Geochemical characteristics of abiogenic alkane gases

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(δ13C1>δ13C2>δ13C3>δ13C4),whereas both regular and reversed hydrogen isotope distribution pattern among C1-C4 alkanes have been reported.The δ13C of abiogenic methane is mainly greater than-30‰ though laboratory synthesized methane can have δ13C 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.