Sedimentary environment and major controlling factors of organic matter-rich shale from the Wufeng-Longmaxi formation in eastern Sichuan Basin, China

Abstract

The major controlling factors of organic matter and its enrichment model of the black shale from the Wufeng-Longmaxi Formation were explored by investigating the vertical variation characteristics, as well as major element and trace element abundances in the Wuxi Bailu section. The results show that the sedimentary tectonic setting of the Wufeng-Longmaxi Formation in the north-east margin of the upper Yangtze platform is located on the active continental margin, which is a passive continental margin and continental island arc. The parent rock in the source area is mainly felsic volcanic rocks mixed with small amounts of sedimentary recycling materials. Due to increased plate activity and a drop in sea levels, terrigenous pyroclastic input increased. The palaeoclimate was semi-humid, and a robust dysoxic-reduction environment and a high level of palaeoproductivity, causing the formation of the organic-rich shale in the Wufeng Formation. At the base of the Longmaxi Formation, the sedimentary water body was affected by global transgression, showing a strong anoxic-reductive environment, and the paleoclimate was a warm and humid condition. The palaeoproductivity level was high, resulting in the formation of organic shale. Due to the sea level drop at the top of the Longmaxi Formation, the sedimentary water was in an oxic-reduced environment, but the input of terrigenous pyroclastic matter increased. Because the paleoclimate was warm and humid and the palaeoproductivity level was high, organic-rich shale was formed. The findings demonstrate that terrigenous clastic input circumstances, palaeoproductivity conditions, and paleoredox conditions had the greatest influence on the enrichment of organic matter in the Wufeng-Longmaxi Formation. Thus, organic matter enrichment was controlled by multiple paleoenvironmental factors.