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Nigerian Cretaceous coal deposits and their petroleum source rock characteristics (Published)

Some Nigerian Cretaceous coal deposits have been mapped and characterized for their petroleum source rock potential based on Rock Eval pyrolysis. These coal deposits which occur entirely within the Anambra Basin/Benue Trough have been mapped at Owukpa, Okaba, Ogboyaga, Omelehu, Lafia-Obi and at Gombe-Kumo. Some of these coal deposits lie deep down at great depths in the Anambra Basin and Benue Trough where they constitute active source rocks. Stratigraphically, the coals in the Anambra Basin and Upper Benue Trough are deposited within fluvio-deltaic Maastrichtian succession of the Mamu and Gombe Formations, respectively, while those in the Middle Benue Trough are embedded in Turonian-Coniacian paralic-shallow marine facies of the Awgu Formation. Although not studied here, Turonian-Coniacian coal deposits also occur in the Lower Benue Trough at Amansiodo and in the Upper Benue Trough at Lamza. Rock Eval pyrolysis results on the coal deposits show Hydrogen Index values of between 206-283mgHC/gTOC for the Anambra Basin coals with Tmax in the immature to sub-mature zone (419-428oC) while the coals in the Middle Benue Trough gave HI values of 54-58mgHC/gTOC and Tmax values that show maturity within the main oil window. Results obtained on the Gombe-Kumo coals are similar to those of the Anambra Basin. Interpretation of the results using the Van Krevelen and HI-Tmax diagrams indicate that the coals as source rocks are characterized by Type II/III kerogens for the Maastrichtian deposits, oil and gas prone and generally sub-mature with respect to hydrocarbon generation at the present depth of study. The coals in the Middle Benue Trough are characterized by Type III/Type IV kerogens, gas prone and mature with respect to hydrocarbon generation. At greater depths, the Middle Benue Trough coals would have completed generation and reached over-cooked facies while the Maastrichtian coals would have entered the main oil window except the Gombe coals which are stratigraphy too shallow.

Keywords: Nigerian, cretaceous coal deposits, petroleum source, rock characteristics