新疆米泉地区煤层气成因及富集模式

Genetic mechanism and accumulation model of coalbed methane in the Midong Block, Xinjiang

  • 摘要: 米泉地区位于准噶尔盆地南缘博格达山冲断褶皱带,构造复杂且深煤层普遍富含CO2,煤层气开发面临巨大挑战。通过系统采集米泉地区西山窑组不同埋深煤样、气样及水样,综合运用煤岩学、气体碳同位素及水化学测试分析技术,揭示了该区2个主要构造单元(八道湾向斜和七道湾背斜)煤层气空间分布规律、成因机制及富集模式。研究结果表明:主力煤层平均实测含气量主要分布在2~12 m3/t,垂向上,随埋深增加含气量增大,呈现垂向分带;平面上,呈现“西南高、东北低,由西南向东北递减”特征。井口产出气CO2平均体积分数达22%,且随埋深增加而增大,1400 m以深CO2体积分数可达75%,七道湾背斜北翼和八道湾向斜两翼主要为微生物产甲烷伴生CO2,八道湾向斜核部表现为煤化作用过程中产生的CO2。七道湾背斜北翼发育生物气水力封堵型气体富集模式,天山雪水和大气降水向下渗透使得微生物降解作用活跃,生物气在中部受水力封堵富集。八道湾向斜发育复合气源构造封盖型气体富集模式,深部热成因气向南北两翼构造高部位运移,同时中部甲烷菌通过CO2还原作用持续生成次生生物气,受逆冲断层的侧向封盖,形成了构造控制下复合气源富集带。该研究为米泉地区和相似复杂构造区低煤阶高CO2的煤层气勘探开发提供了借鉴和参考。

     

    Abstract: Located in the Bogda Mountain fold-and-thrust belt on the southern margin of the Junggar Basin, the Miquan area presents significant challenges for coalbed methane (CBM) development due to its complex tectonics and high CO2 concentrations in deep coal seams. By systematically collecting coal, gas, and water from various depths within the Xishanyao Formation in the Miquan area, and comprehensively applying coal petrology, gas carbon isotope, and hydrochemical testing and analysis technologies, the spatial distribution characteristics, genetic mechanisms, andaccumulation models of CBM in the two primary structural units (the Badaowan syncline and the Qidaowan anticline) in this area have been revealed. The results show that: The average coalbed methane content of the main coal seams is primarily between 2−12 m3/t. Vertically, the gas content increases with the increasing of depth, showing vertical zonation. Horizontally, the distribution is characterized as being higher in the west and south than in the east and north. The average CO2 content in the produced gas reaches 22% and increases with increasing depth. At depths below 1400 m, the CO2 content can reach up to 75%. The CO2 on the north flank of the Qidaowan anticline and on both flanks of the Badaowan syncline is primarily associated with microbial methanogenesis. In contrast, the CO2 in the core of the Badaowan syncline is shown to be generated during the process of coalification. Due to the downward penetration of Tianshan snowmelt and atmospheric precipitation, microbial degradation is active, and biogas accumulates in the central region due to hydraulic blockage. Therefore, the northern wing of the Qidaowan anticline exhibits a hydraulic blockage-type gas accumulation pattern. Due to the migration of deep thermal hydrocarbons toward the structural high areas on the northern and southern wings, and the continuous generation of secondary biogenic gases by methane-producing bacteria in the central area through CO2 reduction, combined with lateral capping by thrust faults, a structurally controlled composite gas source enrichment zone has been formed. Therefore, the Badaowan syncline exhibits a composite gas source structural capping type gas enrichment pattern. A valuable reference is provided for the exploration and development of CBM in the Miquan area and other similar regions with complex structures, low-rank coal, and high CO2 content.

     

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