Origin and Source Characterization of Methane in the Shallow-Water Environment of Southern Lake Tanganyika Rift Basin, Tanzania

Lake Tanganyika, located in the western part of Tanzania between 3°S and 9°S lat, harbours hydrocarbon and non-hydrocarbon gases in its northern and southern shallow-water environment. In this study, a geochemical analysis of stable carbon and hydrogen isotopes and an interpretation of individual gas molecular composition were done in order to determine the origin and composition of the naturally occurring hydrocarbon gases in Tanganyika Basin. Nitrogen, a non-hydrocarbon gas is a major component (76.69%–78.31%) by volume percentage followed by methane (11.68%–12.94%) and other higher hydrocarbons (0.16%–1.63%). The isotopic composition of carbon δ¹³C₁ and hydrogen δ DC₁ ranges from –65.32‰ to –65.81‰ and –272.5‰ to –275.9‰ respectively. The isotopic compositions of ethane (δ DC₂ = –36.7‰ to –35.2‰) and propane (δ DC₃ = –31.3‰ to –27.5‰) reflect the thermogenic origin of these higher hydrocarbons. According to molecular characterization of carbon and hydrogen isotope ratios and δ D-values, methane gas falls in the biogenic origin category and is formed by carbon dioxide reduction. The isotopic composition of CO₂ varies between –8.6‰ and –3.4‰. CO₂ reduction is also regarded as a mechanism of biogenic methane formation based on carbon isotope fractionation factors (greater than 0.16).