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Recently abiogenic alkanes have been found in various locations in the world and other celestial bodies. The chemical composition of abiogenic alkane gases varies widely. The content of methane is low and nearly no C₂₊ is found in the abiogenic alkane gases from fluid inclusions in volcanic rocks or hot springs in China. In the unsedimented submarine hydrothermal vent system C₁/C₂₊ ratios are much greater than those for the thermogenic gases, mostly >800 and in some cases up to 8,000. In the Songliao Basin, China, C₁/C₂₊ of some abiogenic gases are often less than 150. Abiogenic alkane gases which have been found in nature often have carbon isotopic reversal among C₁-C₄ alkanes (δ¹³ C₁>δ¹³ C₂>δ¹³ C₃>δ¹³ C₄), whereas both regular and reversed hydrogen isotope distribution pattern among C₁-C₄ alkanes have been reported. The δ¹³ C of abiogenic methane is mainly greater than -30‰ through laboratory synthesized methane can have δ¹³ C as low as -57‰, and its δD₁ values vary widely and overlap with biogenic gases. High ³He/⁴He ratios often indicate the addition of mantle-derived helium and are related to abiogenic gases. However, some biogenic gases can also have high ³He/⁴He ratios up to 8. The CH₄/³He end-member is often lower than 10⁶ for abiogenic alkane gases while greater than 10¹³ for biogenic gases, and the values between these two end-members often reflect the mixing of biogenic and abiogenic gases.