THE TWO-STAGE FRACTIONATION MODEL OF METHANE CARBON ISOTOPE FROM THE THERMAL SIMULATION EXPERIMENTS OF SOURCE ROCKS IN THE KUQA DEPRESSION OF THE TARIM BASIN

Thermal simulation experiments of hydrocarbon generation on coal,mudstone and carbargilite samples of Triassic and Jurassic from the Kuqa Depression in the Tarim Basin were carried out through the temperature from 260 to 540℃ with 40℃ as increment.Carbon isotope of the generated gas and vitrinite reflectance(Ro) of post-experiment solid product were measured.The relationship between carbon isotopic values of methane in gaseous hydrocarbon and maturity of solid products was also obtained.The results of the experiment showed that there is two-stage model of isotopic fractionation during source rock evolution.During immature to mature phase(Ro1.5%),carbon isotopic values of methane became lighter with the increase of temperature(maturity).On the contrary,during mature to overmature phase,it became heavier with the increase of temperature(maturity).However,both in the above two phases,carbon isotopic values of methane and maturity were in linear relation.