不同成因类型煤型气地球化学特征及其判识意义

通过数理统计前人公开发表的国内外21个盆地或地区的324组煤型气地化数据, 分析不同成因类型煤型气地层分布和稳定碳、氢同位素组成及空间分布特征, 提出多个煤型气成因类型判识图版, 并以实例论证这些图版的可行性.研究结果表明: 与煤层相关的生物成因气不同于常规生物气, 最显著区别在于前者δ13C(CH₄)上限值低, 即生物成因气δ13C(CH₄)＜-60‰, 热成因气δ13C(CH₄)＞-40‰, 混合成因气δ13C(CH₄)介于二者之间.随着有机质演化程度增强, 从生物成因气至热成因气, δ13C(CH₄)、δ13C(CO₂-CH₄)、δ13C(C₂H₆-CH₄)及CH₄/(C₂H₆+C₃H₈)具有变重趋势且相关性明显, δ13C(CH₄)与δ13C(CO₂-CH₄)、δ13C(CH₄)与δ13C(C₂H₆-CH₄)及δ13C(CH₄)与CH₄/(C₂H₆+C₃H₈)是划分煤型气成因类型最可靠的图版. 

Geochemical Characteristics and Identification Significance of Coal Type Gas in Various Geneses

Based on the statistic study of 324 groups of published coal type gas geochemical data from 21 basins or areas from all over the world, stratigraphic distribution, stable carbon and hydrogen isotopic composition, and spatial distribution of coal type gas in various genesis are analyzed. Multiple charts for identifying the genetic types of coal type gas are established. And the feasibility of these charts is demonstrated by an example. The results show that biogenic gases that generated from coal seam are not the same as normal biogas (compared with normal gas). The most significant difference is the δ13C(CH₄) value of microbial gases lower than that of normal biogas. The stable isotopic composition of microbial gases fall within the following ranges: δ13C(CH₄) < -60‰, while for thermogenic gases, δ13C(CH₄) > -40‰. And for mixed gases, δ13C(CH₄) ranges from -60‰ to -40‰. There is a tendency that from biogenic gases to thermogenic gases, δ13C(CH₄), δ13C(CO₂-CH₄), δ13C(C₂H₆-CH₄), and CH₄/(C₂H₆+C₃H₈) gradually become heavier with the increase of the maturity of organic matter. An obvious positive correlation exists among aforementioned parameters. It is concluded that the charts of δ13C(CH₄) and δ13C(CO₂-CH₄), δ13C(CH₄) and δ13C(C₂H₆-CH₄), and δ13C(CH₄) and CH₄/(C₂H₆+C₃H₈) are the most reliable charts for classifying the genetic type of coal type gas.