Abstract

The inland basins of Nigeria comprise the Anambra basin, the lower, middle, and upper Benue trough, the southeastern sector of the Chad basin, the Mid-Niger (Bida) basin, and the Sokoto basin. Organic geochemical and organic petrologic studies indicate that coal beds constitute major potential source rocks in the whole of the Benue trough (Anambra basin inclusive). The generation and production of liquid and gaseous hydrocarbons from coal beds is presently indisputable worldwide.

In the Anambra basin, the coal beds in the Mamu Formation (Campanian–Maastrichtian) have total organic carbon (TOC) contents of as much as 60.8 wt.%, mean hydrogen index (HI) of 364 mg HC/g TOC, vitrinite reflectance (Ro) of 0.54–0.56%, and Tmax 430–433°C. Biomarker data indicate a dominance of high-molecular-weight n-alkanes, very high pristane/phytane ratios, pronounced odd-over-even predominance (OEP), a preponderance of C29 regular steranes, and relatively high contents of C28. In the middle Benue trough, the coal beds of the Turonian–Coniacian Awgu Formation have TOC contents of as much as 79.1 wt.%, Ro of 0.83–1.07%, and mean HI of 281 mg HC/g TOC, unimodal distributions of both low- and high-molecular-weight n-alkanes with no obvious OEP, a predominance of C29 steranes, and relatively high contents of C27 and C28. Coal beds from the Coniacian–lower Santonian Lamja Formation in the upper Benue trough yielded TOC contents of as much as 50.7 wt.%, with HI of 184 mg HC/g TOC, Ro of 0.70–0.73%, low- and high-molecular-weight n-alkane dominance with an unpronounced OEP, high pristane/phytane ratios, and very high contents of C29 regular steranes.

On a basinal evaluation level, incorporating source rock data from the other formations in the respective sectors, plots on the modified Van Krevelen diagram alongside biomarker and maceral data indicate good to fair source rock qualities (oil and gas) in the Anambra basin and middle Benue trough and fair to poor source rock qualities (gaseous to dry) in the upper Benue trough and the Chad basin, with sporadic good to fair source rock qualities in the Lamja Formation (coals) and shales of the Cenomanian–Coniacian Yolde, Dukul, and Pindiga Formations in that part of the Benue trough. Although TOC values and liptinite contents are relatively high in the Mid-Niger (Bida) basin, Tmax values and biomarker data show that hydrocarbons are probably just being generated in the basin and may not yet have been expelled nor migrated in large quantities.

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