The component and isotope characteristics of hydrocarbon in fluid inclusions and its affection on the gas reservoir formation: The case of Ordovician reservoir in the northwest area of Ordos basin

The characteristic and composition of hydrocarbon inclusions of calcite vein and recrystallization dolomite have been identified in gas reservoirs of Kelimo formation and the forth Majiagou formation in the Northwest of Ordos Basin. It can be considered that gaseous hydrocarbon inclusions of Ordovician reservoir in this area is rather abundant and amount to about 30% - 40%. The relative content methane is very high and range from 49.2% mol to 69.7% mol. The relative content of heavy hydrocarbons is very low. It indicates that the process of nature gas migration and accumulation had happened in the geology time. The inclusions opened by heating and carbon isotope of gaseous hydrocarbon in them were analyzed. The carbon isotope ratios of methane in inclusions range from -34.7‰ to -40‰ (average - 37.1‰) and the ones of ethane in them range from -29‰ to -31.1‰(average -30.1‰). Oil type gas is inferred from the gas carbon isotope ratio. In addition, the comparative result of simulated palaeotemperature history data and fluid inclusion homogeneous temperature distribution in this area shows that the nature gas filling period should be early Cretaceous.