Analysis of Gas-Controlling Factors for Middle-Deep Low-Rank Coalbed Methane Accumulation in the Junggar Basin Margin Structural Belt

The Junggar Basin was an important production area of low rank coalbed methane in China. With the breakthrough of deep coalbed methane, the exploration and development of low rank coalbed methane in middle and deep layers in the area is imperative. Taking the Wutong Wozi mining area located at the northeast edge of the Junggar Basin as an example, four wells were deployed and coal rock samples of Xishanyao Formation were collected. By tested the coal rock quality, gas content, gas composition, and isotopes, the relationship between structural evolution, sedimentation, hydrology and gas content was analyzed, then the factors controlling gas accumulation in low rank coal were clarified. The results The results indicated that it had the characteristics of multiple layers of coal seams, large cumulative thickness of coal seams, and low maturity of coal rock. The methane weathering zone was deep, and the gas content had the disadvantage of low gas content and low proportion of methane components. Gas isotopes revealed that the coalbed methane in the area was mainly secondary biogenic gas. By compared the geological characteristics of deep coalbed methane in the region, it was first proposed the concept of endogenous coalbed methane reservoir formation controlled by the tripartite synergy of structure-sedimentation-hydrology. The concept showed that Structural evolution controlled weathering zones, sedimentary cycles controlled gas vertically, hydrological and hydrocarbon generation differences controlled the types of reservoir. In addition, Structural evolution was the basis for determining the exploration potential of deep coalbed methane, then sedimentation and hydrology were the key factors affecting reservoir types. Finally, it was provided a new approach for evaluating the exploration potential of low rank coalbed methane in deep layers similar to the Junggar Basin margin structural belt.