Geochemical characteristics of natural gas of different gas-bearing assemblages in Upper Paleozoic, eastern Ordos Basin

Based on gas molecular and stable carbon isotope analyses of 110 gas samples collected from the Upper Paleozoic in the eastern Ordos Basin,it has been concluded that the lower gas-bearing assemblages such as the Benxi,Taiyuan and Shanxi Formations mainly contain dry gas characterized by high methane content,low C₂₊ alkanes content,high dryness and relatively high non-hydrocarbon content.The He8 gas pools of the middle gas-bearing assemblages mainly contain wet gas characterized by 2 peaks for dryness(0.93-0.94,0.98-0.99) as well as high C₂₊ alkanes content and low non-hydrocarbon content.The Qian5 gas pools of the upper gas-bearing assemblages are characterized by the highest methane content,relatively lower C₂₊ alkanes content,the lowest non-hydrocarbon content and the dryness ranges from 0.96 to 0.99.Carbon isotope values of methane are heavy among different gas-bearing assemblages and those of ethane and propane are in accordance with typical coal gas.Carbon isotopes of kerogen,desorption hydrocarbons of source rocks and cap rocks indicate that the source rocks have the characteristics of typical coal source rocks.Natural gas of different gas-bearing assemblages generates from underlying humic sources.