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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]

Abstract Coalbed methane (CBM) drilling technology is critical in achieving efficient CBM development. The geological conditions for CBM development in China are complex, including special terrain, high-rank coal areas, deep coal seams, multiple superposed coal seams and coal measure gas symbiosis areas, steeply inclined coal seam areas, and tectonic coal seam areas. Therefore, it is critical to apply the optimum drilling technology in each different area. This paper first reviews the current CBM drilling technologies and gas production behaviors in China. It is found that although vertical wells and cluster well groups are common well types for CBM development in China, U-, V- and L- shaped horizontal wells and multilateral horizontal wells have been favored for CBM development in recent years. The paper then discusses the applicability of different well types to geological conditions and proposes a modification method for well design in various geologic conditions. This method uses the coal structure, R o , in situ stress, and ratio of critical desorption pressure to the reservoir pressure as the main inputs. Highlights • The current CBM well technologies and gas production in China were reviewed. • The applicability of different well types to geological conditions was discussed. • An optimization method for well type selection was proposed. [ABSTRACT FROM AUTHOR]

Coal seams are typically self-sourced reservoirs. However, in certain special cases, the coal seams also contain large amounts of external gas from other source rocks. Successful development strategies in relation to Jurassic coalbed methane (CBM) in recent years have confirmed the presence of large amounts of external gas within the coal seams of the Baijiahai Uplift, Junggar Basin, China, which has important exploration and development prospects. However, the genesis and source of Jurassic CBM in the area remain unclear, limiting our understanding of its formation and accumulation as well as the planning of further development strategies. Based on a comprehensive analysis of the geochemical characteristics of Jurassic CBM, combined with research on the burial and thermal evolution history of the Jurassic coal seams, this study discusses the production curves of typical wells as well as regional tectonic styles, genesis, source, and accumulation of Jurassic CBM in the Baijiahai Uplift. Depending on the genesis, Jurassic CBM in the study area is divided mainly into coal-type (humic-type) gas, oil-type (sapropelic-type) gas, and mixtures of them, all of which are thermogenic gases of medium to high maturity. There are two main sources of Jurassic CBM in the Baijiahai Uplift. On the one hand, it comes from the vertical migration of gas generated by Permian and Carboniferous source rocks in the Baijiahai Uplift directly. On the other hand, it comes from the regional lateral migration from the Dongdaohaizi and Fukang Sags. Gas generated by source rocks in the Dongdaohaizi and Fukang Sags is driven by pressure, migrates towards the Baijiahai Uplift through sandstone channels, and accumulates in the Jurassic coal seams through faults. Compared with common self-sourced coal seams, enrichment of the coal seams with large amounts of external gases requires two necessary conditions: (1) Other layers of gas-generating source rocks are present at levels deeper than those of the coal seams, constituting additional gas sources for the coal seams, and (2) an open fault system is developed regionally, providing pathways for gas from other layers to enter the coal seams. • The genesis and source of external gas in deep coal seams are explained. • Identification methods of the gas originates from multiple source rocks. • Reveal the migration and accumulation of external gas in deep coal seams. • The necessary conditions for the formation of external gas are determined. [ABSTRACT FROM AUTHOR]

Organic-rich mudstone, whose formation and accumulation are strongly related to the depositional environment, is a potential source of natural gas. Here, geochemical tests (including total organic carbon (TOC), rock pyrolysis, X-ray diffraction (XRD), and main and trace element analyses) were carried out on mudstone samples from two boreholes in the Jixi Basin to reveal the paleoenvironment and its control on organic matter enrichment. The results show that type III kerogen dominated and that all samples entered the maturity stage with good hydrocarbon generation potential. Generally, the TOC content tended to increase with increasing depth, and the mudstone in the middle (K 1 ch 2) and lower (K 1 ch 1) segments of the Chengzihe Formation was characterized by good quality with TOC > 2.5%. In the Jixi Basin, two common types of mudstones (silica-dominated mudstone and silica-rich argillaceous mudstone) were mainly composed of quartz and clay minerals. These mudstones were rich in Na 2 O, MnO and several trace elements (e.g., Zn, Mo, and Bi) and depleted in other elements (e.g., MgO, CaO, Cr, Co, and Ni). The rare earth element and yttrium (REY) distribution patterns normalized by North American shale compositions (NASC) were flat, with slight negative Ce and Eu anomalies. The ratios of Sr/Cu, Rb/Sr, Sr/Ba and V/(V + Ni) in mudstones indicated a warm-humid paleoclimate, a suboxic environment and a marine-continental transitional fresh-brackish water mass. From the K 1 ch 1 to Muling Formation (K 1 m), although the paleoclimate was more arid and paleosalinity increased gradually, depositional environments were in fresh-brackish water. The paleoclimate and clastic influx are the keys to controlling the enrichment of organic matter. The paleoclimate indirectly affects the preservation of organic matter by influencing the aquatic environment and controlling the level of paleoproductivity, while the strong clastic influx rate directly dilutes organic matter preservation. • Geochemical characteristics of the mudstones from the coal-bearing strata in the Jixi Basin are determined. • Mudstones in the coal-bearing strata were deposited under warm-humid paleoclimate with increasing water salinity. • Mudstones show stable suboxic depositional environment. • Paleoclimate and clastic influx are the keys to controlling the enrichment of organic matter. [ABSTRACT FROM AUTHOR]