TITLE:
Palyno-Stratigraphy and Kerogen Assessment of OMA Well Sediments, Offshore Dahomey Basin, Southwestern Nigeria
AUTHORS:
Omowunmi Idera Agunsoye, Olugbenga Ajayi Ehinola, Olubunmi Christopher Adeigbe, George Unomah
KEYWORDS:
Palynomorphs, Kerogen, Depositional Environment, Spore Coloration Index
JOURNAL NAME:
Journal of Geoscience and Environment Protection,
Vol.10 No.7,
July
19,
2022
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.