Pore Peak Types and Controlling Factors of Micropores in Different Shale Reservoirs

Abstract

The micropores (<2 nm) developed in shale reservoirs play a crucial role in the adsorption and storage of shale gas. In this study, we investigated the organic geochemistry, mineral constituents, and micropore structure characteristics of transitional shale reservoirs in the Yangquan mining area of the Qinshui Basin, China, using total organic carbon (TOC), quantitative X-ray diffraction (XRD), and low-temperature CO2 adsorption. We analyzed the genesis and dominant factors of micropore structure characteristics in marine, terrestrial, and transitional shales. The results showed: (1) In transitional shale, the pore-size distribution (PSD) exhibited a bimodal distribution, and peaks 2 and 3 in transitional shale were mainly organic pores; the peak value of PSD is affected by sedimentary environment, the micropore volume of shale in tidal-flat facies was greater than that of shale in lagoon facies. (2) Regarding the genesis of the micropores, peak 1 was likely a false peak. The value of peak 1 in marine shale was positively correlated with the clay and Illite–smectite mixed layer (I/S), but the interlayer of Illite–smectite mixed layer was approximately 1 nm, suggesting that peak 1 was not a clay mineral pore. In terrestrial shale, the value of peak 1 was positively correlated with TOC, and we observed peak 1 in some terrestrial shale (CL-3, S-1, S-2, and S-3), suggesting that peak 1 was not an organic pore. (3) Maturity and quartz have different effects on clay minerals in different shales, and we found that maturity (R o = 1.4–2.0%) was conducive to developing clay minerals in terrestrial shale, whereas maturity (R o = 0.52–3.66%) was not conducive to developing clay minerals in marine shale; quartz was not conducive to developing clay minerals in marine and transitional shale, but it promoted the development of clay minerals in terrestrial shale.