Petroleum source rock potential evaluation: a case study of block 11a, Pletmos sub-basin, offshore South Africa

Among the several existing geochemical methods in hydrocarbon exploration, the technique linking total organic carbon content (TOC) and Rock-Eval pyrolysis is most widely used, largely as a result of its capacity to rapidly provide vital information regarding the identification of source rocks and their defining characteristic, as well as thermal maturity levels and generative potentials. In this study, data from prospective source units within the southern depocenter of the Pletmos Sub-basin were analyzed using this geochemical method. Cutting samples from six wells in Block 11a were subjected to organic geochemical analysis to understand the occurring hydrocarbon scenario. Based on the results of the investigation, five notable source rock intervals of Kimmeridgian to Turonian ages were identified. The source rocks are shales with both indigenous and non-indigenous organic matter. Their TOC values show a fair to excellent petroleum generating potential, with the Aptian and Kimmeridgian intervals having the highest and lowest, respectively. The Hydrogen Index (HI), along with S-2/S-3 ratio values, typifies a predominance of mixed type II/III (oil/gas-prone) with some type III (gas-prone) and II (oil-prone) kerogens. The trends of the maturity parameters T-max and Ro (vitrinite reflectance) indicate maturities ranging from immature to a late stage of maturity (dry gas window). Two observed breaks in the Ro profile reveal a possible link of maturity to high heat flow that is allied to sedimentation and tectonic uplift during the Late Cretaceous. Generally, based on TOC, S-1,S- and HI, the petroleum potential trend increases with increasing depth, with a striking display of mixed hydrocarbon generating potential. Interpreted hydrocarbon typing is thus recommended to support the well-testing analysis.

Automated summary

Among the various geochemical methods in hydrocarbon exploration, the technique combining total organic carbon content (TOC) and Rock-Eval pyrolysis is widely used due to its ability to rapidly provide important information about source rocks, thermal maturity levels, and generative potentials. This study analyzed data from potential source units in the southern depocenter of the Pletmos Sub-basin using this geochemical method. The analysis identified five significant source rock intervals of Kimmeridgian to Turonian ages, consisting of shales with both indigenous and non-indigenous organic matter. The petroleum generating potential of these source rocks varied, with the Aptian and Kimmeridgian intervals having the highest and lowest potential, respectively. The maturity parameters T-max and Ro indicated maturities ranging from immature to a late stage of maturity, with possible links between maturity and high heat flow during the Late Cretaceous. The petroleum potential trend increased with depth, showcasing mixed hydrocarbon generating potential. Hydrocarbon typing interpretation is recommended to support further well-testing analysis.