Influence of coal rock on tight sandstone reservoirs in coal seam roofs: A case study of the lower jurassic in the Taibei sag, Turpan-Hami Basin, China

Abstract

After shale gas, coalbed methane and gas hydrate, coal measure gas has become an important new unconventional resource, for which exploration to depths of >3000 m are taking place in China. However, owing to unusual geological conditions and as yet inadequate exploration technology the current level of exploration is low. The acidic water medium in coal-bearing strata affects the diagenesis of adjacent tight sandstone reservoirs, thereby influencing reservoir quality. However, the current understanding of the impact mechanism of coal seams on neighboring tight reservoirs is insufficient, constraining the further exploration of coal measure gas. In order to reveal the influence of coal rock on a tight sandstone reservoir in its roof, we study the coal-bearing strata of the Xishanyao Formation in the Taibei Sag of the Turpan-Hami Basin using cast thin sections, physical property tests, XRD analysis, carbon and oxygen isotopes, high-pressure mercury intrusion and low-temperature N2 adsorption. The carbonate cement in the sandstone reservoir of the coal seam roof was formed by the decarboxylation of organic acid. The organic acid, containing CO2 generated by coal rock hydrocarbon generation, has modified the tight sandstone reservoir of the coal seam roof via dissolution. This has improved the physical properties and pore throat structure of the tight sandstone reservoir close to the coal-sandstone interface. The amount of improvement extends to about 2 m into the sandstone. During the upward transport of fluid, the pre-existing carbonate cement dissolves and then precipitates, which causes the CO2 generated by the organic acid decarboxylation to exchange carbon with the carbon isotopes in the pre-existing carbonate cement. Due to the slow upward diffusion rate and limited diffusion distance of the CO2-bearing fluids, the carbonate cement exhibits gradually heavier δ13CPDB values with increasing distance from the coal-sandstone interface.