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<p>The Mafia Basin, on the Tanzania’s eastern coast within the Bigwa-Rufiji-Mafia exploration block, contains a limited natural gas discovery. Assessing its hydrocarbon potential requires understanding source rock quantity, quality, and thermal evolution. This study analyzes Rock-Eval pyrolysis and gamma-ray log data from Well Q to evaluate source rock properties and constrain redox conditions during the Cretaceous–Eocene. The results revealed that the Lower Cretaceous sediments have TOC, S2 and HI values ranging from 0.13 to 0.78 wt%, 0.2 to 0.57 mgHC/g rock and 31.96 to 150.38 mgHC/g TOC, respectively. In contrast, the Upper Cretaceous sediments have TOC, S2 and HI values ranging from 0.49 to 0.54 wt%, 0.2 to 0.47 mgHC/g rock, 37 to 95.5 mgHC/g TOC, respectively. The Paleocene sediments exhibit TOC values ranging from 0.5 to 0.96 wt%, S2 values from 0.2 to 0.66 mgHC/g rock, and HI values from 29.85 to 132 mgHC/g TOC, respectively. Conversely Eocene sediments have TOC, S2 and HI values ranging from 0.23 to 0.53 wt%, S2 from 0.2 to 0.54 mgHC/g rock, and HI from 65.38 to 100 mgHC/g TOC. Cross plots of HI vs. OI and S2 vs. TOC reveal that the Lower Cetaceous, Upper Cretaceous, Paleocene, and Eocene sediments are composed of gas-prone terrestrial organic matter (Type III kerogen). The correlation between TOC and GR data reflects variations in depositional environments; however, these variations appear to have little influence on fluctuations in organic matter content and type, which are more strongly controlled by high terrigenous influxes and sea-level fluctuations, as evidenced by the dominance of terrestrial organic matter. The average Tmax and calculated vitrinite reflectance for the Lower and Upper Cretaceous sediments are &lt; 435&#xa0;°C and &lt; 0.2 ~ Rc%, respectively, indicating that the sediments are immature. Conversely, the Tmax and calculated vitrinite reflectance values for Paleocene sediments range from 410&#xa0;°C to 488&#xa0;°C and 0.22 to 1.62 Rc%, respectively, indicating a transition from immature through mature to post-mature stages. The Eocene sediments are immature, with Tmax and calculated vitrinite reflectance values ranging from 392 to 437&#xa0;°C and from − 0.06 to 0.750 ~ Rc%, respectively. This study identifies Paleocene sediments as the main source rock with fair to good generative potential, possibly extending from Mafia Island and indicating hydrocarbon charge in the Mafia Basin. The results highlight regional variability in Cretaceous, Paleocene, and Eocene source rocks driven by differences in organic matter content, quality, and thermal maturity, leading to variable source potential across sub-basins of the Tanzanian Coastal Basin and the wider East African Margin.</p>

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Source rock characterization of lower cretaceous to eocene sediments, Mafia Basin, Tanzania

  • 期刊论文

  • 发表日期：

    2026年4月8日

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  • Mohamed Mussa Kayamba,
  • John William Gama,
  • Matthew Essien Nton,
  • Meshack Kagya,
  • Emily Barnabas Kiswaka

摘要

The Mafia Basin, on the Tanzania’s eastern coast within the Bigwa-Rufiji-Mafia exploration block, contains a limited natural gas discovery. Assessing its hydrocarbon potential requires understanding source rock quantity, quality, and thermal evolution. This study analyzes Rock-Eval pyrolysis and gamma-ray log data from Well Q to evaluate source rock properties and constrain redox conditions during the Cretaceous–Eocene. The results revealed that the Lower Cretaceous sediments have TOC, S2 and HI values ranging from 0.13 to 0.78 wt%, 0.2 to 0.57 mgHC/g rock and 31.96 to 150.38 mgHC/g TOC, respectively. In contrast, the Upper Cretaceous sediments have TOC, S2 and HI values ranging from 0.49 to 0.54 wt%, 0.2 to 0.47 mgHC/g rock, 37 to 95.5 mgHC/g TOC, respectively. The Paleocene sediments exhibit TOC values ranging from 0.5 to 0.96 wt%, S2 values from 0.2 to 0.66 mgHC/g rock, and HI values from 29.85 to 132 mgHC/g TOC, respectively. Conversely Eocene sediments have TOC, S2 and HI values ranging from 0.23 to 0.53 wt%, S2 from 0.2 to 0.54 mgHC/g rock, and HI from 65.38 to 100 mgHC/g TOC. Cross plots of HI vs. OI and S2 vs. TOC reveal that the Lower Cetaceous, Upper Cretaceous, Paleocene, and Eocene sediments are composed of gas-prone terrestrial organic matter (Type III kerogen). The correlation between TOC and GR data reflects variations in depositional environments; however, these variations appear to have little influence on fluctuations in organic matter content and type, which are more strongly controlled by high terrigenous influxes and sea-level fluctuations, as evidenced by the dominance of terrestrial organic matter. The average Tmax and calculated vitrinite reflectance for the Lower and Upper Cretaceous sediments are < 435 °C and < 0.2 ~ Rc%, respectively, indicating that the sediments are immature. Conversely, the Tmax and calculated vitrinite reflectance values for Paleocene sediments range from 410 °C to 488 °C and 0.22 to 1.62 Rc%, respectively, indicating a transition from immature through mature to post-mature stages. The Eocene sediments are immature, with Tmax and calculated vitrinite reflectance values ranging from 392 to 437 °C and from − 0.06 to 0.750 ~ Rc%, respectively. This study identifies Paleocene sediments as the main source rock with fair to good generative potential, possibly extending from Mafia Island and indicating hydrocarbon charge in the Mafia Basin. The results highlight regional variability in Cretaceous, Paleocene, and Eocene source rocks driven by differences in organic matter content, quality, and thermal maturity, leading to variable source potential across sub-basins of the Tanzanian Coastal Basin and the wider East African Margin.