Geochemical characteristics of the Upper Triassic black mudstones in the eastern Qiangtang Basin, Tibet: Implications for petroleum potential and depositional environment

Abstract

The Upper Triassic successions are important targets for petroleum exploration in the Qiangtang Basin, despite remaining poorly better recognized. The organic and inorganic geochemical characteristics of the Upper Triassic mudstones from the Eer Tuolongba section have been studied in order to estimate their petroleum potential and discuss their depositional environment. The total organic carbon and extractable bitumen contents of these black mudstones are in the range of 0.59–1.81% (average = 1.20%) and 460–1663 ppm (average = 778 ppm), respectively. They are interpreted as being fair to good source rocks. Mixed organic matter sources with land plant and plankton were evidenced by kerogen index (TI), hydrogen index (HI), kerogen H/C and O/C atomic ratios, and C29–C28–C27 steranes. They are characterized by type II2 kerogen, but close to type III kerogen. The medium corrected chemical index of alteration values (CIAcorr = 67–82) indicate that these mudstones were derived from the provenances with mild weathering conditions controlled by the warm and humid climate. Paleoredox indexes (U/Th, Ni/Co, Corg/Ptot) reveal that these mudstones were deposited in the oxic-dysoxic water column. The wide range of Sr/Ba (0.25–1.28), Gammacerane/C30 hopane (0.23–0.68), and Ts/Tm ratios (0.68–1.54) of the studied mudstones reveal the stratified water columns with low to high salinity (marine-brackish). The stratified water columns and dyoxic conditions may be appropriate for the organic matter preservation. Vitrinite reflectance (Ro), pyrolysis peak temperature (Tmax), production index (PI), and biomarker indicators indicate that the organic matter is thermally mature. The studied mudstone samples from the Eer Tuolongba section have a better petroleum potential than those from the adjacent Quemo Co section and well QZ-7, as evidenced by their high hydrocarbon abundances and appropriate thermal maturity, although the organic matter source and type are similar.