Origin and Accumulation of Natural Gases in the Upper Paleozoic Strata of the Ordos Basin in Central China

The natural gases in the Upper Paleozoic strata of the Ordos basin are characterized by relatively heavy C isotope of gaseous alkanes withδ;C;andδ;C;values ranging mainly from-35‰to -30‰and-27‰to-22‰,respectively,highδ;C excursions （round 10） between ethane and methane and predominant methane in hydrocarbon gases with most C;/（C;-C;） ratios in excess of 0.95, suggesting an origin of coal-derived gas.The gases exhibit different carbon isotopic profiles for C;-C;alkanes with those of the natural gases found in the Lower Paleozoic of this basin,and believed to be originated from Carboniferous-Permian coal measures.The occurrence of regionally pervasive gas accumulation is distinct in the gently southward-dipping Shanbei slope of the central basin.It is noted that molecular and isotopic composition changes of the gases in various gas reservoirs are associated with the thermal maturities of gas source rocks.The abundances andδ;C values of methane generally decline northwards and from the basin center to its margins,and the effects of hydrocarbon migration on compositional modification seem insignificant.However,C isotopes of autogenetic calcites in the vertical and lateral section of reservoirs show a regular variation,and are as a whole depleted upwards and towards basin margins.Combination with gas maturity gradient,the analysis could be considered to be a useful tool for gas migration.