Palaeoenvironment evolution and organic matter accumulation of the Upper Triassic mudstones from the eastern Qiangtang Basin (Tibet), eastern Tethys

Abstract

The Upper Triassic successions in the Qiangtang Basin are important targets for petroleum exploration, although they are poorly understood at present. The black mudstones collected from well QZ-16 are the deepest Upper Triassic source rocks to date and have over-mature organic matter (OM) (Ro > 2). In this study, the total organic carbon (TOC) and systematic inorganic geochemistry were determined to reconstruct depositional conditions (palaeoclimate, palaeosalinity, primary productivity, redox conditions, sedimentation rate, and detrital influx) and to assess the OM accumulation mechanism. The TOC contents of the Bagong Fm. mudstones first decrease and then increase from bottom to top (Unit A to Unit D), and Unit C has the lowest TOC contents. These mudstones have relatively obvious correlations between TOC content and the palaeosalinity proxy (Sr/Ba), redox indexes (U/Al, V/Al, Mo/Al, U/Th, and V/Cr) and sedimentation rate indicator ((La/Yb)N), which indicate that OM accumulation was dominated by both high salinity and a slow sedimentation rate in a dysoxic environment. The high Al2O3/TiO2 ratios indicate that the provenance of these mudstones was primarily from felsic igneous rocks. High detrital influx of felsic igneous rock provenance would have diluted the OM and inhibited OM preservation, as evidenced by the negative correlations between TOC content and detrital influx parameters, including SiO2, Al2O3, TiO2, and K2O. The primary productivity (P/Al and Ba/Al) is poorly correlated with TOC for these mudstones, suggesting that its effect on OM accumulation was negligible or limited. The relatively high TOC contents of mudstones from Units A and B are mainly related to the interaction of dysoxic conditions, a high-salinity water column, slow sedimentation rates, and a low detrital influx. An oxidizing water column with low salinity and a relatively high detrital influx and sedimentation rate upward in Unit C led to greater OM degradation and dilution and the lowest TOC contents. The significant TOC content fluctuations in Unit D may be related to vertical variations in the depositional conditions.