ABSTRACT Stratigraphic, geochemical, and biomarker data from the Huallaga Basin suggest that organic carbon-rich shales and limestones of the Upper Triassic to Lower Jurassic Aramachay Formation of the Pucará Group, previously identified as potential hydrocarbon source rocks in Peruvian sub-Andean basins, were deposited under low oxygen or anoxic conditions within a semirestricted basin. Rock-Eval and total organic carbon (TOC) data from surface and subsurface locations show that although most Aramachay Formation shale and limestone outcrop samples have relatively high organic carbon content, the unit has little remaining genetic potential; T max data indicate that the thermal maturity of nearly all outcrop samples ranges from wet to dry gas. Visual kerogen analyses show that type II amorphous kerogen is the dominant type in the Aramachay Formation. Cretaceous rocks within the Huallaga Basin are dominated by type II/III and type III kerogen and generally lack sufficient TOC to be effective source rocks for oil. Geochemical and biomarker data indicate that rock extracts and seep oils were derived from mixed shale and carbonate source facies dominated by marine algal and bacterial organic matter and are similar to “Jurassic” oils described from the Marañon and northwestern Ucayali Basins. Hydrocarbon generation and expulsion models suggest that the generation and expulsion of oil from the Aramachay Formation (likely the middle Aramachay Formation) began from west to east in the Huallaga Basin, starting in the now-exhumed western part of the basin during the Early Cretaceous, extending through the middle Oligocene in the central part of the basin and into the Present in the eastern part of the basin. Estimates of vitrinite reflectance (R o ) based on biomarker data indicate that Marañon Basin oils derived from the Aramachay Formation were likely generated during the peak oil phase of generation; oils in the northwestern Ucayali Basin were generated during the late oil phase of generation. Petroleum extracts from outcrop samples in the northern part of the basin and oils from seeps along the southeastern frontal thrust of the basin indicate a late oil level of thermal exposure. Migration of oils into the Marañon and northwestern Ucayali Basins likely occurred prior to the early Pliocene, when formation of the Andean frontal thrust cut off migration routes from the Huallaga Basin.

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.