二连盆地富气凹陷低阶煤煤层气成因及成藏机制

Origin and accumulation mechanisms of coalbed methane in low-rank coals in gas-rich sags in the Erlian Basin

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^(3)H和^(14)C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,CO_(2)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,δ^(13)C(CH_(4))分布在-70.3‰~-48.0‰,δD(CH_(4))分布在-285.5‰~-189.0‰,δ^(13)C(CO_(2))在-37.6‰~1.94‰变化。煤层水化学类型主要为HCO_(3)-Na型和Cl·HCO_(3)-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1020~47490 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。其中吉尔嘎朗图凹陷早期以乙酸发酵产气为主,晚期转变为CO_(2)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,吉尔嘎朗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。

Inner Mongolia boasts abundant coalbed methane(CBM)resources in low-rank coals.Investigating the origin and accumulation mechanisms of CBM is crucial for the siting and assessment of these CBM resources.This study examined the CBM in low-rank coals in major gas-rich sags of the Erlian Basin through various experiments,including tests of the composition and carbon/hydrogen isotopes of CBM,as well as tests of the water quality,hydrogen/oxygen isotopes,and radioisotopes(^(3)H and ^(14)C)of coal seam water.By and coal seam water,this study revealed the origin,sources,and accumulation mechanisms of CBM in low-rank coals within the study area.Key findings are as follows:(1)The CBM in the Erlian Basin is all dry gas and dominated by methane(CH_(4)).With an increase in the burial depth,the CH_(4) volume fraction increases,whereas the CO_(2) volume fraction first increases and then decreases,peaking at depths between 300 and 500 m.The CH_(4) exhibits generally lighter carbon and hydrogen isotopic values,withδ^(13)C(CH_(4)),δD(CH_(4)),andδ^(13)C(CO_(2))values ranging from−70.3‰to−48.0‰,from−285.5‰to−189.0‰,and from−37.6‰to 1.94‰,respectively.(2)The coal seam water primarily has hydrochemical types of HCO_(3)-Na and Cl·HCO_(3)-Na.The current coal seams feature a relatively stable water environment,with weak hydrodynamic conditions.The coal seam water,with apparent ages between 1020 and 47490 a,principally originates from Quaternary meteoric water,with no or little recharge from modern surface water.(3)The CBM in the Erlian Basin is predominantly composed of original biogenic gas,mixed with a minor amount of early thermogenic gas.With an increase in the burial depth,the stratigraphic environment and types of methanogenic archaea change,with the generation paths of biogenic methane also shifting.In the early stage,the biogenic methane in the Jiergalangtu sag was primarily derived from acetic acid fermentation.In the late stage,it predominantly originated from CO_(2) reduction in the late stage,mixed with a small quantity of low-maturity thermogenic gas.In contrast,the biogenic methane in both Bayanhua and Huolinhe sags was principally sourced from acetic acid fermentation.Besides,the biogenic methane in the Huolinhe sag contains a trace amount of biogenic gas from methyl fermentation.(4)The study area has coal seam conditions favorable for biogenetic gas generation,such as low geotemperatures,low total dissolved solids(TDS)content,and low thermal maturity.The Jiergalangtu sag exhibits a graben-type biogenetic-gas accumulation mode in shallow thick coal seams,while the Bayanhua and Huolinhe sags display a half-graben-type biogenetic-gas accumulation mode with hydraulic sealing in moderately deep confined areas.Identifying favorable target areas for the formation and enrichment of biogenic gas should be the focus of subsequent exploration and production of CBM in the Erlian Basin.Besides,it is practically significant for reserve growth and production addition of CBM in low-rank coals in the basin.