There are different genetic types of natural gases in the deep strata of Songliao Basin, e.g.coal-derived gas, oil associated gas, oil cracking gas, abiogenic alkane gas and CO2. Itprovides an excellent place for studies on mechanisms of genetic types. Three mechanismsare bringing forward in this paper. Firstly, at high thermal evolution stage, the very-low grademetamorphic can produce certain amount of gases, and the carbon isotopes of heavyhydrocarbons (C1+) are reversed at single thermal agent. Secondly, by setting up theradiogenic helium calculation model, the validity of the parameters of CO2/3He and CH4/3Heare confirmed. According to the relationship of R/Ra and CO2/3He, R/Ra and CH4/3He, itverified that some the abiogenic methane are transformed from CO2 by Fischer-Tropschsynthesis. Thirdly, through the evolutionary mode of CO2/3He and CO2, the typical abiogenicmethane from Fangshen 2 was resulted from dramatically lose of CO2. The gas generationintensity of the basement is as 1/20 times as the main source rock is, and the most probableexplored reserves of abiogenic CO2 and CH4 can reach 1663×l08m3 and 500×108m3respectively. Consequently, the target for natural gas exploration in Xujiaweizi fualutdepression should be aimed at the thermalgenic methane beacuse the most probable exploredreserves of it are formed by the main source rocks in this area, which is 7437×108m3. Theclarify of different genetic types of natural gases, their contributions for reserves and theirdistributions, all these results will be a important guiding for natural gas exploration in thedeep strata.
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