Abstract:

There is lack of genetic research on shallow-buried natural gas in South Songliao Basin.  By analyzing the component，carbon isotope and light hydrocarbon of the natural gas samples collected in Quantou Formation，Fulongquan，it shows that in the study area there are two types of shallow gas:oil-gas and bio-degradation gas. The characteristics of the first type natural gas include:the methane carbon isotope value is about -35‰, carbon isotope of methane and its homologs are presented as normal carbon distribution; methylcyclohexane index is about 20%，and cyclohexane index is a little higher than 10%; the R₀ value which was calculated by methane carbon isotope，the heptane value and the isoheptane value are all in high to over mature range，which confirms that it is belong to high mature oil-gas.  The second type natural gas is characterized by rich light methane carbon isotope and rich heavy butane carbon isotope;cycloalkanes is displayed as the main peak in gas chromatogram of light hydrocarbon; cycloalkanes and branched alkanes are highly abundant in light hydrocarbon; all of the light hydrocarbon parameters (methylcyclohexane index， heptane value，K1，etc.)are fluctuated in a wide range，which confirms that it is belong to bio-degradation gas.  This research has great significance on natural gas exploration，resource evaluation and revealing the mechanism of oil and gas accumulation.

Key words: carbon isotope, light hydrocarbon, bio-degradation, Songliao Basin