Assessing the Organic Nature, Potentials and Distribution of Source Rocks, Within the Southern Pletmos Basin (Offshore of South Africa): A 3-D Modelling Study

Abstract

Source rock identification and parameter quantification is a key aspect of petroleum prospect assessment and play risking. Intriguingly, despite the occurrence of exploration activities during the recent decades within the relatively frontier Pletmos basin, prior to this study, little was known about the source rocks that resulted to charging. Accordingly, charging within the basin was postulated to have occurred mostly during the Cenozoic Era. This heightened the need to identify the characteristics, petroleum generative potentials, and existing source rock(s) configurations. Likewise, establish the critical moment(s) of hydrocarbon generation, expulsion and charge within the basin. To redress such snags, this study in a first attempt integrated geochemical source rock analysis with basin-modelling techniques. Based on assessment results of Rock-Eval data (acquired by SOEKOR Ltd) from 89 samples, five Mesozoic source rocks ranging from Kimmeridgian to Turonian were identified. The results demonstrate that TOC values range from 0.5 - 4.0 wt. %, the highest occurring in the Aptian and lowest in the Kimmeridgian. HI values range from 23.7 – 499.5 (mg HC/g TOC) essentially indicating that they are mainly predominated by type III (gas-prone) kerogens and, to a lesser extent contain mixed type II/I1I (oil/gas-prone). Thus, are considered as primarily gas-prone. Tmax values matched with recorded vitrinite reflectance (%VRo); they indicate that source rock maturities range from immature - late mature (dry gas). Reconstructed burial and thermal histories permitted modelling of the thermal maturation and transformation ratios. Results show that maturity trends are in accordance with transformation ratios and they both increase away from the Superior High and generally attain a maximum at the southern portions. At present-day, maturities range from immature – late mature (dry gas) in accordance with the preliminary source rock evaluation results. As expected, the highest extent of kerogen transformation is exhibited by the deepest (oldest) buried Kimmeridgian source rock; the reverse is true for the youngest (mostly-immature) Turonian source rock. Three periods (i.e. ~98Ma, ~80Ma and ~60Ma) are predicted as critical moments for the Kimmeridgian, Valanginian/Hauterivan, and Aptian respectively, implying that hydrocarbon generation preceded charging and the modelled source rocks probably contributed to the hydrocarbons within the area.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018