EFFECT OF PORE STRUCTURE AND COMPOSITION ON GAS SORPTION CAPACITY OF TRANSITIONAL SHALES, NW CHINA

Pore structure and composition directly determine sorption capacity of transitional gas shale (deposited in environments such as lagoons, deltas, beaches and tidal flats), as well as environmental conditions, pressure, temperature and moisture. In this work, organic geochemistry (analysis of enrichment, type and maturity of organic matter (OM)), X-ray diffraction (XRD), N₂ and CO₂ isotherms, and high pressure CH₄ sorption isotherm (up to 18 MPa) were carried out on seven deep Shanxi shale core samples (>3000 m) under dry conditions to investigate the effect of pore structure and composition on gas sorption capacity of transitional shale.

The results show that total organic carbon (TOC) content ranges 0.781-3.82 wt%. Shanxi shale comprises of type III OM and is at over-mature stage. The dominant composition is Clay minerals, averaging 54.6%, and the second is quartz with an average of 39.7%. Feldspar, plagioclase, siderite and pyrite can be observed. The prevailing pore type is silt-shaped, with a smaller amount of ink-bottle-shaped pores, as observed by N₂ adsorption-desorption hysteresis loops. TOC is not the only controlling factor, and clay minerals also contributed to the specific surface area for gas sorption. The relationship between quartz and soft composite (clays and OM) is also observed to impact the gas sorption capacity. In addition, the Langmuir, supercritical Dubinin-Radushkevich (SDR) and Ono-Kondo (OK) model were employed and compared to derive Langmuir volume. The fitting performance demonstrates that the Langmuir model is applicable for shales with measured maxima of excess sorption capacity (MESC) less than 2.36 m³/t, while the SDR model better fits the data for shales with MESC larger than 2.89 m³/t.