Biomarker and stable carbon isotopic study of Eocene sediments of North-Western Potwar Basin, Pakistan

Abstract

Abstract Eocene sediments from the Potwar Basin Pakistan have been characterized geochemically using liquid chromatography, gas chromatograph–mass spectrometry (GC–MS), GC–isotope ratio MS and metastable reaction monitoring (MRM) GC–MS. Biomarker and non-biomarker parameters have been used to evaluate the origin, paleodepositional conditions, lithology and thermal maturation of organic matter. The n -alkanes distribution profiles observed for the studied samples suggest an abundance of low molecular weight compounds in the Upper Eocene sequence, which is consistent with higher marine input. On the other hand, the bimodal n -alkanes distribution observed with depth may suggest terrigenous contribution. An enrichment of the δ 13 C of low molecular weight n -alkanes ( n -C 18 ) reveals marine origin (algal and bacterial) while depleted 13 C values observed for n -alkanes greater than n -C 22 indicate terrigenous origin of these compounds. Despite ample biomarker parameters as to the noticeable marine origin of the organic matter, presence of oleanane and retene as well as the enrichment of the δ 13 C values of the long chain n -alkanes suggest significant terrestrial contribution in the studied organic matter. An interesting feature of the δ 13 C isotopic profiles of n -alkanes higher than C 25 is so called saw tooth profile, in which the odd carbon number compounds were more depleted in 13 C than the adjacent even numbered compounds. This feature may further support the assumption that the source of these n -alkanes lay in plant waxes. Depositional environment of these sediments were typical marine with oxic to suboxic conditions and showed variation in oxicity with depth. Our results indicate that the Upper Eocene sediments have been deposited under oxic conditions while the lower Eocene strata are more likely to be deposited under restricted suboxic reducing marine conditions. Significantly low concentration of rearranged steranes and hopanes suggested carbonate lithology for these sediments. Aliphatic and aromatic hydrocarbons geochemical data indicate level of maturity corresponding to the onset of oil generation. Therefore, it is suggested that these sediments are more likely to be considered as possible hydrocarbon source rocks within the Potwar Basin.