Hydrocarbon potential of shales and lignites of Oligocene-Miocene Ogwashi-Asaba formation exposed in Issele-Uku of Anambra Basin, Southern Nigeria as both conventional and unconventional resources

Abstract

Hydrocarbon potentials of lignites and shales of Oligocene-Miocene Ogwashi-Asaba Formation outcropping along springs at Issele-Uku and Azagba-Ogwashi areas of Anambra Basin, Southern Nigeria were investigated through total organic carbon (TOC), Rock-Eval pyrolysis and organic petrology analyses. Lignite has an average TOC of 47.20 wt.% and shale of 9.15 wt.%, suggesting good to excellent source rocks. Sporinite, resinite, and humodetrinite are the dominant sub-maceral. Ratios of average huminite, liptinite, and inertinite maceral group (H:L:I) in lignite from Nnem-Agadi, Iyiodo, Mgbala, and Nkpunkpa are 55:35:10, 66:24:10, 62:25:13, and 58:30:12, respectively, while for shales in Nnem-Agadi is 57:28:15 and Iyiodo is 55:20:15. Average hydrogen index (HI) of lignite is 436 mgHC/gTOC, and shale is 218 mgHC/gTOC. HI of 783 mgHC/gTOC in one lignite from Nnem-Agadi indicates Type I oil-prone kerogen typical of algae-sourced organic matter, whereas Type II oil and gas kerogen is present in both lignites and shales. Type III gas-prone kerogen is found only in shale. Genetic potential of 106.95 kgHC/t) in lignites and 20.87 kgHC/t in shales indicate excellent and moderate oil and gas potentials. All the source rocks are immature at the current outcrop positions but could have possibly attained maturity in the deeper subsurface area of the basin and contributed to the petroleum systems in the Anambra Basins. Ogwashi-Asaba formation has similar source rocks and organic matter types with the ones generating mixed marine and terrestrial oil within Agbada Formation, in the Niger Delta Basin. These source rocks are significant Oligocene-Miocene conventional and unconventional hydrocarbon resources depending on their current depth in the rift basins in West and Central Africa.