Sequence stratigraphic characteristics and shale gas exploration prospect of Middle Permian Qixia Formation in the Xuancheng-Guangde Basin

Based on the method of sequence stratigraphy, the stratigraphic sequence of the Middle Permian Qixia Formation in the Xuancheng-Guangde Basin was analyzed. Combined with the organic geochemical characteristics of the organic-rich shale, the shale gas exploration prospect was evaluated. The results show that the Qixia Formation in the Xuancheng-Guangde Basin can be divided into three third-order sequences, which represent three relatively large cycles of sea-level rise and fall. Each sequence can be subdivided into the transgressive systems tract, the condensed section, and the highstand systems tract. The organic-rich shale mainly occurred in the condensed section and transitional sections between the condensed section and transgressive systems tract or highstand systems tract. The thickness of organic-rich shales in Qixia Formation varies from 8 to 35 m. The TOC value ranges from 0.77% to 9.49%, and the maturity of organic matter is about 1.428%. Brittle minerals (quartz, feldspar) of shales are relatively enriched, showing good brittleness and compressibility. Accordingly, shale gas accumulation conditions of Qixia Formation are good, especially for the top limestone section of Qixia Formation, which has great resource potential.