Genesis and accumulation mechanism of low rank coalbed methane in gas-rich depressions of Erlian Basin
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摘要: 内蒙古低阶煤煤层气资源丰富，煤层气成因与成藏机制研究对低阶煤煤层气资源选区评价至关重要。以二连盆地重点富气凹陷低阶煤煤层气为研究目标，利用煤层气组分、碳/氢同位素、煤层水水质、氢/氧同位素及放射性同位素3H和14C测试等多种实验手段，分析煤层气、水地球化学特征，揭示低阶煤煤层气成因来源及成藏机制。结果表明，二连盆地煤层气组分以甲烷为主，均为干气，其中甲烷体积分数随埋深增加而增大，CO2体积分数随埋深增加呈先增加后降低趋势，在300~500 m范围出现高值区。甲烷碳、氢同位素普遍偏轻，δ13C (CH4)分布在−70.3‰~−48.0‰，δD (CH4)分布在−285.5‰~−189.0‰，δ13C (CO2)在−37.6‰~1.94‰变化。煤层水化学类型主要为HCO3-Na型和Cl·HCO3-Na型，现今煤层水体环境较为稳定，水动力较弱，煤层水表观年龄在1 020~47 490 a，主要来源于第四纪大气降水，没有或较少有现今地表水补给。二连盆地煤层气主要为原生生物成因气，混有少量早期热成因气，随着埋深加大，地层环境和产甲烷古菌类型发生变化，生物甲烷生成途径发生转变。其中吉尔嘎郎图凹陷早期以乙酸发酵产气为主，晚期转变为CO2还原产气为主，并混有少量低熟热成因气；巴彦花和霍林河凹陷微生物产气途径均以乙酸发酵为主，其中霍林河凹陷还混有少量甲基发酵型生物气。研究区具有适合生物气生成的低地温、低矿化度和低热演化程度的“三低”煤层条件，其中，吉尔嘎郎图凹陷属于地堑式浅部厚煤层生物气成藏模式，巴彦花和霍林河凹陷属于半地堑式中深部承压区水力封堵生物气成藏模式。寻找适合生物成因气形成和富集的有利目标区，应是二连盆地煤层气未来勘探开发的重点方向，也是二连盆地低阶煤煤层气增储上产的现实保障。Abstract: Inner Mongolia is rich in low rank coalbed methane (CBM) resources, and research on the genetic and accumulation mechanism of coalbed methane is crucial for the selection and evaluation of low rank coalbed methane resources. Taking the low rank CBM in the key gas rich depressions of the Erlian Basin as the research objective, the geochemical characteristics of CBM and coal seam water is analyzed and the genesis and accumulation mechanism of low rank CBM is revealed by using various experimental methods, including tests on composition and carbon/hydrogen isotopes of CBM water quality, hydrogen/oxygen isotopes, and radioactive isotopes 3H and 14C of coal seam water. The result shows that the CBM component in the Erlian Basin is dominated by CH4, which belongs to dry gas. The CH4 volume fraction increases with the increase of burial depth, while the CO2 volume fraction first increases and then decreases with the increase of burial depth, and it shows high values between 300 and 500 m. The carbon and hydrogen isotopes of CH4 in the research area are generally lighter, with and δ13C(CH4) ranges from -70.3‰ to -48.0‰ and δD(CH4) ranges from -285.5‰ to -189.0‰, and δ13C(CO2) varies between -37.6‰ and 1.94‰. The chemical types of coal seam water are mainly HCO3-Na type and Cl·HCO3-Na type, and the present water environment of coal seam is relatively stable, with weak hydrodynamic forces. The apparent age of coal seam water is about 1 020-47 490 years, mainly sourced from Quaternary atmospheric precipitation, and there is no or less current surface water supply. The CBM in the Erlian Basin is mainly primary biogenic gas, mixed with a small amount of early thermogenic gas. As the burial depth increases, the geological environment and types of methanogenic archaea change, and the production pathway of biogenic methane also changes. In the Jirgalangtu depression, the biogenic methane is mainly produced by acetic acid fermentation pathway in the early stage, and is converted to CO2 reduction pathway in the late stage, mixed with a small amount of low mature thermogenic gas. In both Bayanhua and Huolinhe depressions, acetic acid fermentation is the main pathway of biogenic gas production, and there is also a small amount of methyl fermentation biogenic gas in Huolinhe depression. The research area has the "three low" coal seam conditions suitable for biogenetic gas generation, including low geothermal temperature, low salinity, and low thermal evolution. The Jirgalangtu depression belongs to the graben type biogenetic gas accumulation model in shallow thick coal seam, while the Bayanhua and Huolinhe depressions belong to the semi-graben type biogenetic gas accumulation model in middle-deep confined areas with hydraulic sealing effect. Finding favorable target areas for the formation and enrichment of biogenic gas should be a key direction for the future exploration and development of CBM in the Erlian Basin, and it is also a practical guarantee for increasing the storage and production of low rank CBM in the Erlian Basin.
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Keywords:
- Erlian Basin /
- coalbed methane genesis /
- low rank coal /
- biogenic gas /
- coal seam water /
- accumulation mechanism
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