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- Volume 10, Issue 1, 2004
Petroleum Geoscience - Volume 10, Issue 1, 2004
Volume 10, Issue 1, 2004
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The influence of intraplate structural accommodation zones on delineating petroleum provinces of the Sub-Andean foreland basins
More LessThe Sub-Andean foreland region is summarized in a series of palaeogeographical maps. Various time-slices show important basin-forming processes and the extent and evolution of principal depositional elements. Based on spatial and temporal changes in palaeodepositional setting, the Sub-Andean region can be subdivided longitudinally into a number of tectonostratigraphic domains. The differential amount of subsidence between two adjacent tectonostratigraphic provinces or sub-provinces has been taken up across, complex, broad, transverse, transcontinental accommodation zones, which probably represent the multiphase reactivation of pre-existing fault systems in the underlying basement. Two predominant sets of basement lineaments are recognized: ENE–WSW and NW–SE. Both sets of lineaments occur as major structural anisotropies throughout the basement rocks of South America, providing zones of weakness, which were repeatedly reactivated and, at least in part, controlled: (a) the geometry of inter- and intra-cratonic rifting; (b) rates of subsidence and uplift along the Andean depositional axis; (c) the position of basin-bounding and intra-basinal highs/arches; (d) the structural geometry of the Andean Deformation Zone, correlating with changes in deformational style and major deflections; and (e) the location of magmatism.
Based primarily on source rock age, it is suggested that these transverse, structural accommodation zones had a profound effect on source rock distribution and hydrocarbon occurrence; subdividing the Sub-Andean region into five petroleum provinces/megasystems: a ‘Northern’ Late Cretaceous (La Luna Formation and equivalents); a ‘Western’ Late Palaeozoic–Early Mesozoic (Permian and Upper Triassic to Lower Jurassic source rocks); a ‘Central’ Palaeozoic (Late Devonian and Silurian); an ‘Eastern’ Tertiary; and a ‘Southern’ Mesozoic.
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Characterization and zonation of a marly chalk reservoir: the Lower Cretaceous Valdemar Field of the Danish Central Graben
Authors Finn Jakobsen, Jon R. Ineson, Lars Kristensen and Lars StemmerikIn the Valdemar Field of the Danish Central Graben, production is from the Upper Hauterivian–Aptian succession (Tuxen and Sola formations) which comprises interlayered pelagic/hemipelagic chalks, marly chalks and marlstones. Based on core data, the reservoir chalks (6–10% insoluble residue (IR)) and marly chalks (10–30% IR) possess porosities in the 20–48% range and matrix permeabilities of 0.1–4 mD. The porosity (ϕ) of these chalks correlates negatively with the IR (particularly clay) content. A permeability cut-off of 0.1 mD was defined for reservoir studies, corresponding to a ϕ of 20–30% and an IR of c. 35%. A detailed reservoir zonation, based on integration of core and petrophysical data constrained within a sequence stratigraphic framework, illustrates the stratigraphical compartmentalization of the field. The distinctive nature of this heterogeneous argillaceous chalk reservoir is illustrated by comparison with Maastrichtian–Danian chalks of the Central Graben. For a given porosity, the matrix permeability of the Valdemar Field chalks is a factor of ten lower than that of an equivalent Maastrichtian chalk, the irreducible water saturation is a factor of ten higher and the capillary entry pressure of the Lower Cretaceous reservoir is 2–3 times that of the younger chalks.
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Seismic characteristics of fluid leakage from an underfilled and overpressured Jurassic fault trap in the Norwegian North Sea
Authors Gunn M. G. Teige and Christian HermanrudPre-drill estimates of hydrocarbon column heights are often uncertain, particularly where filling to structural spill point is questioned. Identification of locations where vertical leakage is concentrated can allow more reliable hydrocarbon column height predictions. The locations of vertical leakage were sought on seismic data over an underfilled and overpressured trap (35/10-2) in the Norwegian North Sea. It was hoped that one single location or one single narrow leakage zone coinciding with the gas–water contact would be found. If such a location or zone existed, it was expected to be in or above a fault plane, as leakage in the 35/10-area is thought to mainly result from shear failure along faults.
The investigation found a zone with pronounced dimming above a triple fault intersection bounding the 35/10-2 structure. This zone stretches further downdip and is positioned above a fault plane. The shallowest part of this zone intersects the top of the reservoir at the depth of the proven gas–water contact. These observations suggest that a discrete leakage zone, of which the shallowest part controls the column height of the structure, has been identified and that the column height is limited by leakage resulting from shear failure. The occurrence of similar seismic features over undrilled structures can lead to safer assessments of hydrocarbon column heights, especially if the observations are consistent with the general knowledge of stress state and leakage processes in the area.
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Anhydrite distribution within a shelf-margin carbonate reservoir:San Andres Formation, Vacuum Field, New Mexico, USA
Authors Matthew J. Pranter, Neil F. Hurley and Thomas L. DavisAnhydrite cement causes significant heterogeneity within the San Andres reservoir of the Vacuum Field where it is associated with faults, fractures, karst zones and highly cemented dolomite intervals. The primary reservoir rocks within the Central Vacuum Unit of the Vacuum Field are dolomitized peloidal packstones, skeletal and ooid grainstones and fusulinid packstones. These rocks alternate with lower reservoir-quality dolomite intervals with variable amounts of anhydrite cement. Nodular and pore-filling fabric-selective anhydrite cements are common within the reservoir interval. Cross-plots of apparent matrix grain-density versus apparent matrix volumetric cross-section (ρ maa–U maa cross-plots), combined with V p/V s seismic attributes and core data, provide insight into the vertical and lateral distribution of anhydrite within the San Andres reservoir. Anhydrite is generally concentrated in thin depositional cycles or intervals that are separated by relatively anhydrite-free cycles that exhibit relatively higher porosity and permeability. Lower V p/V s values correspond to higher percentages of anhydrite and are useful formapping anhydrite distribution. This evaluation of anhydrite distribution providesan estimate of the significant cementation-related heterogeneities within the reservoir that is useful for development-well planning and to target areas of bypassed pay.
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A stratigraphical and palaeoenvironmental analysis of the sub-basaltic Palaeogene sediments of East Greenland
Authors David W. Jolley and Andrew G. WhithamPalaeogene sedimentary strata from beneath the basaltic lavas ofEast Greenland have yielded a suite of palynofloras which determine the ageand environment of deposition of these rock units. These palynofloras include Apectodinium augustum, Deflandrea oebisfeldensis and a pollen assemblage containing common Caryapollenites veripites with Camerozonosporites heskemensis and Stereisporites in assemblages characteristic of the latest Paleocene to earliest Eocene Sequence T40. This allows a correlation to be established, complementing an already established framework based on lava geochemistry, which shows that lava eruption in East Greenland was initiated somewhat later than in the Faroe Islands. It also highlights the presence of a widespread major unconformity in the East Greenland succession, spanning the late Maastrichtian to the end of the Paleocene. The lack of sedimentation throughout the Early Paleocene, and majority of the Late Paleocene, highlights the possibility of Paleocene sedimentary systems bypassing the East Greenland shelf, with clastic deposition forced eastwards towards the area of the Faroe Islands and the Møre and Vøring basins.
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Insights into Cretaceous–Palaeogene sediment transport paths and basin evolution in the North Atlantic from a heavy mineral study of sandstones from southern East Greenland
Authors Andrew G. Whitham, Andrew C. Morton and C. Mark FanningMajor changes in sandstone provenance occurred during the deposition of the Cretaceous–Eocene succession in Kangerlussuaq, southern East Greenland. These changes can be recognized on the basis of provenance sensitive heavy mineral parameters (apatite:tourmaline and rutile:zircon ratios and garnet geochemistry) and the SHRIMP U–Pb dating of detrital zircons. The results support the subdivision of the succession into three units separated by major unconformities spanning the Late Coniacian to Late Campanian and Late Maastrichtian to Early Eocene. Rifting during the deposition of the first unit (Aptian–Late Coniacian) led to rift flank uplift and resulted in the local sourcing of sediment. Thermal subsidence during the deposition of the second unit (Late Campanian–Late Maastrichtian) led to rift flank subsidence and sediment sourcing from outside the immediate region. Renewed rifting immediately preceding the third unit (Early Eocene) resulted in a return to local sediment sourcing. The basin morphology during the deposition of the second unit would have been more conducive for the long-distance transport of sediment into the adjacent Faroe–Shetland Basin than during deposition of the first and third units. The results provide a framework for the identification of Greenland-sourced material in the Faroe–Shetland Basin.
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Mechanical compaction behaviour of natural clays and implications for pore pressure estimation
By N. R. GoultyIn pore pressure estimation and in basin modelling programs it is often assumed that porosity in clay-rich sediments depends on the vertical effective stress. An alternative assumption is that porosity depends on the mean effective stress. Yet triaxial test data on natural clays have shown that porosity depends on both the mean effective stress and the differential stress. Triaxial test data for Winnipeg clay are re-plotted here to quantify the errors in estimated pore pressures that would result if it is assumed that porosity depends on either vertical or mean effective stress. The assumption that porosity depends on vertical effective stress may result in gross underestimates of pore pressures in compressional basins, where horizontal stresses in overpressured zones at depth are greater than the vertical stress. Sections through the yield surface of Winnipeg clay are consistent with a generalized yield locus for clays, normalized for composition as well as volume, based on data from several natural clays. Consequently, a refined equivalent depth method of pore pressure estimation that accounts for porosity dependence on both mean and effective stress could, in principle, be implemented. The method would require some knowledge of yield properties and of all three principal stresses in the subsurface.
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Sand-grade density flow evolution on a shelf-edge–slope–basin-floor complex in the Upper Jurassic Olympen Formation, East Greenland
Authors Rikke Bruhn and Finn SurlykThe uppermost Callovian–Middle Oxfordian Olympen Formation is part of the early syn-rift succession of Jameson Land and Traill Ø, East Greenland and was deposited in prograding shelf-edge, slope and basin-floor environments. The formation consists of: (1) coarsening-upward units of sand-rich, prograding shelf-edge wedges; (2) slope and basin-floor mudstone; and (3) lenticular elongate and sheet-like, massive sandstone bodies deposited by sedimentary density flows on the slope, base-of-slope and basin floor. Systematic spatial and temporal variations in sandstone body geometry reflect down-slope transformations of the density flows. Lenticular and down-slope elongate sandstones mainly occur randomly scattered within prograding shelf-edge sandstone and slope mudstone. They represent short-travelled, hyperconcentrated density flows, filling pre-existing gullies that formed by retrogressive slumping. Sheet-like sandstones encased in mudstone were deposited from unconfined, concentrated density flows transitional to turbidity flows at the base-of-slope and on the basin floor. The Olympen Formation provides a well-exposed example of a sand-dominated shelf-edge–slope–basin-floor dispersal system in which the down-slope transformation of sand-grade density flows and resultant sandbody geometries can be demonstrated effectively. It may serve as a useful field analogue for a poorly understood but important type of hydrocarbon reservoir.
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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)