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Upper Cretaceous mudstones are the most important source rocks in the Termit Basin, SE Niger. For this study, 184 mudstone samples from the Santonian–Campanian Yogou Formation and the underlying Cenomanian–Coniacian Donga Formation from eight wells were analyzed on the basis of palaeontological, petrographical and geochemical data, the latter including the results of Rock‐Eval, biomarker and stable isotope analyses. Samples from the upper member of the Yogou Formation contain marine algae and ostracods together with freshwater algae (Pediastrum) and arenaceous foraminifera, indicating a shallow‐marine to paralic depositional environment with fresh‐ to brackish waters. Terrestrial pollen and spores are common and of high diversity, suggesting proximity to land. Samples from the lower member contain marine algae and ostracods and arenaceous foraminifera but without freshwater algae, indicating shallow‐marine and brackish‐water settings with less freshwater influence.The wide range of gammacerane index values, gammacerane/C30 hopane (0.07–0.5) and Pr/Ph ratios (0.63–4.68) in samples from the upper member of the Yogou Formation suggest a low to moderately saline environment with oxic to anoxic conditions. In samples from the lower member, the narrower range of the gammacerane index (0.23∼0.35) and Pr/Ph ratios (0.76–1.36) probably indicate a moderately saline environment with suboxic to relatively anoxic conditions.Petrographic analyses of the Yogou Formation samples show that organic matter is dominated by terrestrial higher plant material with vitrinite, inertinite and specific liptinites (sporinite, cutinite and resinite). Extracts are characterized by a dominance of C29 steranes over C27 and C28 homologues. Results of pyrolysis and elemental analyses indicate that the organic matter is composed mainly of Type II kerogen grading to mixed Type II‐III and Type III material with poor to excellent petroleum potential. Mudstones from the upper member of the Yogou Formation have higher petroleum generation potential than those from the lower member. Mudstones in the Donga Formation are dominated by Type III organic matter with poor to fair petroleum generation potential.Geochemical parameters indicate that in terms of thermal maturity the Yogou Formations has reached or surpassed the early phase of oil generation. Samples have Tmax values and 20S/(20S+20R) C29 sterane ratios greater than 435°C and 0.35, respectively. 22S/(22S+22R) ratios of C31 homohopanes range from 0.50 to 0.54.The results of this study will help to provide a better understanding of the hydrocarbon potential of Upper Cretaceous marine source rocks in the Termit Basin and also in coeval intracontinental rift basins such as the Tenere Basin (Niger), Bornu Basin (Nigeria) and Benue Trough (Nigeria).

A preliminary sequence stratigraphic geochemical investigation was carried out to assess the areal distribution and hydrocarbon generation potential of marine mudstones of the Upper Cretaceous Yogou Formation in the Termit Basin, Niger. A total of three third‐order sequences (YSQ1, YSQ2, and YSQ3, from bottom to top) and nine systems tracts were identified in the Yogou Formation. Mudstones of each sequences are widely distributed throughout the study area with their maximum cumulative thickness of ~600–900 m in the Dinga and Moul depressions. The ΔlogR method was applied to establish a prediction model for predicting the total organic carbon (TOC) content of the Yogou Formation mudstones. The results of this source rock evaluation indicate that thick offshore mudstones with fair to good hydrocarbon generation potential and dominated Type II to Type III kerogen widely occur in YSQ1, YSQ2, and in the LST (lowstand systems tract) and TST (transgressive systems tract) of YSQ3, while the thin lower shoreface mudstone intervals in the HST (highstand systems tract) of YSQ3 are characterized by good to excellent hydrocarbon generation potential, dominated by Type II kerogen. The present‐day thermal maturation level of source beds from the YSQ3, YSQ2, and YSQ1 has overpassed the main zone of oil generation in the depocenter of Dinga Depression, whereas the YSQ3 and part of YSQ2 source beds were still within the oil window in the depocenter of Moul Depression. The organic matter richness and hydrocarbon generation potential of source beds varies within the Yogou Formation because of changes in organic matter source during evolution of Western Africa rift basins and global sea‐level fluctuations. This study provides a practical sequence stratigraphic geochemical framework which will give a better understanding for the geochemical characteristics of source rock and support improved assessment of petroleum resources.

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