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- Volume 16, Issue 4, 2010
Petroleum Geoscience - Volume 16, Issue 4, 2010
Volume 16, Issue 4, 2010
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Capturing stratigraphic and sedimentological complexity from submarine channel complex outcrops to digital 3D models, Karoo Basin, South Africa
Authors J. K. Pringle, R. L. Brunt, D. M. Hodgson and S. S. FlintABSTRACTSubmarine slope channel-fills form complicated stratigraphy and lithofacies distributions through repeated phases of erosion and deposition. This provides a challenge to accurate 3D modelling, particularly in representing lithofacies transitions within sand-poor areas. In this paper, traditional (sedimentary logs, palaeocurrent measurements, architectural panels) and non-conventional technologies (Light Detection and Ranging; Ground Penetrating Radar) were integrated to quantitatively describe lithofacies distributions and sedimentary architectures from two large-scale outcrops, one base of slope, high sandstone content system (Unit B) and one from a mid-slope, more mixed lithology system (Unit C), in the Laingsburg Formation, Karoo Basin, South Africa.
The workflow described in this study combines digital structural restoration and extrapolation of major stratigraphic surfaces, grouped palaeocurrents and architectural geometries observed at outcrop to create 3D digital models. The model was divided into zones along major stratigraphic discontinuities and populated using lithofacies associations that were adjusted for outcrop rugosity and palaeodispersal direction. Observed channel margin asymmetries, distribution of lithofacies and stacking patterns were all honoured in the digital models.
The Unit C slope-channel system differs from many exposed submarine channels due to the low proportion of sandstone present within the infill. Thin-bedded channel margin lithofacies are preserved through the lateral stepping of channels and allow the correlation of stratigraphy from channel axis to margin and on to overbank areas. In the older, sandier Unit B base-of-slope system, the stratigraphic change in stacking pattern, channel aspect ratio, lithofacies of channel-fills and stratigraphic hierarchy were all captured. This research captured the architectural complexity observed at outcrop to generate more realistic models than could be constructed normally using limited subsurface data.
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Geological controls on Upper Permian Plattendolomit Formation reservoir prospectivity, Wissey Field, UK Southern North Sea
Authors Craig Duguid and John R. UnderhillABSTRACTInterpretation of a large, well-calibrated 3D seismic volume, electrical well-log data and core samples from the Quadrant 53 area of the Southern North Sea provides a new-found basis for understanding the controls on gas production in the Wissey Field, a successful test of the Upper Permian (Zechstein Group; Z3 Cycle) Plattendolomit Formation carbonate play in the Southern Permian Basin (SPB). The petrophysical assessment of wells combined with facies analysis demonstrates that the Plattendolomit Formation represents the northward progradation of an important upward-shoaling carbonate platform. The results show that the highest primary porosity values, lie within the uppermost reservoir sub-units of the Plattendolomit Formation, which consist of brecciated packstones and overlying oolitic grainstones developed at the shelf edge of the Z3 carbonate ramp and sealed by back-barrier lagoonal anhydritic mudstones (ascribed to the Scolt Head Formation). It can now be shown that the play fairway is enhanced where two (WSW–ESE and NNW–SSE-striking) fault trends intersect and cause fracturing of the reservoir interval, providing the additional permeability critical for reservoir production. Seismic stratigraphic studies demonstrate that early (pre-Cretaceous) normal fault sets were formed on both trends and were locally affected by contractional deformation (structural inversion) during the Late Cretaceous. As well as explaining the key geological factors that contribute to production success in the Wissey Field itself, the results provide a foundation for evaluating the prospective potential that the Plattendolomit Formation has elsewhere in the SPB.
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Integrating streamer and ocean-bottom seismic data for sub-basalt imaging on the Atlantic Margin
Authors K. W. Helen Lau, R. S. White and P. A. F. ChristieABSTRACTStacked basalt flows cover much of the NW European continental margin, including potentially prospective sediments of the Faroes shelf. Such flows attenuate seismic energy, hindering sub-basalt structural imaging which is critical for both exploration and tectonic studies. Low-frequency, long-offset reflection surveys have yielded improved images below top basalt, while coincident ocean-bottom seismometer (OBS) data have mapped low velocity zones (LVZ) from tomographic inversion. Image correlation in a common depth domain is challenged by low spatial resolution of the tomographic image, the absence of turning rays in a LVZ and the lack of wide-angle arrivals in the reflection data. We integrate densely sampled reflection data with deep velocities from OBS data to give a common velocity model in a new, iterated, pre-stack depth-migration workflow using complementary constraints from the two datasets. The matched velocity model and depth image enable interpretation of (i) a uniform flood basalt sequence, 2–4 km thick beneath the Fugloy Ridge, with velocities correlating with those in the Lopra 1/1A borehole and (ii) a sub-basalt LVZ with chaotic reflections corresponding to sill-intruded, probably syn-rift sediments. Such a workflow could be extended to targets beneath high velocity salt or basalt and could provide constraints for 3D datasets.
Supplementary material: Overlays of modelled arrival times on all the OBS gathers, similar to Figure 4, can be found at http://www.geolsoc.org.uk/SUP18433
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Overpressure-generating mechanisms in the Peciko Field, Lower Kutai Basin, Indonesia
Authors Agus M. Ramdhan and Neil R. GoultyABSTRACTThe Peciko Field contains gas in multiple stacked reservoirs within a Miocene deltaic sequence. In the deeper reservoirs, gas is trapped hydrodynamically by high lateral overpressure gradients. We have analysed overpressure and compaction in this field by using wireline log, pressure, temperature and vitrinite reflectance data. The top of the overpressure is located below 3 km burial depth, below the depth range for transformation of discrete smectite to mixed-layer illite/smectite. Density-sonic and density-resistivity crossplots for mudrocks show reversals within the transition zone into hard overpressure below 3.5 km depth. Vitrinite reflectance measurements indicate that the start of unloading coincides with the onset of gas generation. Moreover, mudrock density continues to increase with depth in the overpressured section to values above 2.6 g cm–3. We conclude that gas generation and chemical compaction are responsible for overpressure generation, contradicting previous interpretations that disequilibrium compaction is the principal mechanism for generating overpressure in the Lower Kutai Basin. The particular circumstances which make our radical interpretation plausible are that it is a warm basin with lateral reservoir drainage, so the overpressured mudrocks are probably overcompacted as a result of diagenesis.
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Palaeozoic source rocks in the Dniepr–Donets Basin, Ukraine
ABSTRACTThe Dniepr–Donets Basin (DDB) is a major petroleum province in Eastern Europe. In order to understand the regional and stratigraphic distribution of source rocks for the dominantly gas-prone petroleum system, 676 fine-grained rocks from 30 wells were analysed for bulk parameters (total organic carbon (TOC), carbonate, sulphur, RockEval). A subset of samples was selected for maceral and biomarker analysis, pyrolysis-gas chromatography and kinetic investigations. Organic-rich sediments occur in different intervals within the basin fill. Maximum TOC contents (5.0 ± 1.9%) occur in the Rudov Beds, several tens of metres thick. The oil-prone rocks (Type III–II kerogen) were deposited in basinal settings above an unconformity separating Lower and Upper Visean sections. While maximum TOC contents occur in the Rudov Beds, high TOC contents are observed in the entire Tournaisian and Visean section. However, these rocks are mainly gas condensate-prone. Highly oil-prone black shales with up to 16% TOC and hydrogen index values up to 550 mgHC g–1TOC occur in Serpukhovian intervals in the northwestern part of the DDB. Oil-prone Lower Serpukhovian and gas condensate-prone Middle Carboniferous coal is widespread in the southern and southeastern part of the basin. Although no source rocks with a Devonian age were detected, their presence cannot be excluded.
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Validation of Hurst statistics: a predictive tool to discriminate turbiditic sub-environments in a confined basin
Authors F. Felletti and R. BersezioABSTRACTTurbidite sequences within confined basins constitute important hydrocarbon reservoirs world-wide and, for this reason, the discrimination of sedimentary sub-environments based on an objective statistical method is of interest for pure and applied science. We investigated the potential use of the Hurst test as a statistical tool to discriminate sub-environments within geologically complex turbiditic units that fill a confined basin with well-exposed facies transitions and onlaps, at the scale of several stacked reservoirs (Cengio and Bric la Croce–Castelnuovo Turbidite Systems in the Tertiary Piedmont Basin, Oligocene, northern Italy). In vertical stratigraphic sections, the Hurst test determines the degree of clustering of low and high values of sedimentological variables, such as bed thickness, grain size and sand/mud ratio, which are dependent on sub-environments of deposition. We applied the Hurst test to depocentral and marginal sub-areas across the basin (parallel and perpendicular to the main palaeocurrent direction), documenting a different clustering of thick and thin beds, and of high and low values of the sand/mud ratio, in the depocentre–distal sector with respect to the onlap areas.
A new field (onlap sub-environment) could thus be added to the classification diagram of turbidite settings based on the Hurst index. The Hurst phenomenon (clustering of high and low values of the selected variables) was also able to distinguish between proximal and distal (depocentral) lobe settings, and to recognize the fingerprint of the different depositional lobes (fully confined aggrading, prograding, backstepping). The map of turbidite sub-environments obtained by interpolation of the Hurst index is quite comparable to the field-observed facies map, providing impressive robust validation of the Hurst statistics. This method seems to represent a very promising predictive tool for subsurface studies of turbiditic oil fields based on core and log analyses.
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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)