Preliminary study on the origin identification of natural gas by the parameters of light hydrocarbon

The light hydrocarbon composition of 209 natural gas samples and individual light hydrocarbon carbon isotopes of 53 natural gas samples from typical humic-sourced gas and sapropelic-sourced gas in the four basins of China have been determined and analyzed. Some identification parameters for humic-sourced gas and sapropelic-sourced gas are proposed or corrected. The differences of compound-specific delta C-13 value of individual light hydrocarbon between humic-sourced gas and sapropelic-sourced gas have been founded. The humic-sourced gas has the distribution of delta C-13(benzene)> -24 parts per thousand, delta C-13(toluene) >-23 parts per thousand, delta C-13(cyclohexane) > -24 parts per thousand and delta C-13(methyl cyclohexane)> -24 parts per thousand while the sapropelic-sourced gas has the distribution of delta C-13(benzene) <-24 parts per thousand, delta C-13(toluene)< -24 parts per thousand, delta C-13(cyclohexane)< -24 parts per thousand and delta C-13(methyl cyclohexane)<-24 parts per thousand. Among the components of C-7 light hydrocarbon compound, such as normal heptane (nC(7)), methyl cyclohexane (MCH) and dimethyl cyclopentane (Sigma DMCP), etc, relative contents of nC(7) and MCH are influenced mainly by the source organic matter type of natural gas. Therefore, it is suggested that the gas with relative content of nC(7) of more than 30% and relative content of MCH of less than 70% is sapropelic-sourced gas, while gas with relative content of nC(7) of less than 35% and relative content of MCH of more than 50% is humic-sourced gas. Among components of C5-7 aliphatics the gas with relative content Of C5-7 normal alkane of more than 30% is sapropelic-sourced gas, while the gas with relative content of C5-7 normal alkane of less than 30% is humic-sourced gas. These paremeters have been suggested to identify humic-sourced gas and sapropelic-sourced gas.