Occurrence of fluids in high dip angled coal measures: Geological and geochemical assessments for southern Junggar Basin, China

Abstract

The Junggar Basin, Xinjiang has abundant coalbed methane (CBM) resources, and most of the CBM is found in the coal reservoirs with high dip angle (>50°), where the generation, accumulation and extraction of CBM significantly differs with flat coal reservoirs. This study clarifies the effect of high dip angle on the fluid pressure field, in situ stress field, permeability and gas bearing capacity of CBM reservoir in the southern Junggar Basin. The studies show that the high dip angle results in the fluid pressure differentiation of CBM reservoirs at 800–1000 m. The shallow opened fluid pressure system is in an ultralow-normal pressure state, while the deep closed system is characterized by overpressure. The self-sealing property of high dip CBM reservoirs can prevent the gas escaping upward along the inclined bedding. The larger the dip angle, the higher the gas column sealed by the self-physical property. The overburden compaction is reduced for dip formation, which causes the in situ stress of inclined coal seam to decrease but the anisotropy to increase. Furthermore, the high dip angle decreases the gas content per meter of coal seam, and diversifies the gas genesis, which causes the secondary biogenic gas to generate in the shallow seam under the effect of active hydrodynamics, and the thermogenic gas to be better accumulated in the deep coal seam. Due to the desorption hysteresis of the coal seam in downdip direction, the gas production curve of high dip coal seam has obvious bimodal characteristics, and the second peak is significantly higher than the first.