The bitumen formation and Re-Os characteristics of a CO2-rich pre-salt gas reservoir of the Kwanza Basin, offshore Angola

Abstract

Here we present a study on the formation process and Re–Os systematics of the bitumen discovered in a CO2-rich pre-salt gas reservoir, south Kwanza Basin, offshore Angola. The magmatism-derived CO2 may have expanded downwards from the top of the reservoir, creating CO2 concentration and temperature gradients within the reservoir, and leading to the gradients of asphaltene precipitation and thermal cracking. The heat front may have been behind the CO2 front due to the heat exchange with surrounding rocks. Thus, asphaltene precipitation has likely occurred before the thermal cracking. Such bitumen formation process is consistent with the observed decreasing bitumen content and Rock-Eval Tmax with depth, and also the characteristics of the whole rock and bitumen Re and Os concentrations. Five closely spaced bitumen samples define a Re–Os age of 116 ± 29 Ma that is identical to the timing of CO2 charging and thus may indicate the timing of bitumen formation. The heterogeneous 187Os/188Os and lack of mobility of the asphaltene deposit, and the relatively low extent of thermal cracking (Tmax of 425–470 °C) may account for the lack of homogenization of the bitumen initial 187Os/188Os. Given the highly radiogenic initial 187Os/188Os (ca. 1.2–1.8) of the bitumen and limited 187Os/188Os ingrowth within the ca. 15 million years from the deposition of potential source rock (Barremian-Aptian Red and Grey Cuvo formations) to the bitumen formation, the source rock should possess a highly radiogenic initial 187Os/188Os (>1). This is consistent with that of the equivalent lacustrine strata in the conjugated Brazilian marginal basins. Thus, the Os isotope composition indicates that the lacustrine Red and Grey Cuvo formations could be the source rock for the bitumen of the studied CO2-rich pre-salt gas reservoir in the south Kwanza Basin.