Input and preservation of organic matter in the Upper Permian shale from the Lower Yangtze Platform, South China: evidence from organic and inorganic geochemistry

Abstract

During the Late Permian Wuchiapingian–Changhsingian (W–C) transition, significant biotic and environmental changes occurred. These changes had the potential to greatly influence the type, accumulation, and preservation of organic matter within sediments during this period. The Wujiaping Formation, which acts as the contemporaneous heterotopic facie of the Longtan Formation, is regionally found in the Susong–Dongzhi stratigraphic subregion in the Lower Yangtze Platform (LYP). However, the variations in organic matter input and preservation during the Late Permian W–C transition in this specific area have not been comprehensively understood. Consequently, this study involved the collection of Upper Permian Wujiaping and Dalong shale samples in the LYP. Total organic carbon (TOC) content, vitrinite reflectance (R_o), maceral compositions, carbon isotope composition of kerogen, as well as the major and trace elements in the shale samples were all measured in this study. The results of R_o analysis indicate that the maturity levels of shale derived from the Dalong and Wujiaping Formations exhibit maturity levels exceeding 2%, which indicates an over-mature stage. The maceral compositions and carbon isotope analysis of kerogen reveal that the predominant source of organic matter within the Dalong shale is attributed to algal organism. Conversely, the organic matter in the Wujiaping shale primarily comes from higher plants. The major and trace element distributions provide insights into the depositional conditions of the Upper Permian shale, and significant variations in organic matter input and preservation are evident during the Late Permian W–C transition. The findings indicate that the Wujiaping Formation was characterized by a low paleo-productivity, high terrestrial influx intensity, low paleo-salinity, and a pronounced restricted setting under oxic conditions. However, the Dalong Formation is deposited under an anoxic environment, characterized by high paleo-productivity and paleo-salinity, limited terrestrial influx intensity, and increasing upwelling. In addition, the water mass restriction weakened within the Dalong Formation, attributable to a gradual rise in sea level. The shift of organic matter input and preservation was instigated by the persistent regional rise in the sea level during the Late Permian period. In summary, the great input and favorable preservation conditions of organic matter observed in the Dalong shale indicates a promising potential for shale gas exploration.

Graphical abstract