Sedimentary Environment of the Permian Marine Shale in the Kaijiang-Liangping Trough, Sichuan Basin, China: Implications for Organic Matter (OM) Accumulation

Abstract

The Permian shales in the Kaijiang-Liangping Trough within the Sichuan Basin represent a promising frontier for marine shale gas exploration, whereas there has been limited systematic research on their sedimentary environment and organic matter (OM) enrichment mechanisms. Therefore, we present total organic carbon (TOC) analysis, major/trace element analyses and scanning electron microscope experiments for the Permian marine shales from the trough to determine their paleoenvironmental conditions and influencing factors of OM enrichment. The results show that the paleoclimate changed from dry climate to humid and warm climate (P₃w¹ [the first member of the Wujiaping Formation]) and semi-humid to semi-arid climate (P₃w² [the second member of the Wujiaping Formation]) and P₃d-I (Dalong Formation-I) and then to arid climate again during the shale deposition period from the P₂g (Gufeng Formation) to the P₃d-II. The shales with the highest TOC contents (TOC > 3%, P₃d-I and P₂g), lower TOC contents (TOC < 1%, P₃w¹ and P₃w²) and higher TOC contents (1% < TOC < 2%, P₃d-II) were formed under the control of anoxic environment and high paleoproductivity, oxic-suboxic environment and high paleoproductivity, anoxic-euxinic environment and lower productivity, respectively. Only appropriate sedimentation rates promote OM enrichment. Terrestrial input, paleoclimate, volcanic activity and hydrothermal upwelling mainly indirectly affect OM accumulation by influencing paleoproductivity. The degree of redox conditions is the primary factor affecting OM enrichment, followed by paleoproductivity. Nonetheless, anoxic to euxinic environments are most appropriate for OM preservation. Weak volcanic activity can boost paleoproductivity, but severe volcanic activity might introduce excessive harmful compounds that limit organism survival, resulting in a fall in paleoproductivity. Additionally, element P brought by volcanic ashes doesn't contribute to OM accumulation.