Mechanism of Thermogenic Gas Formation From the Cracking of Oil‐Kerogen of the Lower Cretaceous Organic‐Rich Shales in the Lower Indus Basin, Pakistan: Implication From Geochemical and 1‐D Basin Modelling Analysis

ABSTRACT Organic‐rich shale facies of the Lower Cretaceous Sembar Formation in the Lower Indus Basin, southern Pakistan, was studied to assess the original organic matter (OM) input and depositional environment, and to anticipate the impact of thermal maturity levels on the thermogenic gas formation from oil‐to‐gas conversion. The Sembar shales exhibit high total organic carbon (TOC) and sulphur concentrations, with TOC/S ratios exceeding 1 in dominant samples, suggesting deposition under low‐oxygenated marine environmental conditions. The organic carbon enrichment in Sembar shale facies is attributed to low‐oxygen depositional settings combined with extended intervals of warm‐water conditions. Molecular biomarker evidence such as abundant tricyclic terpanes, and a higher proportion of C 27 regular steranes relative to their C 28 and C 29 homologues indicates that the OM within the Sembar shale facies is predominantly sourced from marine algae and bacteria, with minor terrigenous land plant input. This high abundance of aquatic organisms in Sembar shale facies indicates that hydrogen‐rich Types II and II/III kerogen were the original OM input during deposition. This finding contradicts the current measured low hydrogen index (HI) values, between 2 mg HC/g TOC and 138 mg HC/g TOC, indicative of predominantly Types III and IV kerogen in the studied exploration wells. This drastic alteration of the original hydrogen‐rich OM is interpreted as thermal overprinting, wherein the hydrogen‐rich kerogen underwent extensive transformation under elevated maturity levels, ranging from the late oil window to the gas generation window, as supported by Easy%Ro values exceeding 1.35. These elevated maturity levels led to high transformation of hydrogen‐rich kerogen into oil, followed by the cracking of oil into thermogenic gas, with ratios of more than 80% and up to 98% from the Late Cretaceous to the Late Miocene and continuing to the present day. Consequently, these findings highlight the Sembar shale as a promising target for unconventional gas development within the deeper stratigraphic intervals of the Lower Indus Basin.