Quantitative Study On Pore Structure Of Sinian-Cambrian Shale In Yichang Area,Western Hubei

The Sinian-Cambrian shale in western Hubei Yichang area are the research object in this study.On the basis of systematic sample collection,through XRD,TOC,rock pyrolysis,argon ion polishing with high resolution scanning electron microscopy and image analysis,full pore size characterization technique such as gas adsorption and mercury injection,fine reservoir quantitative characterization technique such as FIB-SEM nanoscale three-dimensional modeling,the pore structure characteristics and its control factors of the shale were studied,the main conclusions are as follows:(1)The rock mineral characteristics and organic geochemical characteristics of the shale are clarified.The Sinian and Cambrian shale in the study area is marine over-mature organic-rich shale,and the shale gas resources have great potential.According to the characteristics of core,organic carbon content and mineral characteristics,the Doushantuo shale can be divided into two types of reservoirs: high TOC dolomitic shale and low TOC argillaceous dolomite.The Niutitang shale can be divided into two types of reservoirs: high TOC siliceous shale and low TOC calcareous shale.(2)Four kinds of shale reservoirs were quantitatively characterized,and four kinds of pore structure models were summarized.The results of full pore size characterization show that the shale pores are mainly mesopores,followed by micropores,with a small amount of macropores.The contribution of pore volume below 100 nm is 82-98%,and the contribution of surface area of micropores is 80-91%.Based on high resolution scanning electron microscopy and full pore size characterization,the pore development models of the four reservoirs are summarized.Many pore types can be found,the inorganic pores are mainly mesopores and macropores,while the organic pores are mainly small pores and micropores.The pore development degree of the high TOC shale reservoir is better than that of the low TOC shale reservoir in both formations.(3)The three-dimensional pore network model(PNM)reconstruction and quantitative characterization of inorganic and organic pores were carried out in the low TOC argillaceous dolomite of Doushantuo formation and the high TOC siliceous shale in the Niutitang formation.Results show that the unique platelike interparticle and intercrystalline pores developed in Doushantuo low TOC argillaceous dolomite.From the PNM,the porosity of platelike pores is1.46%,and the pore size is mainly 150-300 nm.The formation time of pores is earlier than that of shale gas,so,this kind of pores provide storage space and potential seepage channels for shale gas.The organic pores in the high TOC shale of Niutitang formation are mainly micropores and mesopores,which are distributed in isolation.The porosity of the organic pores calculated from the PNM is only 0.08%.(4)The controlling factors of the pore development characteristics of two sets of shale have been identified.It is confirmed that the micropores in study area are mainly organic pores,while the inorganic pores contribute most of mesopores and macropores space,and the interparticle pores are the most important inorganic pores.In Doushantuo formation,the development degree of inorganic pores is controlled by the rock types,and the pores are mainly interparticle pores.In Niutitang formation,the development degree of inorganic pores is controlled by the content of clay minerals and calcite,and the pores are mainly interparticle pore between clay minerals and calcite dissolution pores.