AAPG Annual Convention and Exhibition

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Challenge and Response: Identification of Petroleum Systems Across the South Atlantic Equatorial Margins


Halt and identify yourself – the demand of a geochemist to determine the source from which an oil sample derived! In developing our petroleum systems studies of the Atlantic margin basins, a primary focus is to link oils to their source paleo-environments. Initial ideas were tested in Brazilian basins and their Aptian Salt Basin (West Africa) conjugates. As our scope expanded around the Central and South Atlantic, our understanding of the distribution of, and controls on, oil families along the passive margin basins has turned to the Atlantic Equatorial Margins (EqMar). We review the tectono-structural framework that hosts our oil samples; and integration of a complex geochemical evaluation of 1400-odd oil samples into a multi-disciplinary petroleum systems study. Observations include indications that EqMar opening was asymmetric with deep monoclinal basins forming along the African margin between the St Paul and Chain Fracture Zones (FZ) but not on the Brazilian conjugate margin which appears to have retained much or all of the early synrift architecture. Hence the asymmetry a) biased the location of potential lacustrine (pre-rift to early syn-rift) source rocks and b) locally narrowed the width of the optimal marine (post-rift) source kitchens. Post-rift source reflects similar depositional environments and organic matter on both margins, consistent with the global Turonian-Cenomanian oceanic anoxic event.