Genetic type and gas-generation potential of coalbed methane in Bayanhua Sag of the Erlian Basin
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摘要: 二连盆地巴彦花凹陷是内蒙古低阶煤煤层气重点开发试验区，但勘探程度相对较低，对煤层气成因认识不足，在一定程度制约了勘探开发进度。通过对巴彦花凹陷煤层气井气样水样开展气体组分、稳定同位素、水化学及放射性同位素定年等测试，并结合经典天然气成因判识图版厘清气体成因，进一步剖析生气潜力，明确生气关键要素。结果显示： C1/C1-5＞0.99，CO2-CH4系数[CDMI=φ(CO2)/φ(CO2+CH4)]基本小于5%，干燥系数(C1/C2+)介于104~5 540，CH4含量高、重烃及CO2含量低。δ13C (CH4)介于-51.80‰~-67.70‰、δD (CH4)介于-226.20‰~-291.00‰，δ13C (CO2)介于-20.30‰~-37.60‰，为陆相生物成因气特征；判识图版中大部分煤层气样品落在生物成因气区域，甲烷产气途径为乙酸发酵和甲基发酵，CO2主要是微生物产甲烷活动伴生产物。煤层水来源于大气降水，主要为NaHCO3型弱碱性水，δ13CDIC为-2.6‰，δ18O为-16.4‰，结合14C定年表明水为第四纪水，非原生水，为现代混合水。结合全区构造和水文地质条件分析认为，巴彦花凹陷径流区利于乙酸发酵产气，弱径流区利于生物气富集成藏。区内低阶煤储层孔渗性较好，地温适宜，水文地质条件优越，利于生物气的生成，承压区水力封堵型生物气藏发育。水文地质条件是本区生物气形成关键，在煤层气勘探选区中应重点关注。Abstract: The Bayanhua Sag in Erlian Basin is a key experimental area for the development of low-rank coalbed methane(CBM) in Inner Mongolia. However, the exploration degree is relatively low and the understanding of CBM genesis is insufficient, which restricts exploration and development to a certain extent. Through carried out laboratory experiments of gas components, stable isotope, hydrochemical and radioisotope dating of CBM well gas samples and water samples in Bayanhua Sag. And comprehensive use the classical natural gas to clarify genesis. Further analyze potential and identify the key elements of gas-generation. The results show that C1/C1-5>0.99, CDMI is basically less than 5%, the drying coefficient C1/C2+ is between 224 and 5 540. The volume fraction of CH4 is high, while heavy hydrocarbons and CO2 is low. The δ13C(CH4) ranging from -51.80‰ to -67.70‰, δD(CH4) ranges from -226.20‰ to -291.00‰, and δ13C(CO2) ranges from -20.30‰ to -37.60‰, indicating a terrestrial biogenic gas characteristic. In the identification chart, most samples are located in the regions of biogenic gas, indicating that methane pathways are acetic acid fermentation and methyl fermentation, and CO2 is mainly the associated product of microbial methanogenesis. Coal seam water comes from atmospheric precipitation, mainly NaHCO3 type weakly alkaline water, δ13CDIC value is -2.6‰, δ18O is -16.4‰. Combined with 14C chronology shows that the water is quaternary water, non-primary water, but modern mixed water. Based on the analysis of geological structure and hydrogeological conditions in the whole area, the runoff area is conducive to acetic acid fermentation to produce gas, while the slow flow area is conducive to biogas enrichment and accumulation. The low-rank coal reservoir has good porosity and permeability, suitable temperature reservoir and superior hydrogeological conditions, which are conducive to the generation of biogas and the development of hydraulic plugging type biogas in the pressurized area. Hydrogeological conditions are the key to the formation of biogas, and should be paid more attention to the exploration area of CBM.
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Keywords:
- Erlian Basin /
- Bayanhua Sag /
- low-rank coal /
- coalbed methane /
- genetic type /
- gas-generating potential
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