四川盆地东缘—鄂西地区二叠系吴家坪组富有机质页岩孔隙特征及其主控因素OA

The second member of the Permian Wujiaping Formation(Wu 2 Member)in the eastern margin of the Sichuan Basin and western Hubei area represents a critical replacement horizon for marine shale gas exploration in South China.However,the low exploration degree and insufficient basic geological research have resulted in a poor understanding of pore development characteristics and their controlling factors,which restricts the sweet spot selection and efficient exploration of shale gas.Focusing on the Wu 2 Member shale in the study area,this paper systematically analyzes the pore characteristics of different shale lithofacies and their controlling factors by means of core observation,thin-section analysis,argon ion polishing-field emission scanning electron microscopy,and low-temperature gas adsorption experiments.In addition,a pore evolution model is established.The following results are obtained.First,the Wu 2 Member shale in the study area was deposited in a slope-deepwater shelf environment.It is characterized by a mineral assemblage of"high silica,moderate carbonate and low clay",high content of brittle minerals,and high organic matter abundance,dominated by Type Ⅱ2 kerogen.Second,there are mainly four types of lithofacies identified in the Wu 2 Member shale,including carbon-rich siliceous shale,carbon-rich mixed shale,high-carbon mixed shale,and medium-to low-carbon calcareous shale.Third,the reservoir space in the Wu 2 Member shale is mainly composed of secondary organic pores,with a small number of intergranular pores,dissolution pores and microfractures.The pore size is dominated by micropores and mesopores.These shale lithofacies exhibit significant heterogeneity,with carbon-rich siliceous shale being the best in pore structure and reservoir performance,while medium-to low-carbon calcareous shale being the worst in reservoir performance.Fourth,the pore development in the Wu 2 Member shale is synergistically controlled by organic matter type and abundance,mineral composition,and diagenesis.High organic matter abundance and brittle mineral support are crucial for the formation of high-quality reservoirs.Mechanical compaction,cementation,hydrocarbon generation from organic matter pyrolysis,acidic fluid dissolution,and clay mineral transformation jointly control the preservation and modification degree of reservoir pores.In conclusion,this study clarifies the development mechanisms of Wu 2 Member high-quality shale reservoirs,and can provide a theoretical basis for shale gas evaluation and sweet spot prediction in the study area.