四川盆地下寒武统筇竹寺组不同岩相页岩孔隙结构差异及其主控因素OA

The Lower Cambrian Qiongzhusi Formation in the Sichuan Basin is a critical target for marine shale gas exploration in southern China due to its significant potential in shale gas resources.In this formation,however,lithofacies compositions are complex,reservoir heterogeneity is extremely strong,and the pore structures of different lithofacies and their primary controlling factors are not understood,which restrict the sweet spot selection and exploration and development arrangement of shale gas in this region.By integrating core observation,thin section identification and geochemical analysis,this paper presents a"lithology-TOC-sedimentary structure-mineral"four-element lithofacies classification scheme.Then,the micro-,meso-,and macro-pore systems of various lithofacies are finely characterized by means of CO2 and N2 adsorption,and high-pressure mercury intrusion(HPMI)experiments.Finally,the controlling roles of organic matter abundance,mineral composition and other factors on pore structure are analyzed.The following results are obtained.First,there are mainly six types of lithofacies in the Qiongzhusi Formation,including medium-high carbon massive and banded siliceous shale,low-carbon massive and laminated siliceous shale,medium-high carbon massive mixed shale,and low-carbon laminated siltstone.Second,the lithofacies are significantly different in pore structures.Matrix pore system covers three basic types,namely intergranular pore,intragranular pore and organic pore,and three types of typical microfractures are identified,including mineral particle marginal fracture,inter-clay mineral layer fracture,and mineral contraction fracture.Third,pore heterogeneity is synergistically controlled by organic matter,clay minerals and pyrite.In the high-maturity stage,formation compaction promotes the transformation of mesopores into micropores.In conclusion,the proposed four-element lithofacies classification scheme and the insights into pore variability clarify the differential characteristics of the Qiongzhusi shale gas reservoirs,providing a critical theoretical support for the selection and evaluation of Cambrian shale gas exploration areas in the Sichuan Basin.