Natural gas in the Bozhong sag is of organic origin. But there is an abnormal phenomenon in the carbon isotopes of the alkane gas in some gas reservoirs. Their origin is discussed in this paper. The carbon isotope of methane in the QHD30-1N-1well of the Guantao Formation is over-light obviously, which can possibly be attributed to the mixing of the high-mature gas in deep layer and low-mature gas in shallow layer or the migration and diffusion effect of natural gas. The partly reversion of alkane series carbon isotope of natural gas in the CFD18-1-1 well may be mainly caused by the mixing of gases with different types and different sources. That in BZ26-2-1 may be mainly caused by the mixing of gases from one source rock interval of varying types. While that in the BZ22-2-1ST well may be mainly caused by the mixing of same-type gases from different source rocks intervals.
Natural Gas Geoscience