Influence of igneous intrusions on maturation of the Cretaceous-Eocene source rocks of the offshore Tanzanian Basin

Abstract

The offshore Tanzanian Basin contains numerous igneous intrusions emplaced at various stratigraphic levels. Previous studies indicate these intrusions have impacted petroleum systems, affecting key elements such as source rocks, reservoirs, seals, migration pathways, and trapping mechanisms. However, due to the limited number of wells drilled in the region, there have been few studies reporting the associated thermal effects on source rock maturation and their role in hydrocarbon generation. To gain a comprehensive understanding of the intricate relationship between intrusions and the petroleum system, particularly source rock, an integrated geochemical and resistivity log analysis was carried out. The geochemical results show that the Cretaceous-Cenozoic sediments of the study area have low total organic carbon contents (TOC < 1 wt%), kerogen yield (< 1 Mg HC/g), and Hydrogen Index (< 100 Mg HC/g), primarily composed of Type III (gas-prone) to Type IV (inert) kerogens. These sediments have undergone varying levels of thermal maturity, ranging from post-mature (within Cretaceous), matured (in Paleocene) to immature (in Eocene) thermal states. The Cretaceous strata located proximal to the intrusions exhibit significant thermal alteration, resulting in a reduction of both organic matter (OM) content and source potential compared to the Eocene and Paleocene samples. This observation is consistent with the estimated paleotemperature (T) and resistivity log (ILD) along the depth profile, which have mapped local thermal alteration increasing from base Paleocene to Cretaceous. These findings have implications for source rock potential and thermal evolution history in the offshore Tanzanian Basin. This study highlights the necessity for thorough subsurface mapping in the area to identify both younger and older intrusive rocks. These intrusions pose a potential risk in petroleum exploration, especially when they intrude into matured source rock intervals.