Abstract:
Kohat Sub Basin’s crude oils were evaluated for saturated and aromatic HC’s distribution, biomarkers and stable isotopic configurations to establish oil-oil correlation and envisage source input of organic matter, level of bio-degradation, thermal maturity, lithology and source depositional environment. High thermal maturity along with marine source contribution is revealed by unimodal distribution of n-alkane accompanied by CPI and OEP having values approximately unity. The S/S+R C31 homohopane ratio, S/(S+R) with proportion C29 sterane and calculated vitrinite reflectance based on aromatic hydrocarbons shows late thermal maturity (late oil window) of the source rock for Kohat Oils. Chanda, Chanda Deep, Mela and Nashpa disclose mixed source organic matter, while Makori and Makori East oils have predominant marine influence based on isoprenoids/n-alkanes ratio. Regular steranes (C27 to C29) ternary plot suggest mixed organic input for Kohat Oils. Low ratio of 1-MP/9-MP also illustrates a marine source input for Kohat Oils. The lithology of source rock of the crude oils appear to be shale, based on lower values of C29/C30-hopane and higher concentration of diahopane (C30DH). However, Makori and Makori East oil samples have lack of diahopane. DBT/P values further confirm shale as source rock for studied Kohat Oils. Source for Kohat Oils were deposited under oxic/sub-oxic condition presumed on low values of HHI and C35/C34 homohopane ratio (<1). This statement is reinforced by suboxic depositional setting exposed by Pr to Ph proportion of samples (>1). The age of source rock assumed to have produced these oils are Jurassic-Cretaceous based on Oleanane Index. Lack of unresolved complex mixture (UCM) of hydrocarbons confirms non-biodegraded oil samples. Stable carbon isotope ratios ranged from - 23.1‰ to 26.7‰ for the whole oil analysis of Kohat Sub Basin.
