Discovery of oil seeps in the Rukwa Rift Basin, Southwestern Tanzania: origin, thermal maturity, and palaeoredox conditions

Abstract The Rukwa Rift Basin (RRB), a segment in the Western Branch of the East African Rift System in southwestern Tanzania, has attracted attention for hydrocarbon exploration since the 1980s during regional exploration activities. Despite the discovery of significant oil reserves in other segments of the rift system in Kenya (Turkana Basin) and Uganda (Albertine Basin), no hydrocarbon accumulations have been reported in Tanzania (Rukwa Basin). Limited exploration activities were conducted in the basin in the 1980s, the results of which were inconclusive; however, preliminary data analysis indicated the presence of petroleum play elements in the basin. This study reports on geochemical analysis of newly discovered oil seeps in the RRB, using gas chromatography-mass spectrometry (GC-MS) analysis of sediments and water samples. The results reveal a distribution pattern, with the dominance of even-numbered alkanes from n-C12 to n-C32. The isoprenoid values of Pr/Ph are between 0.26 and 5, with the carbon preference indices (CPI) between 0.4 and 1.2. The ratios of Pr/n-C17 versus Ph/n-C18 indicate the dominance of kerogen type I, II, III, and mixed type II/III, with most of the samples showing source affinity to terrestrial organic matter that was preserved under oxic to suboxic conditions. The maturity level is determined to be low (methyl phenanthrene indexes <1) for all samples, with most of the samples presenting methyl phenanthrene indexes lesser than 0.6. The discovery of these oil seeps near the basin margin fault zone provides evidence of a working hydrocarbon system in the RRB.