Source rock maturity and 1-D basin modelling in blocks 2 and 3 sector, Offshore Orange Basin, South Africa

Abstract

The passive continental margins of South Africa are virtually unexplored, although some potential is assumed in the Orange Basin and even proven by the Kudu gas field offshore Namibia and the Ibhubesi gas field offshore South Africa. The present research uses source rock samples and basin modelling to evaluate the hydrocarbon potential in the Cretaceous South African exploration license blocks 2 and 3, Sector, Offshore Orange Basin. With an emphasis on the development of source rock maturity, the study seeks to comprehend the geological processes that led to the formation of the Orange Basin. These organic deposits have average TOC values across the several stratigraphic units, with a relatively fair to good TOC composition ranging from 0.53 to 1.89 wt %. Out of the 23 samples that were examined, 17 had TOC concentrations of more than 1 %, and the remaining 6 had contents between 0.53 and 0.98 wt %. The examined organic sediments have a relatively low sulphur (TS, wt.%) content, ranging from 0.21 to 1.18 wt%, with the Turonian–Coniacian sequence having a slightly higher sulphur content of up to 1.18 wt%. These results show that free petroleum exists (S₁) and that oil generated by kerogen heat cracking (S₂) delivers oil with low petroleum potential. The analysis of the shale deposits shows that their low petroleum potential is supported by the S₂ and S₃ petroleum yields, which are consistent with TOC content. The average hydrogen index (HI) in the study area indicating low-source rock quality capable of producing type III kerogen. A one-dimensional simulation was constructed for four wells, which reveals subsidence and the influence of multi-tectonism on the burial and thermal history, reaching a maximum burial thickness from Cenomanian to Turonian up to 2110 m. The integrated studies between the analyses of source rocks and petroleum basin modelling from the Orange Basin reveal a certain petroleum potential of the Cretaceous source rocks and three petroleum systems in the lower post-rift pattern and an extrapolated topmost post-rift play.