Origins of partially reversed alkane δ13C values for biogenic gases in China

With increasing molecular weight, δ13C values of hydrocarbon gases change in two different manners: the normal order would be δ13C1<δ13C2<δ13C3<δ13C4, whereas the reversed order would be δ13C1>δ13C2>δ13C3>δ13C4. Partially reversed order is common in gas samples from sedimentary basins in China, which can be attributed to one or several of the following four mixing processes: (a) mixing of biogenic and abiogenic gases; (b) mixing of sapropelic and humic sourced gases; (c) mixing of gases from the same types of source rocks with different maturity; and (d) mixing of gases from the same source rock interval of varying maturity.