GEOCHEMICAL EVALUATION OF POTENTIAL SOURCE ROCKS OF PALEOCENE, KOHAT SUB-BASIN, PAKISTAN

Abstract

Kohat sub-basin which exists in north-west of Pakistan have numerous petroleum plays in producing and potential categories ranging in the age from Cambrian to Miocene. Detailed geochemical studies of well cuttings and oil were carried out to check whether Paleocene Hangu Formation, Lockhart Limestone and Patala Formation have their contribution in Mela oilfield reserves or not. The geochemical analysis which were employed include total organic carbon (TOC), Rock-Eval pyrolysis (RE), Organic petrography and biomarkers study. 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 Paleocene formations were determined and then correlated with the geochemical interpretation of Mela oils. Several studies were carried out on source rocks, and it is generally believed that the Patala Formation (Paleocene) plays as a source rock in most of the Kohat sub-basin but the correlation between source rock and crude oil analysis has not yet been confirmed by any reliable geochemical study. The well cuttings of all three Paleocene Formations show fair to good and in some cases very good source rock potential with the type of kerogen is mostly Type-III and Type-II. Vitrinite reflectance values shows the thermal maturity of the well cuttings in oil window. Higher amount of short chain alkanes and long chain alkanes in Mela oil point towards the direction of mixed input. Pr/Ph >1 for Mela-1 shows sub-oxic environment of deposition and ratio is <1 for Paleocene cuttings indicates the presence of anoxic environment of deposition. C19 TCT / C19 TCT + C23 TCT ratio, C24 TeCT / C24 TeCT + C23 TCT ratio and C23 TCT / C30 H ratio indicates that source input for the oil as marine organic matter. The relative abundance of regular steranes; C27:C28:C29 shows mixed source input for all the three formation extracts and Mela oil. CPI values, MP relative concentration and Ts/Ts+Tm ratio shows higher thermal maturity for all three Paleocene formations and Mela-01 oil. Oleanane Index for Mela oils is <20% which indicated Early Cretaceous or Jurassic age Comparison of Mela oil with all three formations generate no correlation.