An integrated organic geochemical assessment and oil-source rock correlation of the cretaceous chichali formation, Kohat Basin, Pakistan

The Cretaceous Chichali Formation in the Chanda-01, Chanda-02, Chanda-03 and Mela-05 wells, along with crude oil from Chanda-01 and Chanda-02 wells located in the Kohat Basin, Pakistan, was investigated to evaluate organic content, depositional environment, thermal maturity, hydrocarbon generation potential, and oil-source correlation using Rock-Eval pyrolysis, organic petrography, gas chromatography (GC), and GC–MS biomarker analyses. Organic matter content in the Chichali Formation is generally low, with the average total organic carbon (TOC) content of 0.54 wt% for the Chanda-02 well, 0.66 wt% for the Chanda-03 well and 0.96 wt% for the Mela-05 well, corresponding to poor to fair quality of source rock in Chanda wells and fair to good quality in the Mela-05 well. Rock-Eval data (Tmax = 432–447 °C for samples considered reliable), hydrogen index values of 42–117 mg HC/g TOC and vitrinite reflectance values of 0.97–1.10% Ro indicate that the organic matter is dominated by Type III kerogen and has reached the main oil window. The non-biomarker and biomarker parameters used for oil–source rock correlation and thermal maturity include the carbon preference index (CPI), pristane/phytane (Pr/Ph), pristane/n-C17 (Pr/n-C17), phytane/n-C18 (Ph/n-C18), the terrigenous-to-aquatic ratio (TAR), C29 sterane 20 S/(20 S+20R), C29 ββ/(ββ + αα), moretane/C30 hopane (M29/H30), Ts/(Ts + Tm), and C31 homohopane 22 S/(22 S+22R), as well as sterane distributions and methylphenanthrene index (MPI). The Chichali Formation is characterized by low TAR values (0.025–0.08), CPI values of 0.9, Pr/Ph ratios of 0.40–0.61, C29 sterane 20 S/(20 S+20R) values of 0.50–0.52, ββ/(ββ + αα) values of 0.50–0.53, Ts/(Ts + Tm) values of 0.43–0.45, and MPI values of 0.78–0.92, whereas the oils exhibit higher Pr/Ph ratios (1.07–1.78), higher TAR values (0.17–0.71), ββ/(ββ + αα) values of approximately 0.50–0.56, Ts/(Ts + Tm) values of 0.48–0.55, and MPI values of 0.92–0.95, indicating slightly greater thermal maturity and a stronger terrestrial organic matter contribution. The integrated biomarker and non-biomarker evidence demonstrates that the investigated oils are not genetically related to the analyzed Chichali Formation. Rather, the oils were probably formed from a more terrestrial-rich source rock located elsewhere in the regional petroleum system.