Study On Carbon Isotopic Evolution Of Marine Natural Gas

In this study,six types of marine source rocks from different regions were selected to simulate hydrocarbon generation and expulsion from different systems and TSR reactions.The gas compositions and carbon isotopes of the gas products in different diagenetic stages were analyzed,and the evolution characteristics and fractionation degree of the gas compositions and stable carbon isotopes of the different types of marine source rocks in different diagenetic stages were compared.The experiments deliver the following results.1)The composition of marine natural gas changes regularly during thermal evolution.When the equivalent vitrinite reflectance VRo<1.0%,the content of methane is low and the dryness coefficient is small.When VRo>1.0%,the methane content began to rise obviously,and the dryness coefficient increase substantially.2)The carbon isotopic values of gas obtained in the semi-closed and semi-open system simulation experiment with the same maturity are lighter than those in the closed system simulation experiment,which indicates that the degree of carbon isotopic fractionation of natural gas is related to the boundary conditions set in the simulation experiment.3)The carbon isotope fractionation characteristics of gas obtained from the simulation experiments of semi-closed and semi-open systems are related to the efficiency of oil expulsion.In the oil generation stage,the carbon isotopic value of the residual oil in the source rock becomes heavier and heavier due to the higher oil expulsion efficiency,which leads to the heavier carbon isotopic value of the hydrocarbon gas generated from the residual oil at high evolution stage.4)Carbon isotopic value “reversal” does occur in the natural gas directly generated from the same source rock.5)The thermochemical sulfate reduction reaction preferred to consume heavier alkanes,which makes the natural gas drier and the carbon isotopes of methane become heavier,thus accelerating the fractionation of carbon isotopes of hydrocarbon and gas.6)The carbon isotopic values of methane and ethane in marine source rocks all become lighter first and then heavier with the increase of maturity,and the turning point occurs at the peak of primary oil generation,while the carbon isotopic values of kerogen,the parent material of hydrocarbon generation,remain basically unchanged.7)The relative fractionation degree between the carbon isotopes of natural gas and the kerogen isotopes of the parent material was measured by the relative fractionation coefficient of isotopes.The relative fractionation coefficients of methane and ethane of typical marine source rocks decrease regularly with increasing maturity in the stage of abundant gas generation(VRo>1.0%).Based on this,the regression equations of the relative fractionation coefficients of ethane of different types of marine source rocks are established.The regression equation of relative fractionation coefficient of ethane in different types of marine source rocks is established.The correlation coefficient between relative fractionation coefficient of ethane and maturity is higher than 0.90.This suggests that it is possible to use this evolution trend to trace the source gas of high-over-mature marine natural gas reservoirs.