dc.description.abstract |
The Niger Delta is a mature basin in Nigeria with petroleum reserve of about 40 billion barrels.
The onshore Niger Delta Paleogene-Neogene, where exploration commenced and was
concentrated before activities shifted to the offshore and deep-offshore Neogene-Quaternary,
has not been fully explored. The shift of exploration focus led to a neglect of work on onshore
Paleogene palynostratigraphy. This study was aimed at investigating the palynostratigraphy,
paleoclimate and hydrocarbon potential of the periods.
Five hundred and eighty-five ditch cutting samples in Oredo Field, OML 111: Oredo-2 (244);
Oredo-4 (195); and Oredo-8 (146) were provided by the Nigeria Petroleum Development
Company, Benin. Sub-samples (25 grams each) were subjected to conventional palynological
maceration procedures for the retrieval and concentration of palynomorphs and palynomacerals,
while standard laboratory procedures were applied for micropaleontological sample preparation
and lithologic descriptions. Retrieved palynomorphs and palynomacerals were mounted on
microscope slides and observed using a high-power research photomicrographic microscope.
The microfauna and lithology were studied using a stereomicroscope. Characteristic
palynological assemblages associated with the recognised systems tracts within the sequence
stratigraphic framework were defined. Biostratigraphic data were processed using conventional
biostratigraphic software with stratigraphic ages cross plotted against depth generated in
chronostratigraphic charts. Data were analysed using descriptive statistics.
The palynomorph assemblages contained abundant and diverse palynomorphs with pollen,
fungal spores, dinoflagellate cysts and pteridophyte spores constituting, respectively, 63.0%,
18.8%, 8.1% and 7.4% of the totality of assemblages in the three wells. Twelve new
palynological zones were established from the stratigraphic distribution of marker species based
on their appearance and extinction datums and local assemblage ranges. Four palynocycles (1,
2, 3 and 4) of alternating wet and dry climatic regimes were identified based on ecological
indicator plant species. The percentage distribution of the palynomacerals revealed the
dominance of amorphous organic matter with phytoclasts and palynomorphs, which aided the
subdivision of the wells into three palynofacies associations: PF-I, PF-II and PF-III. Two distinct
organic facies represented by orange to orange-brown (TAI 3) and yellow to orange (TAI 2)
indicating early mature and immature hydrocarbon potentials, respectively, were recognised.
Recovery of microforaminifera, consisting of 87.0% benthics and 13.0% planktics was generally
low. Two lithostratigraphic units, Agbada and Benin Formations, were delineated. Integration
of palynostratigraphic and foraminiferal data suggested a Middle Eocene (Paleogene) to Early
Miocene (Neogene) age for the analysed intervals. A combination of the palynofacies
associations with other biostratigraphic data indicated palaeoenvironments shoaling from marine
to terrestrial/continental. Two systems tracts – Transgressive Systems Tract (TST) and highstand
systems tract – were recognised. Marine lithofacies with their associated TST in the Eocene are
mature and lie within the oil and gas “windows”. Mapping of 41.0, 39.4, and 38.0 Ma maximum
flooding surfaces with associated 40.1 and 38.7 Ma sequence boundaries were revealed.
A new and more detailed palynostratigraphic zonation comprising twelve zones for the Niger
Delta Paleogene/Neogene was established. Four palynocycles of alternating wet and dry climatic
cycles were revealed. The Eocene shale within the Agbada Formation were matured regarding
hydrocarbon potentials. |
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