Geochemical characteristics and mechanism of organic matter accumulation of marine-continental transitional shale of the lower permian Shanxi Formation, southeastern Ordos Basin, north China

Abstract

The marine-continental transitional shale is widely distributed in the Lower Permian Shanxi Formation of the southeast Ordos Basin. The primary factors controlling organic matter accumulation of the Shanxi shale are still obscure. In this study, we used mineralogy and organic geochemistry, coupled with major and trace element geochemistry to analyze the paleoenvironment and their effects on organic matter enrichment, and established the accumulation model of organic matter in the Shanxi transitional shale. Results show that the mineralogical composition of the Shanxi shale is dominated by clay and quartz (averaging of 62.8 and 33.3%, respectively). The Shanxi shale has relatively high TOC contents and thermal maturity (Ro) (averaging of 1.25% and 2.66%, respectively), and predominantly contains type III kerogen. The significantly low paleoproductivity indicates that this factor plays not the primary role of organic matter accumulation. Ti and Al concentrations indicate a strong terrestrial input, simultaneously bringing abundant terrestrial organic matter, which is favourable for its enrichment. The warm and humid climate condition enhanced higher plant growth and provenance weathering. The δU, U/Th, V/Cr and Ni/Co values show dysoxic-oxic environments, suggesting relatively poor preservation conditions for organic matter. Comprehensive analysis shows that the terrestrial organic matter input, paleoclimate and redox conditions are the primary factors controlling the organic matter accumulation of the Shanxi shale. • The paleoproductivity is significantly low during the Shanxi shale deposition. • The terrestrial detrital input is strong, carrying abundant terrestrial organic matter. • The paleoclimate and redox conditions triggered organic matter accumulation to some extent. • An accumulation model of organic matter in the Shanxi Formation was established.