黄陇煤田大佛寺井田煤层气成因机制研究

彬长矿区大佛寺井田为典型的黄陇侏罗纪低阶煤煤层气田。井田内煤层气井较多，但有关煤层气成因机制方面的研究较少。厘清井田内煤层气地球化学特征及成因机制，对深化煤层气的形成机理认识和科学评价煤层气资源量具有重要指导意义，可为煤层气高、低产井产能差异化分析提供重要依据。采集研究区内6口煤层气井井口排采气样品，22块4号煤层煤样及煤层水和地表水样各1件，开展显微煤岩组分、气体化学组分、碳同位素和水样水质检测，并结合部分研究区相关的文献数据，分析大佛寺井田煤层CH₄碳同位素特征、成因类型及偏轻机理。结果表明:大佛寺井田主采的4号煤层显微煤岩组分中，有机组分含量明显趋高，平均为93.2%，其中，惰质组最具优势，平均68.2%；镜质组次之，平均22.8%，镜质体反射率R_max平均0.65%。煤层气组分以CH₄为主，CH₄体积分数为73.805%~98.006%，平均83.753%；N₂体积分数为1.259%~25.735%；平均15.220%；CO₂体积分数为0.040%~2.380%，平均1.023%；C₂及以上重烃含量平均不足0.0054%；C₁/C_1—n>0.999；CH₄和N₂含量呈明显负相关性，煤层气组分在成藏后期受空气影响明显。δ13C₁为?80.516‰~?62.400‰，平均?73.000‰；δ13C_CO2为?41.693‰~?7.065‰，平均?18.660‰。大佛寺井田煤层气为次生生物成因气，其显著标志为δ13C₁偏轻和重烃含量极少，呈现典型特干气特征，偏轻机理在于其绝大部分由CO₂还原而成，少量由乙酸发酵而成，且在这两种途径的生气过程中，最终均会出现生物甲烷富集轻碳同位素的结果，从而导致δ13C₁偏轻。 

Formation mechanism of coalbed methane in Dafosi mine field,Huanglong coalfield

Dafosi mine field is a typical low-rank coalbed methane (CBM) field in Binchang mining area of Huanglong Jurassic. There are many coalbed methane wells in the field, but few researches are performed on the formation mechanism of coalbed methane. Thus, it is of great guiding significance to clarify the geochemical characteristics and formation mechanism of coalbed methane (CBM) in the mine field, which could provide important basis for the analysis on productivity difference of high- and low-yield CBM Wells. For this reason, the samples of drained gas at the wellhead of six CBM wells, 22 coal samples of No.4 coal seam, 1 water sample from coal seam and one from surface water were collected from the study area. Then, the coal maceral, gas chemical composition, carbon isotopes and sample water quality were detected, and the carbon isotope characteristics, genetic types and lightening mechanism of coal seam methane of (CH₄) in Dafosi mine field were analyzed with reference to the relevant literature data in some research areas. According to the results, the content of organic components in coal maceral of the main coal seam No.4 in Dafosi mine field is obviously higher, with an average of 93.2%. Specifically, the inertinite is the most dominant with an average of 68.2%, and the vitrinite ranks the second with an average of 22.8% and an average reflectance R_max of 0.65%. The coalbed methane is mainly composed of CH₄, which ranges from 73.805% to 98.006%, with an average 83.753%, N₂ with a volume fraction of 1.259% to 25.735% and 15.220% on average, CO₂ with a volume fraction of 0.040% to 2.380% and an average of 1.023%, C₂ and above heavy hydrocarbon with an average fraction less than 0.005 4%. It is shown that C₁/C_1—n is greater than 0.999, the content of CH₄ and N₂ is negatively correlated, and the components of coalbed methane are obviously affected by air in the later stage of reservoir formation. The δ13C₁ ranges from ?80.516‰ to ?62.400‰, with an average of ?73.000‰, while δ13C_CO2 ranges from ?41.693‰ to ?7.065‰, with an average of ?18.660‰. The coalbed methane in Dafosi mine field is secondary biogenic gas, which is obviously marked by light δ13C₁ and low content of heavy hydrocarbon, thus presenting the typical characteristics of extra-dry gas. The lightening mechanism lies in that most of the coalbed methane is produced by CO₂ reduction and a small amount by acetic acid fermentation, and in the process of gas production in these two ways, the enrichment of light carbon isotopes in biomethane will be resulted in eventually, further leading to lighter δ13C₁.