Richness, types, and thermal maturation of organic matter of Jurassic to Eocene rocks of the offshore Coastal Basin, Tanzania

The organic matter richness, kerogen types, and maturity of offshore rocks in coastal Tanzania remain poorly understood, despite extensive research on onshore formations. This knowledge gap hinders a comprehensive understanding of the region’s hydrocarbon potential. To address this, HAWK pyrolysis and vitrinite reflectance analyses were conducted on Jurassic to Eocene rock intervals from the offshore wells Mafia Deep-1 and Songo Songo-5. The findings indicate the Jurassic formations exhibit significant organic matter richness, with Total Organic Carbon (TOC) values ranging from 0.29 to 1.63 wt.%, Hydrogen Index (HI) values between 28 and 129 mg HC/g TOC, and vitrinite reflectance (%Ro) from 0.7 to 0.8, suggesting fair to good organic richness, Type III kerogen, and maturity within the oil window. In the Lower Cretaceous rocks of Songo Songo-5, TOC levels vary from 0.2 to 11.07 wt.%, HI from 6 to 140 mg HC/g TOC, and %Ro between 0.57 and 0.7, indicating fair, good to very good organic richness, Type III kerogen, and oil window maturity. Conversely, the Lower Cretaceous rocks of Mafia Deep-1 exhibit TOC levels from 0.06 to 0.78 wt.%, with HI values peaking at 237 mg HC/g TOC and Tmax > 440 °C, implying poor to fair organic richness, Type III kerogen, and maturity within the oil window. The Upper Cretaceous rocks in Mafia Deep-1 show TOC levels from 0.09 to 0.49 wt.%, HI values ranging from 3 to 96 mg HC/g TOC, and Tmax > 430 °C, indicating poor organic richness, Type III kerogen, and oil window maturity. Similarly, the Lower Cretaceous rocks in Songo Songo-5 display TOC levels from 0.2 to 0.6 wt.%, HI values between 20 and 63 mg HC/g TOC, and %Ro from 0.45 to 0.6, suggesting poor to fair organic richness, Type III kerogen, and immaturity to early maturity. Paleocene rocks in Mafia Deep-1 have TOC values ranging from 0.19 to 0.59 wt.%, HI values from 17 to 63 mg HC/g TOC, and Tmax > 430 °C, indicating poor organic richness, Type III and IV kerogen, and oil window maturity. In Songo Songo-5, Paleocene rocks exhibit TOC levels from 0.2 to 0.6 wt.%, HI values below 50 mg HC/g TOC, and %Ro averaging 0.43, indicating poor to fair organic richness, Type III and IV kerogen, and immaturity. Eocene rocks in Songo Songo-5 show TOC levels between 0.24 and 41 wt.%, HI values up to 121 mg HC/g TOC, and %Ro averaging 0.35, indicating varying organic richness, Type III kerogen, and immaturity. In contrast, Eocene rocks in Mafia Deep-1 have TOC levels between 0.2 and 0.3 wt.%, HI values below 100 mg HC/g TOC, and Tmax ~ 430 °C, indicating poor organic richness, Type III and IV kerogen, and immaturity to maturity. In general, the Upper Cretaceous formations and Paleocene within both wells exhibit limited capabilities for hydrocarbon generation. Conversely, the Jurassic, Lower Cretaceous, and Eocene formations demonstrate significant genetic potential, particularly for gas generation, ranging from fair, good to excellent having S2 values as high as 48.2 mg HC/g rock. This highlights the greater potential of offshore rocks on the contribution to the hydrocarbon reserves. These findings suggest that Jurassic rocks and Lower Cretaceous and in offshore areas serve as good source rocks, exhibiting maturity within the oil window while Eocene is immature possessing good to excellent genetic potential for gas generation, warranting further exploration activities to fully assess the hydrocarbon potential across the whole offshore Tanzania.