--> Abstract: Structural Control from Enigmatic Carbon Dioxide Gas Occurrences in the Southern North Sea Basin: Implications for Play Evaluation and Risk Assessment, by John R. Underhill; #90082 (2008)

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Structural Control from Enigmatic Carbon Dioxide Gas Occurrences in the Southern North Sea Basin: Implications for Play Evaluation and Risk Assessment

John R. Underhill
Grant Institute of Earth Science School of Geosciences, The University of Edinburgh, Edinburgh, United Kingdom

The Southern North Sea is a prolific petroleum province with dry methane gas production from Lower Palaeozoic (Carboniferous and Permian) and Mesozoic (Triassic and Lower Cretaceous) reservoirs within numerous NW-SE striking fault-bound and anticlinal structural closures. Whilst some traps contain a minor component of inert gas, two discoveries (Fizzy and Oak) in the southern part of the basin contain an anomalously high (>40%) and previously inexplicable amount of carbon dioxide. Results of an interpretation of a large, well-calibrated 3-D seismic volume places the discoveries into their proper regional context and provides important new insights into the surprising and enigmatic presence of carbon dioxide gas within their Rotliegend Group, Leman Sandstone Formation host reservoir. It can now be shown for the first time that significant carbon dioxide occurrence is entirely limited to the area immediately adjacent to a graben delimited by well-defined deep-seated, normal faults. Significantly, unlike other structures, which experienced several phases of structural inversion, the traps containing the carbon dioxide gas reside against a major inverted normal fault that only reactivated in compression during the Late Cretaceous. It appears therefore that the extremely unusual occurrence of carbon dioxide is intimately linked to the style and timing of deformation in the Southern North Sea. If this proves to be the case, the study will have produced a robust, testable and unifying model, which can be used to assess remaining plays in the basin. Most importantly, the results provide a new-found basis by which to evaluate and reduce exploration risk, such that future drilling avoids targeting unwanted, carbon dioxide-prone prospects.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery