Gas sources of Ordovician weathered crust gas reservoirs in Central Gas Field in the Ordos basin are an important problem that has not been completely solved. On the basis of a large number of previous research, geochemical characteristics and indexes of mixed origin natural gases i~om Ordovician weathered crust gas reservoirs in Central Gas Field in the Ordos Basin are systematically studied. The gas sources from this gas field are also discussed through the comprehensive analyses of many indexes. The results demonstrate that natural gases from Ordovician weathered crust reservoirs in Central Gas Field in the Ordos Basin have the characteristics of gas components and isotope compositions of high mature and over-mature dl7 gases. In Central Gas Field in the Ordos Basin, there are both coal-ibrmed gas and oil-type gas, the ibrmer of which comes from Permo-Carborrfierous coal measures and the latter originates from lower Paleozoic marine carbonate. The coal-ibrmed gas is mainly located in the eastern part of Central Gas Field while oil-type gas lies in the northern, western and southern parts of Central Gas Field in the Ordos Basin.

The modelling of carbon isotope kinetics of natural gas is a leading research direction in oil and gas geochemistry at home and abroad1 Combined with basin sedimentary burial and geothermal histories, the modelling of carbon isotope kinetics provides a new and effective means for evaluating the origin and accumulation history of natural gas pools1 In this paper, we briefly introduce the modelling of carbon isotope kinetics of natural gas formation and its applications to the assessment of natural gas maturity, the determination of gas source, the history of gas migration and accumulation, and the oil-gas ratio1 We show that this approach is of great value for those applications1 The carbon isotopic characteristics of natural gas are not only affected by gas source and maturity, but also are related to accumulation conditions and the geothermal gradient in a basin1 There are obvious differences in the characteristics of carbon isotope ratios between instantaneous gas and cumulative gas1 The kinetic model of carbon isotope fractionation may be different for different basins, which depends on gas source condition, accumulation history and sedimentary-tectonic history1 Since the origin of natural gas pools in the superposed basins of our country is very complicated, and because natural gas pools have features of multiphase and variable gas-sources, this paper may provide new thought s for the study and evaluation of natural gas1