Characteristics of Carbon Isotope and Origin of Natural Gas in Qaidam Basin

Key words: 
• natural gas /
• carbon isotope /
• genesis /
• Qaidam Basin

Abstract: δ13C1，δ13C2,δ13C3，δ13C4 of〖KG*2〗natural gas isotopic composition are various in different parts of Qaidam Basin. The average〖KG*2〗of them are －23.1‰,－24.1‰,－22.7‰ and〖KG*2〗－23.6‰ in Sanhu area around; －29.8‰,－24.7‰,－23.1‰ and〖KG*2〗－22.9‰ in Chaibeiyuan area;〖KG*2〗－39.2‰,－26.8‰,－23‰ and〖KG*2〗－23‰ in Chaixi area;〖KG*2〗－67.8‰,－39.2‰，－32.3‰ and lack of〖KG*2〗δ13C4 in Sanhu area. δ13C4>δ13C3〖KG*2〗>δ13C2〖KG*2〗>δ13C1 exist between〖KG*2〗component of most natural〖KG*2〗gas isotope, and reversion〖KG*2〗of minor heavy carbon isotopic composition exist. Reginally,δ13C1，δ13C2,δ13C3，δ13C4 of〖KG*2〗natural gas isotopic composition〖KG*2〗decrease gradually from〖KG*2〗Sanhu area around,〖KG*2〗Chaibeiyuan area and Chaixi area to Sanhu area.〖KG*2〗δ13C2-δ13C1〖KG*2〗andδ13C3-δ13C2 of〖KG*2〗natural gas isotopic composition increase gradually from〖KG*2〗Sanhu area around,〖KG*2〗Chaibeiyuan area and Chaixi area to Sanhu area, the average〖KG*2〗of them are －0.8‰ and〖KG*2〗－0.9‰ in〖KG*2〗Sanhu area around, 5.1‰ and 1.3‰ in Chaibeiyuan area，14.5‰ and 2.3‰ in Chaixi area， 27.9‰ and 12.5‰ in Sanhu area.
δ13C4>δ13C3〖KG*2〗>δ13C2〖KG*2〗>δ13C1 exist between〖KG*2〗component of most natural〖KG*2〗gas isotope because natural〖KG*2〗gas of organic geneses are derived from the similar process of thermodynamic action, and reversion〖KG*2〗of minor heavy carbon isotopic composition exist because of the mixed action of natural gas from the different derivation or the difference of duration, temperatures and pressures〖KG*2〗in developing hydrocarbon process from the same source rock. The difference between δ13C2-δ13C1〖KG*2〗and δ13C3-δ13C2〖KG*2〗in different area depend on duration, temperatures and pressures〖KG*2〗of source rock evolution, the higher temperatures and pressures〖KG*2〗and the longer duration of source rock evolution, the more uniform〖KG*2〗δ13C distribution between natural〖KG*2〗gas component. In the meantime, bacteria and types of kerogen have an effect on δ13C2-δ13C1〖KG*2〗andδ13C3-δ13C2 to some extent. The study on carbon isotope geochemical characteristic. can not only divide types of natural〖KG*2〗gas and discuss〖KG*2〗their geneses in theory, but also guide the exploration of natural〖KG*2〗gas.