Abstract We describe an examination of two lines of evidence, tectono-structural evolution and hydrocarbon geochemistry, of asymmetric opening of the Atlantic Equatorial Margin. Our structural mapping used compilations of geophysical data and a review of both published literature and oil company public presentations. Geochemically, we accessed regional non-exclusive oil studies of the conjugate margins of Africa and South America, plus considerable published material. A group of non-exclusive oils was refined to 286, which clustered into five families, all represented along the NE Brazil margin but only one along the West African Transform (WAT) margin. Multiple lacustrine-sourced oils were seen around the South Atlantic, including NE Brazil, but a rich, oil-prone lacustrine source was not indicated offshore Ivory Coast and Ghana. Despite minor evidence of mixed source, possibly lacustrine stringers within an alluvial to marine setting, the predominant source is marine Cretaceous (Cenomanian–Turonian and possibly Albian). We find that opening asymmetry (a) biased the location of lacustrine (Early to mid-Cretaceous prerift to early synrift) source rocks to the NE Brazil margin and (b) locally narrowed the width of the optimal marine (Mid-Late Cretaceous postrift) WAT Margin source kitchens. Burial of the latter has aggravated the risk of late charge from light (condensate and gas) hydrocarbons.

Abstract Marlstone outcrops (~40–50% total carbonate content) of the Upper Cretaceous La Luna Formation in the Middle Magdalena Valley, Colombia, average 4.3 wt% total organic carbon and 2,500 ppm extractable hydrocarbons. Microscopic and elemental analyses of La Luna kerogen suggest that the organic matter in these La Luna Samples is of marine origin and at a maturity level within the oil-generation zone. Geochemical fossil distributions attest to a marine-planktonic and microorganism source, owing to abundant C 27 steranes relative to C 29 steranes, low quantities of C 19 and C 20 tricyclic diterpanes relative to extended (up to C 30 ) tricyclic terpanes, and the presence of ubiquitous hopane triterpanes derived from bacteria.