Sedimentary controls of shale heterogeneity: A case study of Wufeng-Longmaxi shales in the south Sichuan Basin, China

The shales show obvious heterogeneity during the Late Ordovician -Early Silurian in South Sichuan Basin, China. To elucidate the sedimentary control factors of the heterogeneity of the Wufeng-Longmaxi Formation shales and developmental model, we conducted petrological observations, X-ray diffraction analysis, and geochemical studies of the two wells. The findings indicate 10 graptolite biozones are identified in both two studied wells. Notably, the thickness, lithofacies, mineral composition, and total organic carbon content varied noticeably throughout these marine shales within each graptolite biozone. The WF1-LM1 shales mainly accumulated under anoxic conditions that lacked terrigenous input, which was beneficial for the preservation of organic matter. After the LM1 interval, the input of terrigenous material increased, the diluting effect on organic matter was increasingly enhanced. Significant dilution of organic matter occurred after the LM4 interval, when the apparent sedimentation rate increased to over 15.0 m/Ma (49.21ft/Ma). Additionally, the early -late stage of the WufengLongmaxi Formation has different provenance. Within the WF1-LM1 interval, the terrigenous material was composed primarily of carbonate minerals. Above the LM1 interval, the contents of clay, felsic, and silicate clasts increased. In more anoxic conditions, the limited supply of detrital iron from the source region may have led to more acidic pore water conditions during early diagenesis, which restricted early carbonate cementation and resulted in a more clay -rich lithofacies.