A PALYNOSTRATIGRAPHIC STUDY OF THE ENVIRONMENTS AND THERMAL MATURATION OF ONSHORE OREDO FIELD, NIGER DELTA

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.