In order to distinguish the source and migration direction of natural gas by geochemical
characteristics of butane, the components and carbon isotopes of natural gas from major hydrocarbonbearing
basins in China were analyzed. The results showed that: (1) Oil-type gas has i-C4/n-C4<0.8,
δ¹³C_butane <–28‰, δ¹³C_i-butane <–27‰, δ¹³C_n-butane<–28.5‰, whereas coal-type gas has i-C4/n-C4>0.8,
δ¹³C_butane >–25.5‰, δ¹³C_i-butane>–24‰, δ¹³C_n-butane >–26‰. (2) When δ¹³C_i-butane–δ¹³C_n-butane is greater than 0,
the maturity of oil-type gas is generally more than 2.4% and that of coal-type gas is greater than 1.4%,
whereas when the difference is less than 0, the maturity of oil-type gas is generally less than 1.1% and
that of coal-type gas is less than 0.8%. (3) When natural gas migrates through dense cap rocks, the
value of i-C4/n-C4 increases, whereas when it migrates laterally along a reservoir, the value of i-C4/n-C4
decreases. (4) Sapropelic transition zone gas with composition and carbon isotopic signatures similar to
those of oil-type gas in the low thermal evolution stage is found to have a relatively high butane content.
(5) The values of i-C4/n-C4 andδ¹³C_i-butane–δ¹³C_n-butane of gas which has suffered biological degradation
are significantly higher than those obtained from thermogenic and bio-thermocatalytic transition zone
gas. Thus, natural gas of different genetic types can be recognized through component analysis and
carbon isotopic signatures of butane, the natural gas maturity can be estimated from the difference in
carbon isotopic content between isobutane and n-butane, and the migration direction of natural gas can
be determined from i-C4/n-C4 ratios and transport conditions, which can also be used to thermogenic and
bio-thermocatalytic transition zone gas.