Petrographical and geochemical characterization of the Upper Permian Longtan formation and Dalong Formation in the Lower Yangtze region, South China: Implications for provenance, paleoclimate, paleoenvironment and organic matter accumulation mechanisms

The Upper Permian Longtan and Dalong Formations are considered prospective targets for shale gas exploration and production in the Lower Yangtze region, South China. However, the paleoclimate change and the mechanism of organic matter accumulation in the shales are rarely reported. In this study, the varying provenance, paleoclimate, paleoenvironment, primary productivity, and the mechanism of organic matter accumulation of the Longtan and Dalong Formations were investigated using X-ray diffraction (XRD), polished section and scanning electron microscope (SEM), pyrite morphology, and elemental geochemical data. Multiple geochemical proxies indicate that the Longtan Formation was deposited in marine–continental transitional facies under a highly humid climate and intermediate-to-strong weathering conditions. The water column was oxic with low paleoproductivity and high terrigenous influx. The rapid rise in sea level during the deposition of the lower Dalong Formation resulted in an anoxic water column, high productivity, and low terrigenous input under a semiarid–semihumid climate and weak-to-moderate weathering conditions. The oceanic upwelling elevated primary productivity and was crucial in the organic matter accumulation. Therefore, the total organic carbon (TOC) is high in the lower Dalong Formation. However, the later sea level fall at the upper Dalong Formation led to a dramatic change in the sedimentary environment, which is characterized by a moderately humid climate, intermediate weathering conditions, decreased productivity, a dysoxic–oxic water column, and enhanced detrital flux. The correlations between TOC content and geochemical proxies suggest that the enrichment of organic matter in the Longtan Formation was mainly affected by terrigenous influx and sedimentation rate. The high TOC enrichment in the lower Dalong Formation was because of oxygen-deficit water column, high productivity, and low terrigenous influx. In the upper Dalong Formation, a combined effect of the decrease in primary productivity and relatively oxygen-enriched conditions was not conducive to the organic matter accumulation. They gave rise to low concentration of organic matter. However, the positive correlations of TOC vs. Cu/Al and TOC vs. Ni/Al indicate that paleoproductivity was nevertheless the most critical factor controlling the enrichment of organic matter. •The abundance of organic matter in the Longtan Formation and Dalong Formation shows strong heterogeneity.•Hu