低熟气地球化学特征与判识指标

Geochemical Characteristics and Identification Indexes of Low-mature Gases

从目前的研究来看,低熟气的源岩母质类型主要以有机质丰度较高的Ⅲ型煤系为主。低熟气热演化程度(RO)值在0.4%~0.8%之间,根据煤型气CH4碳同位素组成与热演化程度RO之间的"二阶模式"关系,煤型气CH4碳同位素组成应该分布在-54‰^-39‰之间,目前发现的低熟气CH4碳同位素组成主要集中在-49‰^-39‰之间。一般认为煤型气的C2H6碳同位素组成重于-28‰,但是,当考虑低熟气时,将这一指标定为-29‰更为合适。根据煤型气CH4碳、氢同位素关系,低熟煤型气的CH4氢同位素组成分布在-300‰^-216‰之间。天然气轻烃参数中,低熟煤型气的庚烷值分布在16%~22%之间,异庚烷值分布在2.2~5.0之间。生物气不仅在CH4、C2H6碳同位素组成上与低熟气存在明显的差别,同时在轻烃含量的参数上也存在明显的差别。

From the current studies, the coal measures with high organic matter are main types of source rocks of low-mature gases. The thermal evolution indices（R0） of low-mature gases should be within the range of about 0.4 %-0.8%, and corresponding carbon isotope values of methane is within the range of about -49%0--39X0 according the two-stage model of carbon isotope fractionation for coal type gases. It＇s generally believed that δ^13Cethane value of coal-type gas is more than -28 ‰, however, when considering the low-mature gases, the value of -29‰ is more appropriate. According to relationship of carbon and hydrogen isotopic compositions of coal-type gases, the hydrogen isotopic composition of low mature gases should be within the range of about -300‰-216‰. Heptane value of low mature gases are located in between 16 %-22% and isoheptane value are in between 2. 245.0. There exists an obvious difference between biogenic gas and low-mature gases not only in the carbon isotopic compositions of methane and ethane, but also in light hydrocarbon concentration.