Differential accumulation of coalbed methane in medium- to low-rank coal seams in a strongly compressional tectonic setting in the Tarim Basin

Background The Tarim Basin contains abundant coal and coalbed methane (CBM) resources. However, the geological accumulation patterns of CBM in the basin remain poorly understood. This leads to a relative lack of targeted technologies for the exploration and development of both deep coal reservoirs and steeply dipping coal seams, further rendering it challenging to achieve breakthroughs in CBM development in the basin.

Methods Based on test data from 92 coal samples, data on the drilling and production tests of more than 40 exploration wells, and 3D seismic interpretation results, this study systematically investigated the distribution of coal-bearing strata, as well as the characteristics, pore structures, gas-bearing properties, and CBM accumulation patterns of coal reservoirs, in the Tarim Basin using techniques including coal petrography, proximate analysis, low-temperature N₂ adsorption, array acoustic logging, and CT scanning. Furthermore, this study quantified the CBM resource potential and determined CBM play fairways in the basin.

Results and Conclusions  The results indicate that the Tarim Basin exhibits two sets of major coal-bearing strata: the Triassic Taliqike Formation and the Jurassic Yangxia and Kezilenuer formations. The coal reservoirs in the basin are composed primarily of bright and semi-bright coals, which account for 60% to 70% of the total. The reservoirs are rich in vitrinite but depleted in inertinite, with pores dominated by mesopores and micropores. Within the basin, adsorbed gas predominates in shallow parts, accounting for greater than 95%, while adsorbed and free gases coexist in deep parts, representing 82% and 18%, respectively. Four CBM accumulation patterns are identified in the Tarim Basin: the lower source rock-upper reservoir configuration with CBM transported via faults, the in situ, self-sourced pattern in deep parts, the hydraulic sealing pattern in steeply dipping structural zones, and the three-gas coexistence pattern in tectonic slope zones. CBM initially-in-place at burial depths of 2000 m or less stands at 1.29 × 10¹² m³, while that at burial depths exceeding 2000 m amounts to 2.1 × 10¹² m³. Through five-map superposition, two CBM play fairways are determined: the Dibei gentle slope and the western Kubai Coalfield. Both play fairways jointly cover an area of 370 km² and exhibit CBM initially-in-place of 58.2 × 10⁹ m³. Among these, the No. 3 coal seam of the Kezilenuer Formation in the Dibei gentle slope is selected as the optimal core target owing to its considerable thickness, moderate burial depth, high gas content, and favorable sealing conditions. The results of this study enrich the CBM enrichment theory of medium- to low-rank coal reservoirs, providing a geological basis and technical support for selecting optimal play fairways, conducting well placement, and preparing schemes for layered and segmented CBM exploration and development.