The Formation Mechanism of Thick Coal Seams and the Distribution of Coal-rock Gas Sweet Spots in the Tuha Basin

Key words: 
• coal and rock gas /
• genesis of thick coal seams /
• Tuha Basin /
• Xishanyao group /
• coal rock dessert section /
• the water enters and the water retreats

Abstract: [Purpose]As a new type of unconventional natural gas, coal and rock gas has made a major breakthrough in the exploration of the central and western basins in recent years. At present, the research on coal and rock gas mainly focuses on the gas generation capacity and gas accumulation capacity of coal and rock, but the formation mechanism and distribution law of thick coal seams, the carrier of coal and rock gas, are obviously insufficient, which leads to the optimization of favorable coal and rock gas zones, especially the optimization of drilling intervals in thick coal seams of medium and low coal rank in the western basin.[Methods]Based on the description of the coring and macroscopic characteristics of underground coal and rock in the Tuha Basin, the microscopic component identification, major and trace element analysis and rock pyrolysis analysis of coal and rock samples of Xishanyao Formation were systematically carried out, and on this basis, the sedimentary environment, coal facies characteristics and characteristics of coal and rock gas sweet spot sections in thick coal seams were compared.[Results]The analysis of coal and rock microscopic components shows that there are not only various types of coal facies in the Xishanyao Formation in the Tuha Basin, but also various types of coal facies superimposed in the thick coal seam at the same location. The comparative analysis of the main and trace elements in coal and rock shows that there are multi-stage water inflow and water retreat cyclic characteristics in thick coal seams.[Conclusions]The results show that the coal-forming environment is different at different stages in the process of water inflow and retreat, resulting in the difference in the quality of coal and rock between different coal phases, and the cause of the thick coal seam is the longitudinal superposition of coal and rock of different types of coal facies caused by the rise and fall of water bodies in multiple stages. During the Jurassic period, the Tuha Basin was an intermountain continental basin, and different sedimentary facies zones were affected by the depth of the water body, and the ratio of peat production rate to the growth rate of the accommodating space was obviously different, resulting in the difference in the development and distribution of coal seams. In the thick coal seams in this area, the flooded interface and the tolerable spatial transition surface in the process of water retreat can be identified, among which the water degradation transition surface is the layer with the highest organic matter content in coal and rock, and it is also the sweet spot section of coal and rock gas exploration.