Sedimentary microfacies characteristics and organic matter enrichment pattern of the 1st member of the Middle Permian Maokou Formation, southwestern Sichuan Basin

This study aims to investigate the sedimentary microfacies and organic matter enrichment pattern of the 1st member of the Middle Permian Maokou Formation(hereafter referred to as the Mao 1 Member) in the Sichuan Basin. Based on petrological and sedimentary microfacies analyses, we summarize eight sedimentary microfacies(MF1-MF8) in the Mao 1 Member on the Shawan outcrop in Leshan, southwestern Sichuan Basin, encompassing bioclastic calcareous mudstone (MF1), bioclastic marl(MF2), bioclastic argillaceous limestone(MF3), marly limestone(MF4), bioclastic wackestone (MF5), calcareous algal wackestone(MF6), bioclastic packstone(MF7), and calcareous algal packstone(MF8). The sedimentary paleoenvironment and geochemical analyses result in the following conclusions. (1) In the early to middle stages of the Mao 1 Member's deposition, MF2 was predominantly formed, followed by MF7 and MF8. This period was characterized by a transgressive and anoxic environment overall, marked by the presence of upwelling ocean currents and high primary productivity. The middle to late stages of the Mao 1 Member's formation primarily witnessed the occurrence of MF6, MF7, and MF8. This period featured relative regression, with slightly increased oxygen content, weakened upwelling ocean currents, and decreased primary productivity. In the final stage of its deposition, the Mao 1 Member was dominated by MF2, with slightly increased oxygen content and enhanced primary productivity. (2) The organic matter enrichment in the member was predominantly influenced by marine primary productivity and anoxic environment, while under the indirect influence of upwelling ocean currents. Specifically, a setting of higher primary productivity and lower oxygen content is conducive to the enrichment and preservation of organic matter. Furthermore, upwelling ocean currents facilitate the transport of nutrients from deeper parts, thus enhancing the marine primary productivity.(3) MF2 and MF7,characterized by stable regional distributions and depositional environments with higher primary productivity and lower oxygen content, represent favorable sedimentary microfacies for organic matter enrichment in the Mao 1 Member. © 2024 Editorial Department of Oil and Gas Geology. All rights reserved.