Organic matter enrichment of the Late Triassic Bagong Formation (Qiangtang Basin, Tibet) driven by paleoenvironment: Insights from elemental geochemistry and mineralogy

The Upper Triassic dark mudstones in the Qiangtang Basin are key source rocks for petroleum exploration. However, the factors controlling organic matter enrichment in these dark mudstones remain unclear, especially for pro-deltaic core mudstones, which inhibit source rock distribution predictions. Well QZ-7 contains several Upper Triassic dark mudstone intervals (marked by A to G) with total organic carbon (TOC) contents of 0.57-2.73% (avg. of 1.13%). The TOC contents in intervals A, C, E, and G are generally higher than those in intervals B, D, and F. Here, detailed mineralogical and geochemical analyses of these dark mudstones are analyzed to address the organic matter enrichment mechanism. Paleosalinity, redox, detrital influx, sedimentation rate, and productivity proxies suggest that these dark mudstones were deposited in a brackish-salt and anoxic-dysoxic water environment with low detrital influx, sedimentation rate, and primary productivity. Organic-rich intervals of A, C, E, and G generally show higher salinity, lower oxygen levels, detrital influx, and sedimentation rates than the organic-poor intervals of B, D, and F. The significant positive correlations of TOC with paleosalinity proxies and redox indices indicate that anoxic and stratified high-salinity water columns primarily controlled organic matter preservation. The significant negative correlations between TOC and detrital influx proxies and moderate negative correlation between TOC and (La/Yb)(N) suggest that increasing detrital influxes diluted organic matter, while a low sedimentation rate promoted organic matter preservation. The effect of primary productivity on organic matter enrichment was limited, as evidenced by the weak correlation between TOC and P/Ti.