Lower Cretaceous lacustrine source rocks in northern Gabon: effect of organic facies and thermal maturity on crude oil quality

Organic facies variation, which is a key factor affecting the quality of hydrocarbon accumulations, is characterized for the pre-salt freshwater lacustrine source rocks in the Interior Basin and NʹKomi Rift areas in northern Gabon by integrating geochemical and geological data. The Kissenda shales have 0.59–2.23% total organic carbon (TOC) and hydrogen indices (HI) from 458-93 mgHC/gOC. Pyrolysis of isolated kerogen and gas chromatography on rock extracts indicate that these samples are, overall, more waxy than those from the overlying Melania shales which have 0.77–4.79% TOC and HI from 896-212. In addition, hopane/sterane ratios and C30 4-methyl sterane indices increase systematically with depth. It is proposed that the early Cretaceous lacustrine system in northern Gabon was initially oligotrophic and contained primarily bacteria and cyanobacteria with a small population of dinoflagellates and Botryococcus. As trophic level increased through time, the food chain length increased, eukaryotic organisms became more abundant and diversified, steroids increased relative to hopanoids in the sediment, and the preserved organic matter became less waxy. Burial history modeling shows that the Onal-1 structure is charged primarily by waxy oils generated from the lower Kissenda shales. Lower wax, better quality oils are expected to the southwest where the Kissenda shales are more mature and the less waxy Melania shales also become effective source rocks. Thus, variation in organic facies and thermal maturity in the Cretaceous pre-salt source rocks result in commercial production from the Rabi Field vs the subcommercial discovery in the Onal-1 structure.