Organic matter-hosted pore system in the Wufeng-Longmaxi (O3w-S11) shale, Jiaoshiba area, Eastern Sichuan Basin, China

Abstract

The first commercial shale gas with the capability to produce 203,000cubicmetersperday were discovered in the Wufeng-Longmaxi (O3w-S1l) shale (namely, the upper Ordovician Wufeng shale (O3w) and the lower Silurian Longmaxi shale (S1l)) of well JY1, Jiaoshiba area, Eastern Sichuan Basin, China. In this study FE-SEM, CO2 and N2 sorption and MICP were used to characterize the pore morphology, type, size and specific surface area of the Wufeng-Longmaxi shale. We focused our research on well JY1 to establish a standard well cross-section to allow comparison with well studied gas shales such as Barnett, Marcellus, Kimmeridge Clay and others.FE-SEM visible mineral pore sizes range from 4nm to 3416nm with an average of 118nm within the Wufeng-Longmaxi shale. OM-hosted pore sizes vary from 5nm to 1021nm with an average of 68nm. Within the Wufeng-Longmaxi shale, micropore volumes vary from 0.0048cm3/g to 0.0078cm3/g with an average of 0.0059cm3/g; mesopore volumes range from 0.0108cm3/g to 0.0166cm3/g with an average of 0.0139cm3/g; macropore volumes range from 0.00005cm3/g to 0.00274cm3/g with an average of 0.00108cm3/g and surface areas range from 22.05m2/g to 36.37m2/g with an average of 27.68m2/g. The average pore diameters of Wufeng-Longmaxi shales range from 5.31nm to 7.40nm.TOC has a significant effect on bulk shale porosity; porosity measured by FE-SEM ranges from 0.46% to 1.54%, but covers only a small fraction of the shale total porosity as the FE-SEM detection limit is in a μm range. The majority of shale pore space (pores<10nm in diameter) below the FE-SEM detection are performed by CO2 and N2 adsorption, and have an obvious relationship with the TOC. Therefore, the Wufeng-Longmaxi shale has a predominantly organic matter pore system.