Abstract
The Patti Formation which consists of sandstone and shale offers the best potential source beds in the Bida Basin. This inland basin is one of the basins currently being tested for hydrocarbon prospectivity in Nigeria. Fresh samples of shale from Agbaja borehole, Ahoko quarry and Geheku road cut were analysed using organic geochemical and palynological techniques to unravel their age, paleoecology, palynofacies and source bed hydrocarbon potential. Palynological data suggest Maastrichtian age for the sediments based on the abundance of microfloral assemblage; Retidiporites magdalenensis, Echitriporites trianguliformis and Buttinia andreevi. Dinocysts belonging to the Spiniferites, Deflandrea and Dinogymnium genera from some of the analysed intervals are indicative of freshwater swamp and normal sea conditions. Palynological evidence further suggests mangrove paleovegetation and humid climate. Relatively high total organic carbon TOC (0.77–8.95 wt%) was obtained for the shales which implies substantial concentration of organic matter in the source beds. Hydrocarbon source rock potential ranges from 0.19 to 0.70 mgHC/g.rock except for a certain source rock interval in the Agbaja borehole with high yield of 25.18 mgHC/g.rock. This interval also presents exceptionally high HI of 274 mgHC/g.TOC and moderate amount of amorphous organic matter. The data suggests that in spite of the favourable organic matter quantity, the thermal maturity is low as indicated by vitrinite reflectance and Tmax (0.46 to 0.48 Ro% and 413 to 475 °C, respectively). The hydrocarbon extracts show abundance of odd number alkanes C27–C33, low sterane/hopane ratio and Pr/Ph > 2. We conclude that the source rocks were terrestrially derived under oxic condition and dominated by type III kerogen. Type II organic matter with oil and gas potential is a possibility in Agbaja area of Bida Basin. Thermal maturity is low and little, or no hydrocarbon has been generated from the source rocks.
Highlights
The present study is focused on the source rock intervals of Patti Formation in the inland Bida Basin of Nigeria (Fig. 1)
The results show that the source rocks from the Agbaja borehole are richer in organic matter concentration and good source rocks, while source rocks from Ahoko and Geheku are lean in organic matter
The shales from Agbaja and Ahoko sections are dated Maastrichtian and are stratigraphic equivalents. They are older than Agbaja Ironstone and were deposited in a largely non-marine swamp environment inundated by a short-lived sea transgression
Summary
The present study is focused on the source rock intervals of Patti Formation in the inland Bida Basin of Nigeria (Fig. 1). The age of the source beds, paleodepositional environments, paleoredox condition, type and maturity of the organic matters are investigated in this study This is expected to provide platform for more baseline data and advance detail source rock potential evaluation in view of on-going interest of the Federal Government in testing petroleum potential of inland basins in Nigeria and the overarching goal of discovering commercial hydrocarbon plays in the in land basins. Exploration for hydrocarbon in a frontier basin like this requires deep understanding of the stratigraphy and depositional environments of the sedimentary succession, while realistic prediction of the source rocks depends on accurate reconstruction of their depositional and burial history. Given this scenario, knowledge of the source rock characteristics and stratigraphy would assist in hydrocarbon prospecting. This present work is significant because new organic geochemical and palynological data are generated from the Agbaja plateau axis of the basin
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