Kinetic experiments of gas generation for typical samples of marine gas precursors including low-maturity kerogen,residual kerogen and oil as well as dispersed liquid hydrocarbon(DLH)in source rocks were performed by closed system,and the evolution trends of molecular and isotopic compositions of natural gases from different precursors against the maturity(R0%)at laboratory conditions were analyzed.Several diagrams of gas origin were calibrated by using the experimental data.A diagram based on the ratio of normal and isomerous butane and pentane(i/nC4-i/nC5)was proposed and used to identify the origins of the typical marine natural gases in the Sichuan Basin and the Tarim Basin, China.And the maturities of natural gases were estimated by using the statistical relationships between the gaseous molecular carbon isotopic data and maturities(δ13C-R 0 %)with different origins.The results indicate that the molecular and isotopic compositions of simulated gases from different precursors are different from each other.For example,the dryness index of the oil-cracking gas is the lowest;the dryness indices of gases from DLH and kerogen in closed system are almost the same;and the dryness index of gases from residual kerogen is extremely high,indicating that the kerogen gases are very dry;the contents of non-hydrocarbon gases in kerogen-cracking gases are far higher than those in oil-cracking and DLH-cracking gases.The molecular carbon isotopes of oil-cracking gases are the lightest,those of kerogen in closed system and GLH-cracking gases are the second lightest,and those of cracking gases from residual kerogen are the heaviest.The calibration results indicate that the diagrams of ln(C1/C2)-ln(C2/C3)andδ13C2-δ13C3-ln(C2/C3)can discriminate primary and secondary cracking gases,but cannot be used to identify gas origin sources,while the diagram of i/nC4-i/nC5 can differentiate the gases from different precursors.The application results of these diagrams show that gas mixtures extensively exist in China,which involved the ga
WANG YunPeng 1 ,ZHAO ChangYi 2 ,WANG ZhaoYun 2 ,WANG HongJun 2 ,TIAN Jin 1 ,ZOU YanRong 1 &LIU JinZhong1 1State Key Laboratory of Organic Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China
