Geochemical Evaluation of Paleocene Source Rocks in the Kohat Sub-Basin, Pakistan

The Kohat sub-basin is one of the main hydrocarbon-producing sedimentary basins located in the northwest extension of the Indus Basin in Pakistan. It contains numerous proven and potential petroleum from the Cambrian to the Miocene. Conventional petroleum resources have been depleting rapidly over the last couple of years. Therefore, unconventional resources should be explored using a variety of geochemical and geophysical techniques to address the energy demands. Geochemical techniques, including total organic carbon (TOC) assessment, Rock-Eval pyrolysis, organic petrography, and biomarker studies, are essential for evaluating the potential of shale gas reservoirs to delineate future prospects in a basin. The source rock potential of the Paleocene rocks, including the Patala, Lockhart, and Hangu formations of the sub-basin, is evaluated using geochemical analyses on well cuttings from the Tolanj-01 well. The analyses include estimation of total organic carbon (TOC), Rock-Eval pyrolysis, and organic petrography (vitrinite reflectance) to evaluate the organic richness, thermal maturity, kerogen type, hydrocarbon type, and environment of deposition. Other techniques for extractable organic matter (EOM) include solid-liquid chromatographic separation of fractions, gas chromatography (GC-FID)/whole oil chromatography, and gas chromatography-mass spectrometry (GC-MS). The organic matter (TOC, wt %) analysis reveals that 18 (18) samples of the Hangu formation (0.08-1.8 wt %) show poor values, 12 (12) samples of the Lockhart formation (0.05-0.5 wt %) have poor to fair content, and 26 (26) samples of the Patala formation have poor to fair (0.08-0.19 wt %) TOC content. Rock-Eval pyrolysis studies including hydrogen index, oxygen index, T_max, quantities of free hydrocarbons (S₁, mg/g), and hydrocarbons produced from pyrolysis (S₂, mg/g) are determined for the well-cut samples (56) of the Paleocene rocks. The hydrogen index values for the Hangu formation are lower than 200, and those for the Lockhart and Patala formations range between 100 and 250. A maceral analysis is also conducted on these samples, which reveal that the majority of the samples of the Paleocene units present in the basin belong to kerogen types II/III. The thermal maturity of the Hangu and Lockhart formations falls in the late-stage oil window, while that of the Patala formation falls in the peak to late oil window. The genetic potential (GP) for these rock units is also determined based on S₁ and S₂ values, which reveals that it is generally poor except for a few samples of the Hangu and Lockhart formations, which show fair GP values. For the organic petrography (vitrinite reflectance, R₀), one sample from each unit is selected, which shows that the Hangu, Lockhart, and Patala formations fall in the category of the mature oil window with their R₀ (%) values being 0.95, 0.89, and 0.82, respectively. The extracts (EOM) from these rock units are also analyzed to assess the depositional settings, biological source input, biodegradation, thermal maturity, etc. The greater values of pristine to phytane (Pr/Ph > 1) ratios for Hangu (1.33), Lockhart (1.23), and Patala (1.8) indicate an intermediate condition (suboxic), while a cross-plot of Pr/n-C₁₇-Ph/n-C₁₈ shows that the organic matter is deposited in a transitional setting. The ratios between C₁₉TCT/C₁₉ TCT + C₂₃ TCT and C₂₄ TeCT/C₂₄TeCT + C₂₃TCT biological source inputs are mainly of marine origin. Similarly, the ternary diagram of regular steranes (C₂₇-C₂₈-C₂₉) shows a greater marine input. Lower values of the carbon preference index (CPI1) for Hangu (0.95), Lockhart (0.91), and Patala (1.04) indicate higher thermal maturity of the Paleocene rocks. Similarly, the methylphenanthrene index (MPI-1) values, Moretane index, and Pr/n-C₁₇ vs Ph/n-C₁₈ plots also show higher thermal maturity for these rock extracts.