Rapid transgressive shale characteristics and organic matter distribution of the Upper Ordovician-Lower Silurian Wufeng-Longmaxi Formations in southern Sichuan Basin,China
SHI Zhensheng1,2, WANG Hongyan1,2, ZHAO Shengxian3, ZHOU Tianqi1,2, ZHAO Qun1,2, QI Ling1,2
1 PetroChina Research Institute of Petroleum Exploration and Development,Beijing 100083,China; 2 National Energy Shale Gas R & D(Experiment)Center,Hebei Langfang 065007,China; 3 PetroChina Southwest Oil & Gas Field Company,Shale Gas Research Institute,Chengdu 610051,China
Abstract：The characteristics and formation of the MF(maximum flooding)black shale are one of the focuses of fine-grained reservoir geology research. Seismic interpretation,well correlation,X-ray diffraction whole rock analysis,total organic carbon(TOC)test and field emission scanning electron microscope analysis show that the MF black shale of the Longmaxi Formation in the southern Sichuan Basin is in the basal part of the Longmaxi Formation,corresponding to the graptolite belt LM1. The shale has an average content of 49.3% quartz(85% clay-sized),10.5% calcite,8.4% dolomite and 23.4% clay minerals. The quartz content increases while the clay mineral content decreases basinward. The MF black shale formed during the stage of rapid relative sea level rise,with a thickness of 0.5-2.8 m and gradually thickening basinward. The average TOC content is 5.4%,which exhibits a gradual decrease towards the basin and forms four distinct stacking patterns in the vertical TOC content profile. The mineral composition and thickness distribution of the shale are closely related to the rapid transgression,biology and volcanism during the sedimentary period. The rapid transgression has led to a decrease in terrestrial sediment input,resulting in a reduction in shale thickness. Additionally,biological activity and volcanic influences have resulted in a prevalence of microcrystalline quartz and an increase in shale thickness towards the basin. The high TOC content of this shale is related to anoxic water,low sedimentation rate,and high primary productivity. Anoxic water body enhances preservation of organic matter. Low sedimentation rates can weaken the dilution of organic matter,while high primary productivity can increase the supply of organic matter. The planar variation and stacking style of TOC content in this set of shale are related to water depth. With increasing water depth,there is an amplified degradation and recycling of organic matter during sedimentation,leading to a decline in TOC content. Simultaneously,as the water depth rises,the sediment accommodation space also increases,resulting in distinct stacking patterns of TOC content.
SHI Zhensheng,WANG Hongyan,ZHAO Shengxian et al. Rapid transgressive shale characteristics and organic matter distribution of the Upper Ordovician-Lower Silurian Wufeng-Longmaxi Formations in southern Sichuan Basin,China[J]. JOPC, 2023, 25(4): 788-805.
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