Depositional Environments and Geochemistry of the Upper Triassic to Lower Jurassic Pucará Group, Huallaga Basin, Peru

Abstract

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 Pucara 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; Tmax 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 Maranon 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 (Ro) based on biomarker data indicate that Maranon 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 Maranon 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.