Integrated geochemical study of Chichali Formation from Kohat sub-basin, Khyber Pakhtunkhwa, Pakistan

摘要

An integrated geochemical study was performed for the assessment of the hydrocarbon potential, environment of deposition, thermal maturity and the organic matter’s source of the Chichali Formation in the Kohat sub-basin of Pakistan. The analytical techniques used included the total organic carbon (TOC), Rock–Eval (RE), organic petrography, column chromatography (CC) and gas chromatography mass spectrometry (GC–MS). The quantity of the organic matter (i.e., TOC), Rock–Eval parameters (such as the original hydrogen index, oxygen index and Tmax) and maceral analyses revealed that the shales of the Chichali Formation have poor to good petroleum source potential with Kerogen type II presently shown as type III (hydrogen index, oxygen index and Tmax) due to thermal maturation and with higher marine organic matter. The extracts of the rock samples have high amount of short-chain n-alkanes with high ratios of tricyclic terpanes to hopanes (TCT/H), C27 to C29 stranes and low ratios of pristane to phytane (Pr/Ph), C19/C23 TCT and C20/C23 TCT. These ratios and lack of terrestrial biomarker (oleanane) are pointing toward algal/marine organic source deposited under anoxic environment. The dibenzothiophene-to-phenanthrene ratios (DBT/P) versus Pr/Ph cross-plot also confirms the anoxic environment with sulfate poor mixed shale/carbonate lithology. The drill cuttings show relatively high maturity compared to outcrop samples indicated by n-alkanes ratios, isoprenoids vs n-alkanes cross-plot, methyl-phenanthrene index (MPI-1), methyl-dibenzothiophene ratios and absence of saturate biomarkers. All the above findings reveal that the Chichali Formation had mature algal source with anoxic environment of deposition and may prove to be a poor to good hydrocarbon source rock.