Geochemical characteristics and influencing factors of deep coalbed methane in the Linxing-Shenfu block

Identifying the origin and geochemical characteristics of deep coalbed methane (CBM) is critical for its exploration and production. This study examined the desorbed gas from deep-CBM parameter wells in the Linxing-Shenfu block on the eastern margin of the Ordos Basin. It systematically tested the composition, content, and carbon isotopes of CBM in coal seams Nos. 8 and 9. In combination with the degree of thermal evolution, gas-bearing properties, coal seam burial depths, geological structures, and hydrogeologic conditions, it delved into the geochemical characteristics and influencing factors of deep CBM. The results show that the deep CBM in the study area consists primarily of CH₄, CO₂, N₂, ethane, propane, isobutane, n-butane, isopentane, and n-pentane. Among them, CH₄ exhibits the highest content, with volume fractions ranging from 75.98% to 96.20% (average: 86.49%), succeeded by ethane (average: 5.97%), CO₂ (average: 5.18%), and propane (average: 1.24%), exhibiting subtle differences in gas content compared to shallow CBM. The deep CBM manifests δ¹³C₁ (CH₄), δ¹³C_CO2, δ¹³C₂, and δ¹³C₃ values ranging from −51.9‰ to −38.2‰ (average: −42.84‰), from −8.6‰ to −0.63‰ (average: −4.66‰), from −27.36‰ to −18.70‰ (average: −25.27‰), and from −25.96‰ to −14.69‰ (average: −22.93‰), respectively. It predominantly proves to be thermogenic gas, displaying lower δ¹³C1 (CH₄) values and higher δ¹³C_CO2 values compared to shallow CBM. On a horizontal plane, its δ¹³C₁ values show a growing trend from southeast to northwest in the Linxing area. The study area kept subsiding from the Carboniferous to the end of the Triassic, with the Middle Jurassic and the Early Cretaceous identified as the primary hydrocarbon-generating phases. Specifically, the Middle Jurassic witnessed a prolonged duration, a low average rate, and moderate hydrocarbon yield of hydrocarbon generation. In contrast, the hydrocarbon generation during the Early Cretaceous featured a higher rate, a significantly increased yield, and a peak intensity, establishing the e Early Cretaceous as a dominant hydrocarbon-generating stage. Due to the controlling effects of gas fractionation, the characteristics of reservoir pores and fractures, reservoir pressure, and the roof and floor tightness of coal seams, the δ¹³C₁ values increase with the increasing maximum vitrinite reflectance (R_max), gas content, and burial depth. They are relatively high in synclinal cores, the stress concentration zones of faults, and fold flanks. However, they are lower in the stress release zones of faults and anticlinal cores, suggesting inferior CBM storage conditions, unfavorable for gas accumulation, in these zones. Influenced by groundwater dissolution, the δ¹³C₁ values are relatively low in areas with strong hydrodynamic conditions, while showing an insignificant downward trend in zones with weak fluid dynamics or fluid stagnation. The comprehensive investigation into the geochemical indicators of CBM leads to the conclusion that the Linxing and southeastern Shenfu areas are enriched in CBM, especially the former. This study can serve as a reference for target area selection and well placement in future CBM exploration and exploitation in the study area.