Molecular organic geochemistry and origin of oil in the Potwar Basin, Pakistan

Abstract

The Potwar Basin (PTB) is a prominent geological feature located in northern part of Pakistan, and is considered as one of the active and productive regions for petroleum and gas exploration in Pakistan. In this study, eight crude oil specimens originating from three distinct fields within the PTB were comprehensively examined to decipher the environmental conditions, source of organic matter (OM) and oil-oil correlations using gas chromatography coupled with flame ionization detection (GC-FID) and gas chromatography-mass spectrometry (GC–MS). The molecular marker and hierarchical cluster analysis of PTB oils reveal two different crude oil families. Family-I showed relatively low values of Pr/Ph, C₁₉TT/C₂₃TT and C₂₅TT/C₂₄TeT, C₂₇-C₂₉ regular steranes, dibenzothiophene (DBT), fluorene (FL) and dibenzofuran (DBF), as well as C₂₆/C₂₈ TAS (20S) and C₂₇/C₂₈ TAS (20R). These results suggest that Family-I crude oils derived from marine environment of suboxic water bodies with higher contribution from green algae or planktonic microbes. However, Family II contains comparatively high values of the aforementioned molecular parameters, indicating that crude oils mainly originated from a lacustrine environment with a higher contribution of plant organisms under oxidizing conditions. Saturated hydrocarbon maturity parameters such as CPI and OEP, and C₃₁22S/(C₃₁22S + C₃₁22R) and C₃₂22S/(C₃₂22S + C₃₂22R) indicate PTB crude oils are thermally mature, while aromatic indices reveal that Family-II crude oils are high mature. The present research defines that PTB crude oils contributed from mixed organic matter sources of marine and lacustrine majorly from marine sedimentary environment with the bloom of algae and/or higher plants.