Abstract: | Molecular nitrogen (N2) content of marine saporpelic ytpe gasesderived from Lower Paleozoic source rock is high, ranging from 1.61~ 36.2 percent of molecular nitrogen . N2 content of wet gases ranges from 10.1% to 36.2 %, while dry gases less than 10 %, it is to say that N2 content of wet gases is higher than that of dry gases. Why is there large difference of N2 content between wet gases and dry gases derived from the same type source rock? What relationship it has with gas migration and accumulation ? Based on the composition and isotope geochemical characteristics ofassociated gases and non-hydrocarbon gases, as well as rare gas in nature gas , N2 genesis and origin are ascertained in this paper. The study shows that natural gases of middle-high N2 content belong to organic genesis, which is originated from lower Paleozoic marine carbonate source rock. It is indicates that N2 content difference between wet and dry gas has close relationship to source rock, maturity and natural gas accumulation history. For the same type source rock, N2 content in gas is controlled by source rock maturity and entrapment conditions . Because of the difference of nitrogen existence form in organic matter of carbonate and mudstone or shale source rock, nitrogen is mainly combined in kerogen by chemical bond for carbonate source rock, but for mudstone or shale source rock, besides chemical bond, nitrogen is bonded in clay mineral by ion substitution form during organic matter maturation. Thermal simulation shows that nitrogen release mechanism is different for the different source rock. N2 release capacity reachs peak at the generation peak for carbonate source rock, N2 content of gases decreasing at the high-super maturity stage. While for mudstone or shale source rock, N2 release capacity reachs peak at the high-super stage. Therefore, N2 content of natural gas entrapped at different maturity stage of source different, N2 content variation in natural gases can be used to reveal natural gas accumulation history, and also to evaluate trap effectiveness. Lower N2 content of dry gas in Tabei area demonstrates that oil and gas containing higher N2 content at the generation peak have been lost, and that natural gas captured in the present traps is mainly originated from high-super maturity source rock,The above conclusion is supported by heavy methane carbon isotope and large dryness of the gas, and also supported by geologic conditions of Tabei area. Higher N2 content and lighter methane carbon isotope of wet gas in Tazhong area shows that gas maturity is lower than that of dry gas in most part of Tabei area, natural gas with high N2 content formed by source rock at the generation peak is captured by trap. |