Abstract: The black cherts in the middle Permian Gufeng Formation in Chaohu area in Lower Yangtze, South China are rich in organic matter. This formation is the potential source rocks of petroleum and shale gas exploration in China. The study on the organic matter accumulation of the Gufeng Formation is important for the petroleum exploration. Here we analyze total organic carbon (TOC) content and biomarkers characteristics of organic matter in the Gufeng cherts of middle Permian in the Pingdingshan, Chaohu area, so as to address the organic matter accumulation mechanism. Our results show that the organic matter thermal mature proxy C29 ααα 20S/(20S+ 20R) ratios range from 0.37 to 0.46, and C31 22S/(S+R) ratios range from 0.53 to 0.59. These suggest that the thermal mature level is the early stage of“oil window”. The n-alkane compounds are mainly composed of short-chain n-alkanes and peak at C18. Hopane contents are
higher than the terpane. C27 ααα 20R sterane is higher than the C28 and C29 ααα 20R. These suggest that the organic matter of the Gufeng Formation originates from phytoplankton algae and bacteria with a small amount of terrestrial high plant. The Pr/Ph ratios of the organic matter in the Gufeng Formation is less than 0.6, and the average gamacerane index is 0.2. These suggest that the redox conditions of paleo-water column during the deposition of the Gufeng Formation were moderate stratification and anoxia. The organic-rich Gufeng Formation is probably related to this stratification and anoxia during warm climatic conditions. Associated with the suddenly cooling, enhanced upwelling increased greatly the primary productivity greatly and yielded more large-scale organic matter accumulation.

Key words: Gufeng Formation, TOC, biomarker, organic accumulation, Chaohu area