Influence of rock properties and prediction on the methane storage capacity in marine-continental transitional shale and coal from northern China

Abstract

Systematic understanding of geological controls on methane storage capacity in marine-continental transitional shale can help to predict the potential gas resources in such shale reservoirs. In the present study, seven transitional shale samples were first collected from three different basins (Qinshui Basin, Ningwu Basin, and eastern Ordos Basin) in Northern China and characterized for their composition, thermal maturity, pore structure and CH4 adsorption capacity. Their total CH4 storage capacities (including adsorbed and free gas) at pressures up to 10 MPa were determined using an ingenious method from previous studies. Based on this, we investigated the properties controlling gas adsorption and total gas storage in these shales. Meanwhile, several coal samples from the same strata unit were analyzed through analogous processes and treated as the naturally occurring extreme total organic carbon (TOC) cases of transitional shale during discussion. Our findings indicate that the transitional shale’s composition (typically represented by the TOC and total clay content) directly influences the pore development and then controls the CH4 adsorption, which is consistent with the previous findings for marine shale. Though the positive correlations between CH4 adsorption capacity and both TOC and total clay content were observed for the studied shale samples, the observations demonstrate the dominance of organic components in rock for gas adsorption when the transitional shale and coal samples are considered together. In addition, the positive correlation between clay minerals and CH4 adsorption may be overshadowed, even reversed. Based on the method used, the total gas-in-place (GIP) storage in these transitional shales shows a stronger positive correlation with total clay content than TOC content. This is mainly attributed to the meso- and macroporosity of clay minerals. For the studied shale samples, the relationship between thermal maturity and CH4 storage capacity can be represented by a quadratic polynomial function, which is likely due to considerable variation in rock composition. Furthermore, it is emphasized that predicting the total GIP in shale and coal reservoirs using these correlations should be approached with caution, regardless of the method (direct/indirect) used to determine the GIP.