also applied to a real data of a vertical magnetic anomaly from the Pima copper mine, Arizona, USA. The results are consistent with the known information about the causative source.
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Optimising Overdetermined Non-linear Inverse Problems
More LessMany geophysical problems are both non-linear and overdetermined (ie there are more data points than model parameters). A common strategy is to apply linear least-squares inversion iteratively. Although each observed datapoint contributes towards the solution, not all do so in a positive manner - some (because of the nonlinearity of the problem) make the parameter change values very poor estimates of the correct values. This paper examines strategies for producing optimised results based on using only subsets of the observed data. Conventional approaches, such as the use of the data resolution matrix, are tested and found to give poor results. The method is demonstrated using different gravity models.
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