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浅层复合(混合)天然气藏广泛存在于含油气盆地中，有效鉴别浅层复合(混合)天然气藏中天然气的成因可为浅层气勘探提供有价值的信息。我国含油气盆地浅层天然气组分及碳同位素组成分布特征表明，浅层天然气在总体上显示相对较高的甲烷含量和相对较低的甲、乙、丙烷碳同位素比值。浅层复合(混合)气藏中天然气的来源决定了气藏不同的复合(混合)类型，如以生物气为主与低温热成因气的复合，以低温热成因气(生物-热催化过渡带气)为主与成熟/高成熟热成因气的复合，以及低熟油伴生气与微生物降解气的复合。浅层复合(混合)气无论是甲烷碳同位素分布特征还是组分组成特征大部分都与生物气与热成因气的混合气相似，但从重烃组分含量，乙、丙烷碳同位素比值及其差值的不同，可以判断浅层复合(混合)气藏中天然气各组分的主要来源。

Genetic Identification of Natural Gas in Shallow Combination Reservoirs in China

The shallow combination (mixed) natural gas reservoirs are widely distributed in oil and gas bearing basins and a valuable information may be provided for the shallow gas exploration through effectively identifying natural gas geneses in these reservoirs.According to the distribution characteristics of the chemical and carbon isotope compositions of shallow gases in the oil and gas bearing basins in China,it is shown that they have relatively high methane content and relatively low methane,ethane and propane carbon isotope ratios overall.The different combination (mixed) types of gas reservoirs are dependent upon different natural gas geneses of the shallow combination (mixed) gas reservoirs,such as the combination of biogenic gas (occupying a major part) with low temperature thermogenic gas,that of low temperature thermogenic gas or bio thermocatalysis transitional zone gas (occupying a major part) with mature/high mature thermogenic gas and that of low mature oil associated gas with microbiological degradation gas.Not only the distribution characteristics of methane carbon isotopic composition but also chemical composition characteristics of shallow combination (mixed) gas are mostly similar to those of mixed gas coming from biogenic gas and thermogenic gas.The main source,however,of each component of natural gas in shallow combination reservoirs may be identified according to the heavier hydrocarbon content and the ethane and propane carbon isotope ratios and their differences.