Petrological, palynological analysis and Geochemistry of Maastrichtian Patti Shale in some parts of the southern Bida Basin, Nigeria: Implications for provenances and hydrocarbon studies

Abstract

Godwin Okumagbe Aigbadon , Azuka Ocheli2,  Tope Shade Alege1, Esther Onozasi David

The Patti Formations' shale and mudstones are the principal petroleum source rock of the Lokoja sub-Basin. Though only a
few works on Patti Shales in the Ahoko outcrops have been published, no significant work has been done in the Agbaja-Ibaru region of the basin. Arising from the need to analyze the provenances and hydrocarbon potentials of the shales of the Patti Formation in the basin, this study used petrological, palynological, and geochemical analyses. This study will employ palynological, petrological (Scan Electron Microscope, SEM and X-ray diffractogram, XRD, X-ray fluorescence spectroscopy, XRF),
and geochemical techniques to assess the sources of the basin's rock potential and depositional environment. Field observations
results revealed the lithology of the Patti Formation to comprise siltstone, claystone, mudstones, and dark shales. According to the XRF results, the primary elemental oxides are SiO₂, Al₂O₃, and Fe₂O₃. The X-ray diffraction (XRD) results for all shale samples examined revealed that the principal mineral components are quartz (32-49%), kaolinite (26-38%), and illite (0.2-5.26%). The shale is classified as quartzose shale because it has a high amount of quartz. The diagnostic palynoflora association showed the occurrence of Cingulatisporites ornatus, Tricolporopollenites sp., Monocolpites marginatus, Cyathidites minor, Distaverrusporites simplex, Longapertites sp., Rugulatisporites apparatus (Campanian), Tubistephanocolpites cylindricus, Longapertites microfoveolatus, Echitriporites trianguliformis, Cyathidites minor and Gleicheniidites sp., indicating Maastrichtian age and predominantly Continental (Terrestrial) environment of deposition. Total organic carbon (TOC) levels range from 0.46 to 2.56%, with a mean of 1.15%. The genetic potential (GP) ranges from 0.162 to 1.235Mg/g, with a mean of 0.619Mg/g, indicating a moderate to relatively good source of the gas-rich rock. The hydrogen index (HI) varies from 7.8 to 173.9 mgHC/gTOC, with a mean of 67.29 mgHC/gTOC indicating Type III organic matter. The plots of producible hydrocarbon (S₂) against TOC and HI against maximum temperature (Tmax) confirm the presence of type III (gas-prone) and type IV (inert) organic matter. As a result, the shale is slightly mature and successful at producing oil and gas.