| Summary: | Tight sandstone gas belongs to unconventional natural gas. There is no unanimous agreement about the source of tight sandstone gas source. Both the Yuanba and Tongnanba gas fields in the Sichuan Basin produce tight sandstone gas from Xujiahe Formation gas reservoirs. The geochemical data such as the carbon and helium isotopes of the 14 gas samples from the Xujiahe Formation of 9 wells in the Yuanba gas field show that the gas mainly consists of coal-derived gas from the self-source type, oil-type gas from other source, and a small amount of abiogenic gas, suggesting combination gas sources. The similar geochemical data of 5 gas samples from the Xujiahe Formation of 5 wells in the Tongnanba gas field indicate that the gas consists of oil-type gas from other source, accompanied by a small amount of abiogenic gas, also being of mixed origins. Therefore the Xujiahe Formation tight sandstone gases of the Yuanba and Tongnanba gas fields do not come from coal measure alone. They have a variety of sources, mainly the coal-derived gas or oil-type gas of organic origin or the mixture of them. Key words: Yuanba gas field, Tongnanba gas field, Xujiahe Formation, tight sandstone, coal-derived gas, oil-type gas, gas source
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