High-resolution sequence stratigraphic characteristics and organic matter enrichment mechanism of fine-grained sedimentary rocks in the Tiesiao-Datangpo Formations in northeastern Guizhou

The Datangpo Formation of the Nanhua System is a set of high-quality source rocks and potential unconventional oil and gas reservoirs, which has important geological historical research and oil and gas exploration significance. However, research on its sedimentary process and high-precision sequence stratigraphy remains relatively limited. The current study, using the core description and thin section observation from Well ZK513 in northeast Guizhou as the primary research methods, employed updated classification criteria for fine-grained sediments to meticulously classify and identify 14 distinct lithofacies, including quartz sandstone, pebbly argillaceous sandstone, grey mudstone, grey pebbly sandy mudstone, grey argillaceous sandstone, black carbonaceous-siliceous sandy mudstone, black pebbly carbonaceous-siliceous argillaceous sandstone, grey quartz sandstone, black manganese mudstone, black argillaceous siltstone, black grey argillaceous siltstone, grey-black argillaceous siltstone, grey silty mudstone and grey-white mudstone. Based on the characteristics of lithofacies combination, three sedimentary facies—intra-shelf basin, deep-water shelf and shallow-water shelf—were identified. These three facies were subsequently subdivided into seven sedimentary subfacies: foreshore, ice sea shallow continental shelf, manganese-rich basin in continental shelf, deep-water basin in continental shelf, margin of basin in continental shelf, deep-water continental shelf and shallow-water continental shelf. Based on the analysis of lithofacies and sedimentary facies, it is considered that LF9 black manganese mudstone and LF10 black argillaceous siltstone in the continental intra-shelf basin facies are favorable facies and intervals for hydrocarbon source rocks. Through the study of lithofacies and sedimentary facies, five third-order sequences SQ1-SQ5 and four third-order sequence boundaries SB1-SB4 were identified. According to the lithofacies superposition pattern and quantitative statistics of sedimentary structure, four typical parasequence types were summarized in the third-order sequence. Based on the superposition form of parasequence, several parasequence sets in the third-order sequence were identified, and then five transgressive (TST)-highstand system tract (HST) cycles and five internal maximum flooding surfaces MFS1-MFS5 were identified, and then the single well sequence stratigraphic framework was established. Based on the data of lithofacies, sedimentary facies and sequence of Well ZK513 and other wells in the study area, the filling evolution model of the sedimentary period of Datangpo 1 member in northeast Guizhou was established by well comparison. The distribution of the basin in the continental shelf of the favorable facies area in the study area and the distribution pattern of the organic-rich interval LF9-LF10 in the facies area were predicted.