Geochemical characteristics and genesis of high sulfur natural gas in the fourth member of Majiagou Formation in the Ordovician sub-salt reservoirs,Ordos Basin
A major breakthrough has been made in the exploration of subsalt natural gas in the Ordos Basin.High-sulfur natural gas content has been gradually drilled in the fourth member of Majiagou Formation.The geological and geochemical characteristics indicate that its genesis and source are still unclear.Natural gas samples from the fourth member of Majiagou Formation,shale gas from Wulalik Formation and Upper Paleozoic tight sandstone gas were collected systematically.The composition and carbon-hydrogen isotopes composition of natural gas were determined,and the genesis and source of natural gas were studied in combination with the geological background in the central and eastern parts of the basin.The results showed that the natural gas in the fourth member of Majiagou Formation is dry gas dominated by methane.The average drying coefficient was 0.97,the average ethane content was 1.07%,the average content of propane was 0.32%,and the average content of heavy hydrocarbon gas was 0.38%.The average concentration of hydrogen sulfide is 124 774×10-6,which is the product of TSR,which is obviously different from the Upper Paleozoic tight sandstone gas and the shale gas of Wulalik Formation.The content of hydrogen sulfide gradually decreases from west to east in the research area.The carbon isotopic composition of methane in the natural gas of the fourth member of Majiagou Formation ranges from-44.8‰to-36.4‰,with an average value of-40.3‰.The carbon isotopic composition of ethane ranges from-27.7‰to-19.1‰,with an average value of-24.3‰.The hydrogen isotopic composition of methane ranges from-172‰to-140‰,with an average value of-162‰.The isotopic composition of ethane hydrogen ranges from-140‰ to-112‰,with an average of-119‰.The natural gas in the fourth member of Majiagou Formation has a light carbon isotope composition of methane and a heavy hydrogen isotope composition of methane,indicating that the natural gas in the fourth member of Majiagou Formation has a different gas source from the Upper Paleozoic coal-type gas.Using the carbon-hydrogen isotopes composition of methane as the main identification parameters of high sulfur natural gas,it is identified that natural gas in the fourth member of Majiagou Formation is characterized by"self-generation and self-accumulation".The gas source rock is a marine source rock developed in the gypsum-bearing strata of the Majiagou Formation.TSR promotes the conversion of hydrocarbons to non-hydrocarbons,the conversion of heavy hydrocarbon gas to methane,and also leads to changes in the carbon isotopic composition of hydrocarbons.The carbon isotope composition of ethane in some gas reservoirs is significantly heavier than that of kerogen,which is beyond the influence range of thermal evolution,highlighting the great influence of TSR on ethane carbon isotope.Based on the geological background of the formation and evolution of the Majiagou Formation subsalt natural gas,it is estimated that TSR can cause a change in the carbon isotope composition of ethane from 5‰to 10‰.
Ordos BasinMajiagou FormationGenesis of natural gasCarbon-hydrogen isotopes compositionHydrogen sulfide
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