Research Progress of Microscopic Pore–Throat Classification and Grading Evaluation of Shale Reservoirs: A Minireview

The grading evaluation of shales helps in understanding theshale reservoir formation and sweet-spot screening of physical properties.Reservoir formation of shale is closely related to the size, distribution, andconnectivity of shale pore-throats. Therefore, characterization andclassification of microscopic pore-throats are the basis for shale reservoirgrading. However, previous microscopic pore-throat classification schemesfor materials science are unsuitable for shales with strong heterogeneity. Inrecent years, the author's team has proposed a new classification method formicroscopic pore-throats of shale reservoirs based on the comprehensivecharacterization of numerous shales with varied maturities in different basins.Results show that the microscopic pore-throats for all shales can be dividedinto micropores, small pores, mesopores, and macropores. The component,size, and distribution of microscopic pore-throats in shales in different basinsdiffer due to the differences in mineral composition and diagenetic evolutionof shales caused by varying depositional environments and burial histories.Correspondingly, the demarcation points of different types of pore-throatsare different. After clustering statistical analysis of different types ofmicroscopic pore-throats, shales can be divided into grade I, II, III, and IV reservoirs, regarded as good, medium, poor, andnonshale reservoirs, respectively. Grading evaluation criteria for shale reservoirs were then established. Further, a new technique forevaluating the reservoirflow zone index (FZI) and dividingflow units based on logging data was constructed using the relationshipof permeability with porosity in the same hydraulicflow unit. The grading evaluation criteria can be applied to single wells,multiwells, and planes using logging data. The application in numerous shale oil and gas exploration areas in China shows that theestablished classification scheme can be applied to the grading evaluation and sweet-spot screening of shale reservoirs