Application of butane geochemistry of natural gas in hydrocarbon exploration

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-C-4/n-C-4 < 0.8, delta C-13(butane) <-28aEuro degrees, (delta) C-13 (i-butane) <-27aEuro degrees, (delta) C-13 (n-butane) <-28.5aEuro degrees, whereas coal-type gas has i-C-4/n-C-4 > 0.8, (delta) C-13(butane) >-25.5aEuro degrees, (delta) C-13 (i-butane) >-24aEuro degrees, (delta) C-13 (n-butane) >-26aEuro degrees. (2) When (delta) C-13 (i-butane)-(delta) C-13 (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-C-4/n-C-4 increases, whereas when it migrates laterally along a reservoir, the value of i-C-4/n-C-4 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-C-4/n-C-4 and (delta) C-13 (n-butane) (delta) C-13 (i-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-C-4/n-C-4 ratios and transport conditions, which can also be used to thermogenic and bio-thermocatalytic transition zone gas.