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The southern Junggar Basin, Xinjiang, has abundant coalbed methane (CBM) resources. Currently, the Baiyang River development pilot test area (BYR block for short) in the Fukang east block has achieved large-scale CBM development, but the productivity characteristics and its controlling factor are still unclear. Based on the field production data of the BYR block and experimental tests, this paper summarizes the gas and water production characteristics and presents the analysis results of the geological and geochemical responses to the productivity of CBM wells. The productivity of CBM wells in the BYR block was generally characterized as medium-to-low yield. The productivity was jointly controlled by the burial depth, structure condition, thickness and number of co-production coal seams, and hydrogeological conditions. The gas production first increased and then decreased with the increase in the burial depth of the coal seam, and a burial depth between 750 and 1000 m was the most beneficial to increasing the gas production due to the good gas preservation conditions and suitable permeability and stress conditions. The total thickness of the co-production coal seams had a positive effect on the productivity of gas wells, but the productivity was also affected by the number of co-production coal seams and interlayer interference. In the BYR block, the co-production of the nos. 41 and 42 coal seams was the most favorable combination form for CBM drainage. The productivity of CBM wells had a good response to the Na+, K+ and HCO3− concentrations but a poor response to δD-H2O and δ18O-H2O. Based on the concentrations of the main ions and TDSs of the coal seam water, a productivity response index δ* was established, and there was a good positive correlation between the productivity and δ*. [ABSTRACT FROM AUTHOR]

The reservoir physical properties, pore types, diagenetic characteristics and reservoir quality controlling effect of the Xishanyao formation coal measure sandstone in the southern margin of the Junggar basin were discussed in this study based on thin section observation, high pressure mercury injection, low-temperature nitrogen adsorption and scanning electron microscope observation. The result shows that the porosity and permeability of the sandstone are generally low with a medium-high texture maturity and low compositional maturity. The sandstone storage space is mainly composed of residual intergranular pores, secondary dissolution pores, inter-crystalline pores and micro-fractures. The diagenetic stage of coal measure sandstone is in the mesodiagenesis A1-A2 stage, and their diagenetic interaction types mainly include compaction, cementation and dissolution. The reservoir quality of the coal measure sandstone deteriorates by compaction due to high matrix content and plastic debris content. Because of the large amounts of organic acids generated during the thermal evolution of the coal measure source rock, the coal measure sandstone suffers from strong dissolution. The secondary dissolution pores formed by the massive dissolution of feldspar, lithic fragments and early carbonate cementation in the sandstone significantly improved the reservoir quality. In the coal measure sandstone, clay mineral cementation is the most developed cementation form, followed by quartz cementation and carbonate cementation. Although kaolinite cementation and dolomite cementation can generate a small number of inter-crystalline pores, cementation deteriorates the reservoir quality. The Xishanyao formation coal measure sandstone formed in a lacustrine-delta environment, and its composition and texture make it susceptible to the influence of compaction and dissolution during diagenesis. [ABSTRACT FROM AUTHOR]

Coal seam with high dip angle has relatively high permeability anisotropy, which has great impact on the production of coalbed methane. In this study, a laboratory method was established to evaluate the influence of dip angle on coal permeability with three steps. Firstly, five groups of coal columns with different dip angles were obtained by wire cutting technology from the southern Junggar Basin. Then, all samples were tested for initial (no confining pressure) porosity and permeability. Finally, some samples with different dip angles were selected for the permeability test under confining pressure. According to the test process and results, numerical simulations were established to explain how dip angles affect permeability under the experimental conditions. In the initial permeability test, as the dip angle increases, although the porosity was almost the same, the coal samples showed a decreasing trend in permeability. This trend is caused by the anisotropy of coal samples, which can be characterized by numerical simulation. Under confining pressure, the permeability decreases exponentially with increasing confining pressure. When the inclination angle gradually increased from 0° to 90°, the rate of permeability decrease had a tendency of first decrease and then increase. The reason for this phenomenon is that the strain, including the shear strain, can be changed anisotropically by the dip angle, which is proved by good agreement between test results and simulation results. The results are helpful to analyze the law of permeability change in coal seam with high dip angle. [ABSTRACT FROM AUTHOR]

This paper mainly focused on the genesis of coalbed methane (CBM), CO 2 origin and the CBM accumulation mechanism in the southern Junggar Basin using a combination of geochemical and geological analysis methods. From this work, it is found that the degree of early coalification and the current structural and hydrogeological conditions jointly determine the regional differences in CBM genesis, accumulation and gas-bearing capacity. Microbial (biogenic) gases are widely developed in the whole study area, while their generation types and contributions vary greatly in different areas and structural belts. The CBM in the eastern Urumqi (ER) and Jimusaer (JMSE) areas is primary microbial (biogenic gas) that have been generated through CO 2 reduction pathway and mixed with a small amount of early mature thermogenic gas. The Baiyang River (BYR) area has a mixed gas source of early mature thermogenic gas, primary microbial gas and secondary microbial gas. The CBM in other areas is mixed gas of thermogenic and secondary microbial origins. Abnormally high CO 2 concentration and extremely positive δ13C-CO 2 are all related to microbial methanogenesis. The CO 2 in the ER and JMSE areas and partial CO 2 in the BYR area are produced by the microorganism-mediated degradation of sedimentary organic matter. The selective consumption by microbial reduction and differential dissolution of groundwater result in an increase in CO 2 concentration with increasing burial depth in the ER and BYR areas. The CO 2 in other areas is originally derived from the deoxidation reaction of early coalification which then undergoes secondary biotransformation. The CO 2 intervention commonly runs through the whole process of CBM accumulation in low-rank coal in the study area. Due to isotope fractionation caused by groundwater flow and differential modification of microorganisms, the vertical variations of δ13C-CH 4 and δ13C-CO 2 are positively correlated with burial depth, and the genetic types and gas-bearing characteristics of CBM have obvious vertical zoning under the condition of high-dip structures. • Multiple genetic types of gases coexist in the southern Junggar Basin. • The early coalification, structural and hydrogeological conditions control CBM accumulation. • Abnormally high CO 2 and extremely positive δ13C-CO 2 are related to microbial methanogenesis. • The CO 2 in SGR area was originally derived from the deoxidation reaction of early coalification. [ABSTRACT FROM AUTHOR]

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. • The effect of high dip angle on pressure, in situ stress, permeability and gas bearing capacity of coal seam were clarified. • The gas self-sealing model of high dip CBM reservoirs was established. • The gas genesis specialty in high dip coal seam was revealed. • The gas productivity characteristic of high dip CBM reservoirs was summarized. [ABSTRACT FROM AUTHOR]