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- Volume 24, Issue 1, 2018
Petroleum Geoscience - Volume 24, Issue 1, 2018
Volume 24, Issue 1, 2018
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History of the development of Permian–Cretaceous rifts in East Africa: a series of interpreted maps through time
More LessEast Africa represents the most rifted portion of crust on the planet, having been subjected to numerous phases of extension from Permian to Recent times. The first rifting phase commences in the Permian as the ‘Karoo’ set of narrow half-graben, many formed by lateral shear. Peak rifting appears to be of Middle Permian age for rifts south of Tanzania. In Kenya, Ethiopia and Somalia, this rift population merges into a set of rifts with peak rifting of earliest Triassic age.
A further phase of rifting is seen in the Toarcian–Aalenian. Many of these overlie Permo-Triassic rifts but others are displaced towards what will become the continental margin. Three unrelated populations of Late Jurassic–Early Cretaceous rifts are observed, including those in Somalia, a series of pull-apart basins on the Davie Ridge and a poorly documented set in southern Mozambique. The Anza Rift, with peak rifting in the Late Cretaceous associated with the building of rift shoulders kilometres in height, is proposed here to be an isolated plume-derived rift.
Evaluation of the petroleum potential associated with these rifts relies on an accurate assessment of each in terms of their age and affinity to well-documented systems.
Supplementary material: A table of East Africa Permian–Mesozoic rift basins and references is available at https://doi.org/10.6084/m9.figshare.c.3902653
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Re-imagining and re-imaging the development of the East African Rift
More LessThe East African Rift (EAR) has fascinated and challenged the geological imagination since its discovery nearly a century ago. A new series of images showing the sequential development of faulting and volcanism along the Rift from 45 Ma to present offers a regional overview of that development. The EAR is the latest phase of the extensive Phanerozoic rifting of the East African continental plate, interwoven with the lithospheric fabrics knitted together during its complex Proterozoic past. South of 5° S, the EAR variously follows or cuts across the Karoo rift trends; north of 5° S, it is almost totally within new or reworked Neoproterozoic terranes, while the Karoo rifts are almost totally outside them. The compilations raise several aspects of rift development seemingly in need of re-imagining, including tight-fit reconstructions of the Gulf of Aden, and the projection of Mesozoic rifts from Yemen to Somalia. Overall, the rifting process does not accord well with a mechanistic paradigm and is better imagined within the Prigoginian paradigm, which accepts instability and disorder within natural processes such as mantle plumes. The structural complexity of Afar and its non-alignment with magnetic anomalies suggests that the seafloor spreading process is, in its beginnings at least, more chaos than order.
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The development of the East African margin during Jurassic and Lower Cretaceous times: a perspective from global tectonics
More LessThe eastern margin of Africa resulted from the first successful trans-Gondwana rupture which retraced, in part, the earlier unsuccessful Karoo rift system. Widespread volcanism in southern Africa (182 Ma, Toarcian) presaged NW–SE-directed extension between East Gondwana and West Gondwana (Africa). Rifting turned progressively north–south in orientation, leading quickly to ocean growth off Somalia and off central Mozambique while, elsewhere, strike-slip within the stretched margin came to predominate. East Gondwana, including Madagascar, was demonstrably still intact at 151.4 Ma (M22, Kimmeridgian) but, as the two large continental fragments disengaged from each other, pure north–south movement became possible. After about 140 Ma (Berriasian), East Gondwana itself started to fragment off Western Australia but little separation occurred as far west as Madagascar before Aptian times (126.1 Ma). Nevertheless, the geometry of the Australia–India opening required that, in the interval 140 – 120 Ma, Madagascar–India pursued a path against Africa different from that of Antarctica. The arcuate Davie Fracture Zone, 1800 km in length, functioned as a pure strike-slip transform off the Tanzania–Mozambique coast for this fragment until the early Aptian demise of the Somali mid-ocean ridge. The active transform east of the Lebombo in southern Africa, meanwhile, relocated progressively eastwards, finally to outboard of the Mozambique Ridge at 136 Ma (Valanginian), leaving most if not all of the stretched continental crust and its volcanosedimentary load attached to Precambrian Africa.
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Geology and hydrocarbon potential of the East African continental margin: a review
Authors Ian Davison and Ian SteelThe East African margin has a complex structure due to multiple phases of rifting with different stretching directions. The main phase of rifting leading to Indian Ocean opening lasted from the Late Pliensbachian to the Bajocian (c. 183 – 170 Ma). This occurred during impingement of the Bouvet hotspot which weakened the lithosphere sufficiently to allow continental break-up. Thick salt and marine shales were deposited during the Toarcian in the Majunga, Ambilobe and Mandawa basins and the onshore Ogaden Basin; marking the onset of the Indian Ocean marine incursion, when good quality oil-prone source rocks were deposited at this time. The recent giant gas discoveries in Tanzania and Mozambique are believed to be sourced from overmature Jurassic or, possibly, deeper Permian age Karoo shales. The margin from the Lamu Basin in the north to the Zambesi Delta in the south is covered by thick Tertiary and Cretaceous sediment derived from the East African rift shoulders, and Lower Jurassic source rocks are predicted to be in the gas window along most of the margin. However, the margins in South Africa, south Mozambique, northern Somalia and Madagascar are less deeply buried, and have better oil potential.
The large Tsimimo and Bemolanga tar sand deposits and the recent announcement of an oil rim in the Inhasorro Field indicate that there are good oil-prone source rocks in the Karoo rifts and in the Albian Domo shales; and the search for oil continues with companies exploring in areas where Jurassic source rocks may be less deeply buried, and/or potential Albian–Turonian-aged source rocks are sufficiently buried to generate oil.
Supplementary material: Figures S1–S3 are available at: https://doi.org/10.6084/m9.figshare.c.3894931
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Ages of Norwegian oils and bitumen based on age-specific biomarkers
Authors Z. Matapour and D. A. KarlsenNorwegian oils are generally considered sourced primarily from the Kimmeridge Clay equivalent shales such as the Draupne, Mandal, Spekk and Hekkingen formations, with secondary contributions from the mid–lower Jurassic, and also from the Triassic in the Barents Sea (Botneheia Formation). Still, as most of our age inferences concerning source-oil correlation are based on facies-specific biomarkers, a number of proposed correlations have been questioned.
Thus, source to oil correlations were frequently made on the basis of facies parameters, and rightfully so, but facies-specific signatures in oils will transgress age – and, in principle, not correlate with the phylogenetic evolution. This means that one could, in principle assign an oil to ‘the wrong’ age – when one is, in fact, linking it to a known source rock signature.
A series of 40 oil samples and core extracts, which cover a wide range both stratigraphically and geographically, have been analysed. In this paper, we present for the first time a Norwegian oil-age map based on age-specific biomarkers among the nordiacholestanes and triaromatic steroids parameters, and delineate also where we find Cretaceous- and Palaeozoic-derived oils. The reasons for this distribution pattern, compared to that of Mesozoic oils on the Norwegian Continental Shelf (NCS), are discussed.
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Application of attribute-based inversion and spectral decomposition with red–green–blue colour blending for visualization of geological features: a case study from the Kalol Field, Cambay Basin, India
Authors Surajit Gorain and ShalivahanThe Kalol Field in the Cambay Basin of India was discovered in 1961 and has been producing through more than 608 wells from Kalol reservoirs but the cumulative production up until 2008 was only 95.19 MMbbl. Reasons for low recovery have been ascribed to poor reservoir facies and limited reservoir thicknesses. In the Kalol Field, several thin clastic reservoirs are sandwiched within the main reservoir and exhibit lateral variations in lithology.
Attribute-based inversion (ABI) is effective at delineating potentially prospective areas and reveals the existence of a good reservoir facies cluster in the south, SE and NW corners of our study area, near the Kalol Field. However, this method cannot decipher the depositional setting or the finer details of facies elements. To obtain a smooth and fine-tuned facies model, geostatistical modelling is adopted taking the ABI output (seismic-facies model) as the initial model. The advantage of geostatistical modelling is that it always honours the input data and respects the positional variograms of the geology.
Adding spectral decomposition with red–green–blue (SD-RGB) colour blending reveals the existence of a meandering river in the study area. This river is interpreted to have deposited crevasse splays and channel facies along the river banks. These two facies are the main producing contributors to a well that has maintained a higher production than any other well in this field. This facies-based approach is also effective in determining the reservoir geometry and quality consistent with the interpretation of the depositional environment.
In ABI, 3D attribute volumes of petrophysical properties are calculated using a genetic algorithm inversion and artificial neural network using a non-linear correlation between seismic and log properties. The calculated 3D attribute volumes of petrophysical properties are subsequently utilized for seismic facies classification. In contrast to ABI, SD-RGB colour blending has been solely utilized for co-visualization of different band-limited amplitude volumes from spectral decomposition. Conventional seismic inversion has now been replaced by an integrated approach combining ABI, geostatistical modelling and SD-RGB colour blending in an effort to delineate the remaining potential of the field, and to improve the geological success and ultimate recovery.
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Combining basin modelling with high-resolution heat-flux simulations to investigate the key drivers for burial dolomitization in an offshore carbonate reservoir
Authors A. Mangione, H. Lewis, S. Geiger, R. Wood, S. Beavington-Penney, J. McQuilken and J. CortesThe Eocene El Garia Formation in the offshore Hasdrubal Field was originally a nummulitic limestone in which subsequent burial dolomitization has significantly enhanced permeability. Identification of the reservoir's petrophysical property distributions requires knowledge of the spatial extent of its dolomitization, in turn requiring understanding of the processes that caused the dolomitization. Some of this understanding can be derived from measurements but others need to be simulated. In this study, the former are used as guides and we focus on the latter, evaluating the character of the dolomitizing fluid's movement and temperature patterns by using basin modelling to develop heat-flux simulations to represent the time of dolomitization. Basin modelling reconstructs the region's geology at the time of dolomitization, while heat-flux simulations recreate the appropriate conductive and convective heat and mass transport through these systems. Potential key drivers are rock mass and fault-zone permeability, and the position and shape of any salt domes. The results suggest that salt dome shape and position is the dominant control, the salt dome localizing convective systems which also use convenient faults so that hotter upwelling fluids pass through the Hasdrubal reservoir and are instrumental in the development of burial dolomitization.
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Fuzzy fusion of geological and geophysical data for mapping hydrocarbon potential based on GIS
Authors Zhou Ziyong, Yu Hangyu and Gu XiaodanEffective fusion of multiple data, including geographical, geological, geophysical, geochemical and dynamic data for hydrocarbon potential mapping, involves both a fusion algorithm and a convenient modelling platform. In this study, fuzzy logic and a geographical information system (GIS) are used to fuse geological and geophysical interpretations in mapping the gas potential of the Kazakhstan Marsel Territory Carboniferous system based on the assumed gas-accumulation model. Non-linear membership functions are used to transform the input data, while the gamma operator is used to combine the multiple datasets. Finally, the Carboniferous system targets, the Visean (C1v) and Serpukhovian (C1sr) units, are mapped. Gas testing in situ validated our results.
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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)