Confirmation and significance of high quality and oil prone source rocks in the southeast African offshore

Abstract

The offshore region of East Africa has undergone extensive geological evolution since the Permian period, characterized by two stages of rifting phases followed by a subsequent stage of passive margin development. The stratigraphic record in this area is comprehensive, indicating the formation of multiple series of source rocks. Recent discoveries have identified over 200 trillion cubic feet of natural gas in the Ruvuma Basin, in addition to a modest quantity of light crude oil found onshore in Mozambique. However, the precise origins of hydrocarbons in this region remain unclear. In this study, the paleo-position restoration technique for oil and gas fields was utilized to reconstruct the paleo-positions of key wells and reservoirs during critical periods of source rock development. Three significant Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) wells drilled in the offshore region of East Africa were selected for source rock assessment. The geochemical data obtained from these wells were systematically analyzed, resulting in the creation of continuous geochemical profiles that elucidate the characteristics of the thick, high-quality, oil-prone marine source rocks formed in the southern offshore region of East Africa during the Upper Jurassic to Lower Cretaceous periods. These source rocks exhibit high total organic carbon (TOC) and hydrogen index (HI) values, indicating a robust capacity for hydrocarbon generation. They are widely distributed and predominantly consist of oil-prone Type II1-II2 kerogen. The sequence of source rocks with elevated TOC exceeds 115 m in thickness. Carbon isotope data from methane and ethane in natural gas samples suggest that the hydrocarbons in the onshore and southern offshore areas of East Africa are derived from these Upper Jurassic to Lower Cretaceous marine source rocks, thereby indirectly indicating significant exploration potential for these source rocks. Based on seismic sections, a hydrocarbon accumulation model has been developed for the southern offshore region of East Africa. This model comprises multiple series of source rocks that provide sufficient hydrocarbon supply, effective source-reservoir-seal assemblages, and near-source hydrocarbon migration and accumulation. The findings of this study enhance confidence in hydrocarbon exploration within the study area and further delineate potential areas and directions for future exploration efforts.