Quantitative Evaluation Of Factors Influencing Continental Shale Reservoir Capacity Of Xujiahe Formation In West Sichuan Depression

Whether shale gas can be exploited successfully depends on the reservoir capacity and seepage capacity of shale itself.In addition,the occurrence phase of shale gas has a great influence,and the important factors determining these two aspects are the size and shape of shale pores.The pores structure of shale is an important index for evaluating high quality shale gas reservoirs and selecting favorable areas.In this paper,a series of experiments have been carried out on Xu-5 shale of Xujiahe Formation in West Sichuan Depression,including organic geochemistry experiment,low temperature gas adsorption experiment,XRD analysis experiment and scanning electron microscope observation experiment.The results show that the average total organic carbon content?TOC?of the shale in the study area is 1.38%,and the organic matter type is mainly type III.The average maturity of organic matter is 1.31%.The main mineral components of shale in the study area are quartz and clay minerals,among which illite is the most important component,up to 39.31%.Because of the existence of micropores in shale poress,field emission scanning electron microscopy?FESEM?was used to observe shale samples.It was found that the best poress types of shale in the study area were intergranular poress and intragranular pores,while the poorly developed ones were organic pores.Subsequently,the whole pores size of the continental shale samples in the study area was characterized by low temperature gas adsorption?N₂ and CO₂?and high pressure mercury.The following conclusions were drawn:mesoporous is the main contributor of pores volume,accounting for about 60%.Through investigation and research,the following five major controlling factors of reservoir capacity are selected:TOC,R_o,siliceous mineral content,clay mineral content and carbonate mineral content.Because the main factors affecting shale pores reservoir capacity are shale pores volume and specific surface area,the evaluation parameters R_V?pores volume parameter?and RS?specific surface area parameter?are set up,and the evaluation model of continental shale reservoir capacity in the study area is established to evaluate the shale of different lithofacies in the study area.The conclusions are as follows:the better reservoir capacity lithofacies of Xu5 member are clay shale facies and siliceous shale facies,while the mixed shale facies and calcareous shale facies are poor.Finally,according to the lithofacies of the section in the study area,favorable reservoirs are selected,mainly distributed in the middle of the Xu 5 member and the middle of the lower segment.