Origin and evolution of a CO2-Rich gas reservoir offshore Angola: Insights from the Gas Composition and isotope analysis

Abstract

The pre-salt sections of the offshore Kwanza Basin, Angola are potentially petroliferous reservoirs like their Brazilian counterparts on the other side of the South Atlantic Ocean. In this study, a pre-salt reservoir of the offshore Kwanza Basin was found to contain solid bitumen and gas of high portions of CO2 (ca. 83%) and methane (ca. 16%). The chemical and isotopic (C, H and He) compositions of the gas indicate that the CO2 is primarily mantle-derived while the gaseous hydrocarbons are mainly late mature thermogenic gas. The Barremian-Aptian Red Cuvo Formation with sapropelic and humic organic matter and an average TOC content of 1.7% could be the principal source of the bitumen and gaseous hydrocarbons in the reservoir. Rapid oil generation since the source rock sedimentation could have occurred as a result of the fast and thick salt deposition or the high thermal gradient created by the crustal thinning and magmatism during the South Atlantic Ocean opening. The δ13C of gaseous hydrocarbons also indicates their mixed origins of sapropelic and humic organic matters and possible abiotic processes. This reservoir is similar in the origin of CO2 and geological background to some pre-salt reservoirs in the Brazilian basins. Such similarities with prolific petroleum systems may imply great petroleum potential in the pre-salt sections of the offshore Kwanza Basin, Angola.