Abstract:
The Kohat sub-basin is a part of Himalayan foreland basin in the north-west Pakistan having various proven and potential plays, ranging in age from Cambrian to Miocene. A detailed geochemical investigation of rock and oil samples was made to assess that whether or not the shales of the Cretaceous Chichali Formation have contributed to the petroleum reserves of Mela oil field. The geochemical analysis which were employed include total organic carbon (TOC), Rock-Eval pyrolysis (RE), Organic petrography, biomarkers and stable carbon isotopes. From the results of these analysis the organic richness, thermal maturity, kerogen type, hydrocarbons type, age and environment of deposition of the organic matter in the Chichali shale was determined and then correlated with the geochemical interpretation of Mela oils. The outcrop and well cutting samples of Chichali shales proved fair to good source rock potential. The kerogens determined showed type III affinities. Vitrinite reflectance values also showed maturity of outcrop samples. Saturates/aromatic ratio in the extracts of Chichali shales is high as compare to Mela oils which evident marine source input for Chichali shales and terrestrial influence for Mela oils. High abundance of short chain n-alkanes, low values of Pr/Ph (<1), low values of different tricyclic terpanes, high TCT/H ratios, high C27 steranes, low ratio of Ts/Ts+Tm and absence of terrestrial biomarker (Oleanane) showed that Chichali shales had algal/marine organic source and deposited under anoxic environment of deposition. In contrast Mela oils have high abundance of long chain n-alkanes, high Pr/Ph (>1), high values of various tricyclic terpanes, low tricyclic terpanes to hopanes ratios, high C29 steranes, high concentration of oleanane signifying suboxic environment with more terrestrial source input. A cross-plot of dibenzothiophene/phenanthrene (DBT/P) versus the pristane/phytane (Pr/Ph) ratios showed lacustrine sulfate poor or mixed shale/carbonate environment for rock samples while marine shale or other lacustrine environment for Mela oils. Marginally terrestrial organic input is suggested for Mela oils based on stable carbon isotope values. Nalkanes ratios, isoprenoids vs n-alkanes, absence of saturate biomarkers in well cutting and different ratios of aromatic compounds indicates high maturity of the source in well cuttings and oils compared to outcrop samples. Oleanane Index for Mela oils is <20% which indicated Cretaceous or younger source rock. Based on C28/C29 –sterane ratio Early-Cretaceous or younger source is suggested for extracts of outcrop samples as well as Mela oils.
