Geochemical Characteristics and 1-D Basin Modelling Study of the Early Cretaceous and Paleogene Succession in the Southern Indus Basin, Southern Pakistan

Abstract

Early Cretaceous and Paleogene shale and limestone sediments in the southern Indus Basin were investigated by geochemical data and 1-D basin modelling. Most of the shales from the Early Cretaceous and Paleogene formations exhibit total organic carbon (TOC) content between 0.51 wt. % and 6.06 wt. %, overall, indicating organic matter richness capable of generating hydrocarbons. However, the limestone samples of the Paleogene formations have lower TOC values in the range of 0.36–0.97 wt.%, inferring poor to fair petroleum source rock. The studied shale and limestone sections exhibit also varying hydrogen indices (HI) ranging from 27 to 430 mg HC/g TOC and different kerogen pathways, ranging from Type II to Type IV. Generally, most of the samples with the Lower Cretaceous and Paleogene formations consist mainly of hydrogen-poor Type III and IV kerogens, with HI values range from 27 to 206 mg HC/g TOC, while some other samples belonging to the Paleogene formations exhibit Types II and II/III kerogen (HI from 219 to 430 mg HC/g TOC). The dominance of such kerogen shows the presence of oil- and gas-prone source rocks, with high potential for gas generation. Maturity-related indicator of Rock-Eval T_max shows different thermal maturity levels, ranging from immature to post-mature. Most of the Lower Cretaceous Goru shales are more mature than other Paleogene sediments, and rank from main oil to gas generation windows, reaching the generation efficiency. This is probably attributed to the deep burial of the Goru Formation reaching a depth up to 4050 m. Therefore, the preliminary geochemical results of the Goru shale unit were integrated into a basin modelling analysis using three exploratory wells to simulate the timing of oil and gas generation. In this case, the simulated basin models reveal that the Goru source rock system currently attained the main oil and gas generation windows, with computed vitrinite reflectance values between 0.75 and 2.00 Easy %Ro. The simulated models indicate that commercial amounts of oil have been generated from the Goru source rock system since the early Palaeocene, as demonstrated by the TR ratio of up to 62%. Moreover, oil was cracked into thermogenic gas during the late Eocene to present-day, with computed vitrinite reflectance of up to 2.00 Easy %Ro. The oil and gas generation was increased with increasing the burial depth, thus, an intensive hydrocarbon exploration and production program is highly recommended in the deeper stratigraphic succession of the Goru source rock system in the southern Indus Basin.