Source-rock evaluation of the Triassic – Jurassic interval of the Tanga Basin, coastal Tanzania

Abstract

The Triassic-Jurassic interval is known to be the main source of oil and gas generation across the world. In the Tanga Basin of Tanzania, however, the potentiality of these strata for oil and gas generation has remained poorly understood. This study presents the organic geochemical analysis of the source rock within the Triassic-Jurassic interval of the Tanga Basin, integrating rock-eval pyrolysis, vitrinite reflectance and spore coloration analyses to assess the organic richness, quality and maturity for hydrocarbon generation. The results reveal that the Triassic and Jurassic shales are organically rich, with average total organic carbon (TOC) values of 1 wt % and 2 wt %, respectively. Triassic shales demonstrate the dominance of kerogen types II (oil), III (gas pone), II/III (gas with minor oil), and IV (inert) as indicated by hydrogen index values between 365 and 15 mg HC/g TOC. The hydrogen index (HI) values in the Jurassic interval range from 87 to 162 mg/HC/TOC, suggesting the dominance of kerogen type III (gas prone). Pyrolysis Tmax, vitrinite reflectance (Ro) and spore color index (SCI) indicate that the Triassic interval is thermally mature into a gas generation window, with average Tmax values of 450 °C, Ro ∼ 1.7% and SCI ∼8.5. The Jurassic interval is also revealed as thermally mature, with average values of Tmax ∼ 452 °C, Ro ∼ 0.7% and SCI ∼ 7.2, indicative of oil window. The positive findings of this study lay out an important foundation to follow up studies and extended exploration for hydrocarbon accumulations in the poorly explored Tanga Basin and across the coastal Tanzania in general.