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- Volume 13, Issue 2, 2007
Petroleum Geoscience - Volume 13, Issue 2, 2007
Volume 13, Issue 2, 2007
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Return to rifts – the next wave: fresh insights into the petroleum geology of global rift basins
More LessRift basin exploration has provided the oil and gas industry with almost one third of discovered global hydrocarbon resources. The maturity of prolific passive margin sequence plays has necessitated a shift in technical emphasis to understanding petroleum geology of the deeper precursor rift basin megasequences. Modelling of these petroleum systems relies extensively on theoretical stretching hypotheses for whole-crust evolution and heat flow prediction. Several alternatives have emerged to account for anomalous rift-related thermal stress, asymmetry of structural styles and origin of thick syn-rift ‘sag-basin’ subsidence patterns at passive continental margins. These observations cannot be predicted by the pure shear uniform stretching model first introduced by McKenzie in 1978. Newly acquired, 2D long cable deep seismic records provide empirical evidence that supports more complex, polyphase, depth-dependent stretching origins for rift-basin formation. Heterogeneities are believed to be inherent in the brittle upper crust, in the ductile lower crust and lithospheric mantle and so result in the complex distribution of accommodation space recorded by syn-rift megasequence deposition during episodes of orthogonal or oblique extension. No single stretching model uniquely describes the varied structural response of the anisotropic crust to plate-scale extension. A hybrid of phased simple and pure shear deformation mechanisms separated by regional lateral, and possibly vertical crustal discontinuities, may explain the apparent paradox of along-strike co-existence of uniform versus depth-dependent stretching structural geometries.
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Mesozoic Gulf of Mexico basin evolution from a planetary perspective and petroleum system implications
More LessThe lack of a sharp boundary between North and South Atlantic stress fields recorded in the arcs of transform faults and ridge segments suggests a gradual merging of mantle stresses within a broad Central Atlantic plate boundary zone. The nature of descending slabs deep in the mantle beneath North and South America suggests that this intra-American plate boundary zone has existed since the Early Cretaceous, at which time it was located beneath the Gulf of Mexico Basin. Simple Euler sums of North and South America to Africa rotation poles validate the concept of merging stress fields, providing a geologically reasonable trajectory and rotation data for the Yucatan microplate with respect to Africa. The new Yucatan rotation geometry is consistent with initiation of back-arc spreading in the western Gulf of Mexico Basin during the Late Berriasian or Early Valanginian, c. 140 Ma, triggered by a strengthening South Atlantic stress field. Continued spreading and rotation of Yucatan likely persisted through the Late Albian, c. 100 Ma. These findings are supported by Early Cretaceous deposystem architecture, basin margin reef trends and source-rock distribution. Kinematic analysis predicts that most Gulf of Mexico seafloor (c. 60%) was created during the Cretaceous period of stable normal geomagnetic polarity, c. 125–83.5 Ma (the ‘Cretaceous Quiet Zone’). Salt-lubricated detachment faulting in the young Gulf of Mexico likely covered newly formed oceanic crust with large allochthons of Oxfordian–Valanginian strata.
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Petroleum systems in rift basins – a collective approach inSoutheast Asian basins
Authors Harry Doust and H. Scott SumnerThis paper synthesizes some of the main conclusions reached in a recent regional review of the Tertiary basins of Southeast Asia, carried out by Shell. Four distinctive types of petroleum systems, correlating with the four main stages of basin evolution (early to late syn-rift and early to late post-rift), are developed widely in the basins. These petroleum system types have characteristic interbedded environmentally controlled source, reservoir and seal lithofacies which, in combination with the structural trap style, determine the hydrocarbon habitat. Variations in the tectonostratigraphic evolution consequent on differences in, for example, basin palaeogeographical position and proximity to late Tertiary collision events, are reflected in differences in the representation of the four petroleum system types. In turn, this is reflected in the overall hydrocarbon volumes found, the average field sizes and the ratio of oil to gas. The recognition of analogous petroleum systems (‘petroleum system types’) and reservoir lithofacies play types in well-explored basins can facilitate prediction of hydrocarbon prospectivity in less well-known rift/post-rift basins and plays, and thereby contribute to future exploration evaluation in these provinces.
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A Middle–Upper Miocene fluvial–lacustrine rift sequence in theSong Ba Rift, Vietnam: an analogue to oil-prone, small-scale continental rift basins
Authors L. H. Nielsen, H. I. Petersen, N. D. Thai, N. A. Duc, M. B. W. Fyhn, L. O. Boldreel, H. A. Tuan, S. Lindström and L. V. HienThe small Neogene Krong Pa graben is situated within the continental Song Ba Rift, which is bounded by strike-slip faults that were reactivated as extensional faults in Middle Miocene time. The 500 m thick graben-fill shows an overall depositional development reflecting the structural evolution, which is very similar to much larger and longer-lived graben. The basal graben-fill consists of thin fluvial sandstones interbedded with well-oxygenated lacustrine siltstones in the basin centre, while very coarse-grained fluvial sandstones and conglomerates dominate at the basin margins. With increased subsidence rate and possibly a higher influx of water from the axial river systems the general water level in the graben rose and deep lakes formed. High organic preservation in the lakes prompted the formation of two excellent oil-prone lacustrine source-rock units. In the late phase of the graben development sedimentation rate outpaced the formation of accommodation space and fluvial activity increased again. During periods when the general sedimentation rate was in balance with the creation of accommodation space the environment changed frequently between lake deposition and intermittent vigorous fluvial activity. It is likely that the resulting interbedding of fluvial sandstones and lacustrine sediments reflects variations in precipitation. In periods of little precipitation the lakes diminished and lake bottoms became exposed. After heavy precipitation, transverse river systems transported sands from the rift shoulders across the exposed lake bottom and fluvial sands were deposited on lake bottom sediments. Subsequently, lake level rose due to increased water supply from the axial river and the sands were drowned and topped by transgressive lacustrine mudstones. These sandstones may function as carrier beds, whereas the braided fluvial sandstones and conglomerates along the graben margins may form reservoirs. The Krong Pa graben thus contains oil-prone lacustrine source rocks, effective conduits for generated hydrocarbons and reservoir sandstones side-sealed by the graben faults toward the footwall granites. In addition to the structural and climatic signals recorded by the graben-fill, sediment partitioning among the partly isolated basins along the rift axis seems to have been important.
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Overpressure development in rift basins: an example from the Malay Basin, offshore Peninsular Malaysia
By Mazlan MadonThe Malay Basin is a Tertiary transtensional rift basin located in offshore Peninsular Malaysia. A study of the subsurface pressure data has revealed at least two major overpressure compartments that are sealed by regional shale units. The main, basin-centre overpressure compartment has a domal shape and, in profile, shows a convex-up top-of-overpressure surface. In the basin centre, the top of overpressure is generally between 1900 m and 2000 m depth, and is confined mainly to a particular stratigraphic unit, the Middle Miocene unit E. It appears that the top of overpressure is influenced by the underlying regional shale seal in unit F, and that the mild overpressure encountered in unit E represents the overpressure transition zone. Due to the convex-upward top-of-overpressure surface, overpressure is generally encountered at shallower depths in the basin centre compared to the basin flanks. A smaller overpressure compartment is also identified on the northeastern flank of the basin. This compartment is sealed by the onlapping, transgressive shale of unit L (Lower Miocene) and occurs at a depth of 2600–3000 m. Hence, regional shale seals have a strong influence on overpressure distribution.
Disequilibrium compaction is believed to be the primary causal mechanism for the overpressure in the basin centre. Overpressure development is the consequence of high subsidence and sedimentation rates during basin extension. Low subsidence and sedimentation rates on the basin flanks do not generate overpressure. The domal shape of the top-of-overpressure surface is thus a result of different rates of subsidence and sedimentation across the basin.
The basin's overpressure history is simulated by means of a simple model. Modelling results indicate that the basin overpressure developed very early, during the syn-rift phase (c.30–21 Ma), when sediment burial rates were very high (>1000 m Ma−1). Overpressure build-up occurred rapidly during the syn-rift phase but has started to dissipate gradually since the post-rift phase began 21 Ma ago, when sedimentation rates were well below 1000 m Ma−1. This suggests that disequilibrium compaction as an overpressure-generating mechanism was effective during only the syn-rift phase of basin development. Sedimentation rates during the post-rift phase (generally less than 500 m Ma−1) were not high enough to generate overpressure. The pre-existing overpressure is thus able to dissipate through the sedimentary column, causing the build-up of pore pressure in the post-rift section. Thus, the overpressure in the post-rift strata is probably of secondary origin, derived from excess pressure dissipated from the underlying syn-rift strata. Overpressure generated by disequilibrium compaction during the syn-rift phase has been modified and re-distributed as the basin evolved through the post-rift phase.
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Cretaceous tectonostratigraphy and the development of the Cauvery Basin, southeast India
Authors Matthew P. Watkinson, Malcolm B. Hart and Archana JoshiThe late Jurassic to early Cretaceous rifting between India/Australia and India/Antarctica resulted in the formation of a number of NE–SW-trending basins in the Indian Precambrian crystalline basement. The Cauvery Basin is the southernmost basin along the eastern margin of the Indian Sub-Continent, covering much of this part of India and extending a considerable distance offshore. The basin comprises several ‘depressions’, or sub-basins, with the Ariyalur–Pondicherry Depression in the north. The exposed successions are in the southern part of this sub-basin.
The result of fieldwork (1994–8) has been a reassessment of the lithostratigraphy and the tectonostratigraphic history of the Ariyalur outcrop. Three major sedimentary groups were identified: the syn-rift Gondwana Group (of early Cretaceous age), the syn-rift Uttatur Group (of Albian to Coniacian age) and the post-rift Ariyalur Group (of Santonian to Maastrichtian age). Both microfaunal and macrofaunal information were used to develop a biostratigraphic framework for the basin and a new tectonostratigraphic model. This new model for the development of the basin is significantly different to that used by the Oil and Natural Gas Commission of India. Structures exposed onshore, which have been interpreted as Albian reefs, are interpreted here as irregularly shaped limestone olistoliths/olistostromes produced by intra-Cretaceous rifting and slumping within the basin. The paper discusses this model for the basin history which is calibrated by updated foraminiferal (and macrofossil) biostratigraphy.
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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)