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- Volume 8, Issue 3, 2002
Petroleum Geoscience - Volume 8, Issue 3, 2002
Volume 8, Issue 3, 2002
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An experimental study of the texture of deformation bands: effects on the porosity and permeability of sandstones
Authors A. E. Lothe, R. H. Gabrielsen, N. Bjørnevoll Hagen and B. T. LarsenWe have investigated the texture and formation of deformation bands in relation to permeability and porosity. Video image analysis of the Brumunddal sandstone showed a decrease in the number of large pores in the deformed zones. The frequency of small pores is increasing in the intermediate zone, compared to the undeformed rock and the central zone of a deformation band.
Triaxial compression tests were performed on Red Wildmoor sandstone with constant confining pressure (8 MPa). Axial P-and S-wave velocities measured during loading showed structural changes in development of a deformation band: Stage I and II – closure of micro-cracks and pores and tighter grain packing parallel to the maximum stress direction and simultaneously dilation perpendicular to the maximum stress direction. Stage III – both the P-and the S-wave velocities decreased, reflecting tighter grain packing and development of micro-fractures. These observations are supported by permeability measurements taken before, under and after triaxial compression, with recovering of permeability due to elastic effect and static reduction due to tighter packing and ultimately grain size reduction. NMR images of oil-saturated samples after loading to failure show: stage III – grain size reduction; stage IV – secondary fracturing; and stage V – development of a slip plane.
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Detailed analysis of probe permeameter and interval pressure transient test permeability measurements in a heterogeneous reservoir
Authors Kirsty Morton, Simon Thomas, Patrick Corbett and David DaviesThe goal of reservoir characterization is to generate models that allow the accurate prediction of future well performance and the estimation of reserves. Of particular importance to the characterization of permeability heterogeneity is an accurate understanding of vertical flow behaviour in a reservoir. Upscaling techniques applied to conventional core plug and probe permeameter permeability anisotropy are compared with Interval Pressure Transient Tests (IPTT) conducted with multiprobe formation testers. The aim is to investigate the scale of the dynamic permeability measurement and review the representivity of the IPTT estimate in a field example. We illustrate improvement in permeability anisotropy estimation by a combination of dynamic measurements with static measurements.
Further to this, we consider the permeability prediction from continuous downhole measurement. High-resolution measurements such as probe permeametry data and borehole imaging can produce the most complete data coverage at the well and have similar measurement scales. Predicted permeabilities are compared with IPTT results by upscaling to a length scale appropriate to the IPTT and the geometry of the formation.
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Determination of facies from well logs using modular neural networks
Authors Alpana Bhatt and Hans B. HelleZonation of well logs and the correlation of zones between wells are primary tasks in sub-surface geological and engineering analysis. We propose in this paper an artificial neural network (ANN) approach for objective clustering and identification of facies from well logs. The method relies upon combining back-propagation neural networks in ensembles and modular systems, where the multi-class classification problem of facies identification has effectively been reduced to a number of two-class problems.
Based on the neural network responses using synthetic logs from a realistic model, we optimized the architecture and training procedure of the component networks in the modular system, where the building blocks are simple three-layer back-propagation ANNs. Ensembles of ANNs are trained on disjoint sets of patterns using soft overtraining to ensure diversity and generalization. Recurrent ANNs are shown to enhance the facies continuity by effectively removing ambiguous or spurious classifications.
The performance of the technique was demonstrated using synthetic data and it was then used to detect four different facies within the Ness Formation in the North Sea. An average hit rate of above 90% in wells not used for training the network is slightly to significantly better than the performance published for similar classification experiments.
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Quaternary lithology and shallow gas from high resolution gravity and seismic data in the central North Sea
Authors Christian Bauer and Christine FichlerHigh resolution marine gravity data and 3D post-stack seismic data from the central North Sea have been jointly interpreted. The accuracy of the gravity data allowed the detection of density contrasts related to a Quaternary sub-glacial melt-water channel, a shallow gas accumulation and a Tertiary gas chimney. The combination of gravity and seismic data is shown to particularly improve the detection of the shallow gas accumulation.
The interpretation included visual correlation of gravity images and seismic data performed in a seismic workstation environment, as well as 2.5D gravity modelling along selected seismic profiles. The successful application of this method on shallow targets requires a limited complexity of the shallow strata as well as targets defined by a distinct density contrast and a reasonable size. Data requirements include high-resolution bathymetric and free air gravity data. Bouguer gravity data, which are commonly used in exploration, cannot be used here as densities may vary within the uppermost layer below the sea bottom.
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Predicting distribution of remobilized aeolian facies using sub-surface data: the Weissliegend of the UK Southern North Sea
Authors Anna C. Strömbäck and John A. HowellAeolian sandstones of the Permian Rotliegend Group comprise the principal gas reservoirs of the Southern North Sea Basin. The upper portion of the reservoir interval comprises a unit informally called the ‘Weissliegend’. This unit comprises mass-flow and in-situ soft-sediment deformed deposits formed when the Zechstein Sea catastrophically flooded the Rotliegend sand sea. The Weissliegend typically exhibits poorer reservoir properties than the underlying Rotliegend reservoirs and its thickness and distribution are notoriously difficult to map.
Hanging vertical sections on a regionally extensive, intra-Rotliegend super-surface has led to the recognition of a preserved topography at the top of the Rotliegend. The thickness differences within the unit between the super-surface and the top Rotliegend (termed here the Upper Aeolian Unit, UAU) represent the original dune topography modified by flood-related processes, and provide an insight into the nature and scale of the aeolian bedforms that existed within the basin prior flooding. The relationship between known Weissliegend distribution and this topography provides a predictive tool for understanding Weissliegend distribution.
Mapping the preserved topography reveals preserved bedforms up to 85 m high, with a bedform spacing of 8 to 10 km. The dunes deposited ‘transverse type’ strata but the scale and spacing of the bedforms are too big compared with modern transverse dunse. Instead a more complex model is proposed. The Weissliegend has highly variable thickness (0–26 m) and is commonly thicker within the interdune hollows.
The model for a preserved topography at the top of the Rotliegend is further supported by thickness trends in the overlying Zechstein units which increase into the proposed interdune lows.
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Correlation of reservoir sandstones using quantitative heavy mineral analysis
Authors Andrew Morton, Robert W. O’B. Knox and Claire HallsworthHeavy mineral analysis is one of a group of provenance-based methods that complement traditional biostratigraphic correlation of clastic reservoirs. A variety of processes give rise to stratigraphic changes in sediment composition, including source area uplift, unroofing, changes in climatic conditions, extent of alluvial storage on the floodplain and the interplay between different depositional systems. Heavy mineral analysis is a reliable and proven technique for the correlation of clastic successions because prolonged and extensive research has provided detailed understanding of the effects of processes that alter the original provenance signal during the sedimentary cycle, such as hydrodynamics and diagenesis. The technique has been successfully applied to a wide range of clastic reservoirs, from fluvial to deep marine and from Devonian to Tertiary, using a combination of different types of parameters (provenance-sensitive mineral ratios, mineral chemistry and grain morphology). The application of heavy mineral analysis as a non-biostratigraphic correlation tool has two limitations. The first is that valid correlations cannot be made in sequences with uniform provenance and sediment transport history, but this is a problem inherent with all provenance-based methods. The other is that the technique can be applied only to coarse clastic lithologies and is not suitable for fine-grained sediments or carbonates.
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The effect of faults on the 3D connectivity of reservoir bodies: a case study from the East Pennine Coalfield, UK
Authors W. R. Bailey, T. Manzocchi, J. J. Walsh, K. Keogh, D. Hodgetts, J. Rippon, P. A. R. Nell, S. Flint and J. A. StrandThe connectivity of reservoir bodies has been tested in unfaulted and variably faulted, high resolution 3D geocellular stratigraphic models (16 million cells in a 20 km × 20 km × c. 600 m volume) from the East Pennine Coalfield, UK. These deterministic stratigraphic–structural models are underpinned by high density borehole and mineplan data. The stratigraphy extends from mid-Westphalian A to base Westphalian C and represents a low net:gross (0.03–0.27) coal-bearing, delta top/fluvial reservoir analogue. The models contain up to 1467 post-depositional normal faults with maximum displacements ranging from 1 m to 140 m. Using two sets of facies as net, the effect of fault size on inferred reservoir connectivity has been assessed for stratal intervals with different sedimentary architecture, interval net:gross and thickness.
Faults often disconnect reservoirs in thinner, lower net:gross intervals containing only laterally discontinuous, channelized bodies. By contrast, faults generally increase reservoir connectivity in thick, relatively high net:gross intervals containing laterally extensive, tabular reservoir bodies. The most significant factor for fault-related reservoir connectivity is the dimensionality and scale of reservoir bodies in relation to the displacement distribution along faults. Populations of small, sub-seismic, faults (<5 m throw) can reconnect reservoir bodies that are dissected by larger displacement, seismic resolvable, faults (>20 m throw).
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Tectono-thermal evolution of the Qaidam Basin, China: evidence from Ro and apatite fission track data
By Qiu NanshengIn the Qaidam Basin, on the Northeastern Qinghai–Tibet Plateau, the thermal evolution of western and northern sub-basins was analysed using apatite fission track (AFT) data and vitrinite reflectance data from 27 wells. This study shows that thermal gradients decreased during the evolution of the basin, but there existed some differences in thermal evolution between the west and north of the basin, due to different tectonic movements. The thermal gradients in Paleocene–Miocene in the west and in Jurassic times in the north were more than 30°C km−1, exceeding present-day gradients. These thermal data can provide useful parameters for the study of the Qaidam Basin and Qinghai–Tibet Plateau. Palaeotemperature data are the critical parameter for hydrocarbon generation calculations. The results in this study provide a foundation for hydrocarbon generation history modelling and petroleum resources assessment in the Qaidam Basin.
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Influence of clay and silica on permeability and capillary entry pressure of chalk reservoirs in the North Sea
Authors B. Røgen and I. L. FabriciusThe permeability and capillary entry pressure of chalk reservoirs are controlled by their porosity and specific surface area. Measured permeabilities are in the range 0.025–5.3 mD and are successfully predicted by use of the Kozeny equation. In this paper we focus on the factors that control specific surface area. Fifty-nine Tor and Ekofisk Formation chalk samples from five North Sea chalk reservoirs were investigated. All contain quartz and clay minerals, most commonly kaolinite and smectite, with trace amounts of illite. The contents of calcite and quartz are inversely correlated and both are independent of the content of clays. We thus infer that the main part of the silica is of biogenic origin.
The specific surface area of the chalk is mainly controlled by clay content. The specific surface area of calcite is determined by the individual calcite crystal size and is not dependent on stratigraphic variations in fossil size. The specific surface area of calcite increases with increasing content of quartz and clays. These constituents may inhibit recrystallization of calcite and thus preserve high specific surface area. Our data accord with the following specific surface areas (m2 g−1): calcite between 0.5 and 3.5, quartz about 5, kaolinite about 15, and smectite about 60.
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Volumes & issues
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Volume 30 (2024)
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Volume 29 (2023)
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Volume 28 (2022)
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Volume 27 (2021)
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Volume 26 (2020)
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Volume 25 (2019)
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Volume 24 (2018)
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Volume 23 (2017)
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Volume 22 (2016)
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Volume 21 (2015)
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Volume 20 (2014)
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Volume 19 (2013)
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Volume 18 (2012)
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Volume 17 (2011)
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Volume 16 (2010)
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Volume 15 (2009)
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Volume 14 (2008)
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Volume 13 (2007)
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Volume 12 (2006)
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Volume 11 (2005)
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Volume 10 (2004)
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Volume 9 (2003)
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Volume 8 (2002)
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Volume 7 (2001)
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Volume 6 (2000)
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Volume 5 (1999)
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Volume 4 (1998)
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Volume 3 (1997)
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Volume 2 (1996)
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Volume 1 (1995)