Gases with different origins occur in the Xujiaweizi Fault Depression. Among them, the abiogenic gases have a typical carbon isotopic reversal trend (δ13C1>δ13C2>δ13C3>δ13C4), the carbon isotope of methane is greater than-30‰, and the R/Ra greater than 0.5. Thus it is argued that the abiogenic gases in the Xujiaweizi Fault Depression can not simply be a result of two-source mixing. The mixing of coal-derived and oil-derived gases can not lead to the carbon isotopic reversal trend, which provides further evidence for the abiogenic origin of the deep-layer gas.