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.