Palyno-Stratigraphy and Kerogen Assessment of OMA Well Sediments, Offshore Dahomey Basin, Southwestern Nigeria

Abstract

Biostratigraphy has remained a tool for basin and environmental reconstruction in sedimentology. This study aims to further delineate and classify the lithology, environment of deposition, and the biozonation of microorganisms found in the studied well. Thirty-four (34) ditch cutting samples from OMA well, offshore Dahomey Basin, were considered for their textural and palynomorphs/palynofacies characterization. The samples were observed and processed according to the standard palynological procedure for geologic samples using hydrochloric and hydrofluoric acid digestion. The stratigraphic sequence of the well ranges from shale to sandstone to argillaceous sandstone. The shales appear highly fissile and laminated in all shades of grey and black while the sandstone colour ranges from light-grey/brown to brown and very deep brown. The sandstone contains some mica flakes and likely pyrites in fine-medium, sub-angular to rounded grains. A total of 28 diverse palynomorphs were reported from the palynological analysis with low abundance. The low recovery of pollens was likely due to limited terrestrial influence and the reasonable amount of dinoflagellate cysts and microforaminifera wall present indicated a marginal marine to a distal offshore environment. Two zones correlated for this study are Danea mutabilis (Early Paleocene) and Dinogymnium euclaense (Upper Maastrichtian) zones. Palynomaceral classifications through visual, microscopic kerogen assessment resulted in high preservation of organic matter which suggests a reducing environment of deposition. The dominances of 48% amorphous (I), and 36% exinite (II), with a considerable amount of the 5% vitrinite (III), and 11% inertinite (IV) kerogen types, indicate that the organic matter is mainly sapropelic and mixed humic-sapropelic types which are favorable for hydrocarbon generation and primarily of marine origin. The spores/pollen colour ranges from light yellow/brown, brown transitioning into darker brown on a scale of 4/5 transitioning to 5/6. Medium degree of maturation (4/5) to late degree (6/7) of maturation was inferred. The studied interval of the OMA well sediments agrees with the established stratigraphic interval of the offshore, Dahomey basin, within the Upper Maastrichtian to Late Paleocene age range and supports high preservation of organic matter sufficient and mature enough to generate hydrocarbon.