BIOSTRATIGRAPHY AND PALEOECOLOGY OF FORAMINIFERA AND PALYNOMORPHS IN THE OPOLO – 5 WELL, NIGER DELTA BASIN, NIGERIA

Abstract

Ditch cutting samples (3125ft to 8150ft), from the Opolo – 5 well, a shallow offshore well from the Niger Delta basin, was analysed for the occurrence and attributes of microfossils, (foraminifera and palynomorphs) with the main purpose of establishing biozones and reconstructing the paleoenvironments of the sediment deposition. The foraminifers retrieved consist of eight (8) planktonic species and thirty-two (32) benthonic species, aside the indeterminate planktics and benthics. The planktonic species identified were Globigerinoides immaturus (LeROY), Globigerinoides ruber (d’ORBIGNY), Globigerina praebulloides (BLOW) and Globoquadrina dehiscens etc. The benthonic species include Lenticulina inornata, Quinqueloculina lamarckiana, Heterolepa pseudoungeriana, Reticulophragmium venezuelanum, Alveolophragmium crassum, Cyclammina cancellata, etc. The palynomorph assemblages recorded were better preserved and diverse than the microfaunal content. Sixty–seven (67) species of pollen and spores were identified, two of which are species of algae, Botryococcus braunii and Pediastrum spp., four (4) dinoflagellate species and one acritarch, Leiosphaeridia spp., was also recorded. The abundance, range and depth at which these forms occur are clearly shown on the biostratigraphic distribution chart. Two planktonic foraminiferal zones were erected, the Globoquadrina dehiscens zone and the Globigerinoides ruber zone. Based on the stratigraphical distribution patterns of benthonic foraminiferal species identified in this well, four informal benthonic zones have been suggested. Four ‘Floral’ zones were established based on the inferences from phytoecological groupings and the occurrence of paleoenvironmentally significant species. The Opolo-5 well was dated as Late Miocene to Early Pliocene, based on the record of index foraminiferal species, Globoquadrina dehiscens, and palynomorphs Retistephanocolpites gracilis, which is a Pliocene/Miocene boundary marker. Generally, the evidence from the data collated from the foraminifera associations, palynological data and sedimentological inferences, all point in the same direction, that the Opolo-5 well is a shallow marine well ranging from inner to outer shelf. No one ecological factor played the most role in determining the distribution of foraminifers in Opolo – 5 well, it is therefore a network of factors, the major being salinity, temperature and depth, all other factors where probably minor. A predominantly humid tropical climate with heavy rainfall is inferred for the Late Miocene period with an alternation of dry and wet climates (cyclicity of vegetation and continental climate) in later periods, that is, in the Early Pliocene.