Evaluation of Hydrocarbon Potential in the Subathu Sediments of Jammu Sub-Himalaya in the Western Himalayan Foreland Basin, India

ABSTRACT The Subathu Formation is the only known Cenozoic marine sequence in the Jammu Sub-Himalayan fold-thrust belt, which is the south-eastern extension of the petroliferous Potwar Basin in Pakistan. By analogy, these sediments have been targeted for exploration by ONGC to evaluate their potential for oil and gas, having drilled two deep exploratory wells that did not encounter the Subathu sediments in the subsurface. The presence of oil and gas seepages indicates it is highly prospective and a promising target for exploring the hydrocarbon potential of these sediments in the Jammu Sub-Himalaya. The paper presents the results of geochemical analysis, organic petrography, and biomarker studies carried out on outcrop samples. These shales show high to very high (0.63% to 60.01%) TOC, high S2 (36 to 98 mg HC/g rock), and several samples show HI ≤ 167 mg HC/g TOC. The HI values indicate predominantly Type III and Type IV kerogen (gas-prone), typical of coaly shales. A significantly high (>1%) extractable organic matter (EOM) indicates a strong to excellent oil-prone potential of the source rock. Biomarker ratios (Pr/nC17 < 0.1; Ph/nC18 < 0.5) suggest that the organic matter is predominantly of mature Type III kerogen, deposited under reducing marine conditions. Vitrinite is the predominant maceral (61 to 91%) with a subordinate amount of Inertinite and Liptinite, characteristic of Type III organic matter of terrestrial origin, and minor Type II organic matter indicating influence of marine source also during deposition of these sediments. The Tmax values (461 to 467°C) align with VRo values (0.50% to 1.26%), suggesting that the Subathu coaly shales attained a thermal maturity equivalent to sub-bituminous to medium bituminous rank. The VRc values (1.3% to 2.0%) obtained from aromatic fragmentogram analysis indicate that the samples have attained thermal maturity levels corresponding to the wet gas window of hydrocarbon generation. The Subathu coaly shales are organically rich, mature to overmature, Type III kerogen with significant potential for hydrocarbon generation and exploration.