Abstract: The Bongor Basin, located in the northwest part of the Central African Shearing Zone in Chad, is one of the most proliferous basins of the MesozoicCenozoic era. In this study, chemical composition and carbon/hydrogen isotopic analysis were performed on natural gas samples by routine geochemical methods to investigate the origin and maturity difference of natural gas in different structural units of the Bongor Basin. The results showed that the chemical compositions of the natural gas analyzed are mainly hydrocarbon and nonhydrocarbon gases with trace amount of rare gases. Hydrocarbon gases belong to organic gas, such as biogenetic and thermal cracking gases, while inorganic gases included N2, CO2 and H2, etc. Gasgas and gassource correlations were obtained to determine the organic source of the natural gas. It showed that the minor composition dissimilarity in these gases are attributable to facies variations in different structural units, namely the M, P and possibly K group dark clay shale facies; whereas gases generated by organic source rocks in different structural units exhibit large difference in maturity, simply because they were generated in different thermal evolution stages.

Key words: Bongor Basin, isotope, natural gas, maturity, genetic type, Chad