1D basin modelling of upper jurassic to upper cretaceous source rocks in the Southern Pletmos Sub-Basin, Outeniqua Basin, offshore South Africa

Abstract

The Pletmos sub-basin in the Outeniqua Basin, offshore South Africa, is divided by the Superior, Pletmos and Plettenberg faults into northern and southern depocentres. In this study, geological and organic geochemical data together with 1D basin modelling were used to investigate the burial, thermal and maturation histories, and the timing of hydrocarbon generation and expulsion of clay-rich source rocks in the southern depocentre. The modelled source rocks were of Kimmeridgian, Valanginian, Hauterivian, Aptian and Turonian ages. Source rock burial histories at the Ga-A4, Ga-E1, Ga-E2, Ga-H1, Ga-Q1 and Ga-Q2 wells in the Ga-A gas field were reconstructed with subsequent modelling of source rock thermal and maturation histories. The models were calibrated using vitrinite reflectance data from six offshore wells and constrained by palaeotemperature, heat flow and kinetic parameters. The modelling results indicate three principal phases of hydrocarbon generation (132–89, 89–45 and 45 Ma to present day), with a critical moment occurring at approximately 23 Ma during the Oligocene. The deepest syn-rift Kimmeridgian and Valanginian source rocks reached late-mature to over-mature conditions and are interpreted as the principal contributors to hydrocarbon charge in the basin. These results indicate that the petroleum system in the southern Pletmos sub-basin is predominantly gas-prone to gas-dominant, reflecting the advanced maturity of the deeper syn-rift source intervals and highlighting the continued exploration potential of deeply buried petroleum systems in the Outeniqua Basin