Advance on exploration-development and geological research of coalbed methane in the Junggar Basin

The Junggar Basin is extremely rich in coalbed methane (CBM) resources. Nearly 30 CBM exploration projects have commenced, and more than 270 CBM wells have been drilled. Some pilot test areas for the CBM development in Baiyang river, Sigong river and the eastern Urumqi river have been built, and large-scale CBM development and utilization is being promoted. In the Junggar Basin, many blocks show the momentum of high production in a single CBM well. For multiple, thick and high dip angle coal seams, the technologies including drilling along coal seam, reservoir transformation, and optimized drainage have been continuously promoted and applied, which leads to the development of CBM industry in Xinjiang. This paper systematically summarizes the special geological conditions of CBM accumulation in the Junggar Basin, scientifically analyzes the current status and trends of CBM exploration and development in the early stage, and comprehensively presents the geological problems faced by CBM exploration and development, in order to provide a reference for the efficient development of low- and medium-rank CBM in Xinjiang and even in China. The structural uplift, hydraulic dissipation, and the deep extension of the CBM aeolian oxidation zone in the basin margin are geological factors that restrict the distribution of CBM resources and the potential of CBM exploration and development, and cannot be ignored. The deep coal measure gas resources in the basin center, including CBM, have huge resource potential. It is imperative to carry out comprehensive geology research on coal measure gas and expand the exploration and development field of coal measure gas. The CBM in many blocks at the basin margin is rich in CO2 and presents an abnormal phenomenon that the CO2 concentration increases with the increase of coal seam depth. Taking the coupling effect of hydrodynamic field and hydrochemical field as the main guide, and the difference of gas composition and occurrence as clues, the source, mixing degree and accumulation contribution of multi-source and multi-stage gas have been identified, and the abnormal CO2 enrichment has been revealed. Furthermore, the multi-source, multi-stage and multi-factor coupling accumulation mechanism of CBM under the leading action of fluid field is being explored. The thickness of the two sets of coal measure strata in the Xishanyao Formation and the Badaowan Formation in the basin is huge. Under the control of the coal accumulation environment, it has different spatial distributions between the unified, independent and mixed multilayer overlapping gas-bearing systems. Based on the coupling of sedimentation, structure and hydrology, the basic accumulation model of CBM is summarized. However, many issues need to be studied in depth, including the effective determination of fluid pressure system, reasonable layer selection and combined development, advantageous geological configuration of CBM accumulation and its regional, local and stratigraphic change rules. Most of the dip angles of coal reservoirs at the basin margin exceed 35°, and the steeply inclined coal reservoirs change rapidly with depth in a small space. The fluid movement mode, stress state in the reservoir, etc. affect the pressure drop propagation and CBM production process. As a result, the changes in reservoir mechanical properties and physical property, transformations in fluid phase state and saturation relationship, gas/water output mechanism and law during the CBM development process need to be further investigated. © 2021, Editorial Office of Journal of China Coal Society. All right reserved.