阜康矿区煤层气成因探讨

Discussion on genesis of coalbed methane in Fukang Mining Area

以准噶尔盆地南缘阜康矿区为研究区,对研究区内10口煤层气井的排采气进行了气体组分、甲烷碳同位素、乙烷碳同位素及二氧化碳碳同位素测试,并结合研究区特殊的地质环境特征对煤层气的成因进行了判别。研究发现:阜康矿区煤层气组分包括CH4(81.79%)、CO2(14.36%)、N2(2.28%)和C2+(0.99%),煤层气甲烷碳同位素值δ13C1分布范围为-58‰~-49‰,平均值为-53‰,乙烷碳同位素值δ13C2分布范围为-32.2‰~-23.1‰,平均值为-28.1‰,二氧化碳的碳同位素值δ13C(CO2)分布范围为+8.5‰~+14.2‰,平均值为+12.9‰;研究区煤层气为热成因和次生生物成因的混合成因气,其中的热成因气为热降解成因,次生生物气主要是醋酸发酵形成的产物;急倾斜煤层和火烧区为微生物进入煤层提供了通道;温度适宜、偏酸性且矿化度较低的地下水环境为微生物提供了生存环境;地下水渗流方向与煤层气运移方向相反,具有水力封堵控气作用;火烧区滞水层与煤层顶底板一同对煤层气藏进行有效圈闭,保证了混合成因气的储存。

The research area is Fukang Mining Area in the southern margin of Junggar Basin. The gas composition, methane carbon isotope, methane carbon isotope and carbon isotope of CO2 were tested in 10 CBM wells in the research area, and combined with the special geological environment characteristics of the area to discriminate the origin of CBM. It is found that the CBM components in Fukang Mining Area include CH4(81.79%), CO2(14.36%), N2(2.28%)and C2 +(0.99%). Among them, the distribution range of methane carbon isotope (δ13 C1)is-58‰ to-49‰ with an average value of-53‰;the distribution range of ethane carbon isotope value (δ13 C2)is-32.2‰ to-23.1‰ with an average value of-28.1‰;and the distribution range of carbon isotope value δ13 C(CO2)of carbon dioxide is +8.5‰ to +14.2‰ with an average value of +12.9‰. The CBM in the study area is of mixed origin gas, among which the thermogenic gas is of thermal degradation origin, and the secondary biogas is mainly the product of acetic acid fermentation. The steep coal seam and burning area provide a channel for microorganism to enter the coal seam. The flow direction of groundwater is opposite to that of CBM, and which has the function of hydraulic blocking and gas control. The groundwater environment with suitable temperature, slightly acidic and low salinity provide the living environment for the microorganism. The stagnant water layer in the burning area and the roof and floor of the coal seam effectively trap the CBM reservoir to ensure the storage of the mixed gas.