Geological characteristics and gas accumulation condition of deep coal-rock gas in Carboniferous Benxi Formation,Western Ordos Basin,China

Abstract

Previous studies on the coal-rock gas accumulation has largely emphasised geological characterisation and controlling condition of coal-bed buried below 2000 m, leaving a gap in understanding how gas accumulate in deep coal-rock (>2000 m). The enrichment of free gas in deep coal-rock reservoirs is different from the enrichment of adsorbed gas in coal-bed buried below 2000 m, and has relatively unique geological condition and distinct gas accumulation models. To fill this gap, low-temperature carbon dioxide adsorption, low-temperature nitrogen adsorption, mercury intrusion porosimetry, scanning electron microscopy, and 3D-CT-scan were carried out to characterise the pore structure and cleat of deep coal-rock reservoirs. The organic geochemical indicators, storage capacity and gas-bearing characteristics of deep coal-rock reservoirs have also been revealed. The deep coal-rock display a lustrous semi-dark to bright sheen, with both face and end cleats. The microscopic composition is dominated by vitrinite, with a variety of minerals interspersed with the organic components. The reflectance of the vitrinite is relatively high, reaching up to 1.65, indicating high maturity, typical of coking coal and lean coal. The deep coal-rocks exhibit a dual-porosity system, consisting of matrix pores and cleats. Micro-pores make up 84.8% of the deep coal-rock and serve as the primary reservoir space for adsorbed gas. Meso-pores make up the smallest proportion, at only 6.9%, while macro-pores average 8.3%. Natural fractures are rare in the deep coal-rock. In the matrix of the deep coal-rocks, open gas pores, pores from residual plant tissues, and inorganic mineral-related pores such as intercrystalline, intergranular, and intragranular pores can all be observed. The average conventional porosity is 5.35%, with relatively large variations in permeability. The gas content of the deep coal-rock ranges from 3.27 to 19.18 m³/t, with an average of 9.33 m³/t. The proportion of free gas is 18.1%, while adsorbed gas can account for up to 81.9%. The accumulation of deep coal-rock gas is controlled by three key factors: hydrocarbon generation conditions, reservoir conditions, and preservation conditions. The coal quality in Western Ordos Basin is slightly lower than that of the Eastern Ordos Basin, but the stability in the distribution of thick coal-rocks (average 8.56 m) provides a strong foundation for the accumulation of deep coal-rock gas. The reservoir and preservation conditions of deep coal-rock in the study area are comparable to those in the eastern basin, demonstrating significant exploration potential. The findings of this study are valuable for future exploration of coal-rock gas, as these findings will enhance the understanding of the petroleum systems in deep coal-rock.