Palynostratigraphy and correlation of three wells in the deep offshore Cenozoic Niger Delta

Abstract

Recently hydrocarbon exploration and production in the Niger Delta Basin have shifted to offshore areas and this has required risk appraisal for their successful implementation. In order to improve our understanding of the petroleum system of these areas ditch-cuttings samples selected from three deep offshore wells A, B and C are subjected to palynological and sedimentological analyses to create a palynomorph biostratigraphic framework and microfloral zones. Palynostratigraphic data and lithofacies data are integrated for the first time (in this study area) with those of the wireline well log data for the better understanding of the sequence stratigraphy of this region. In general, the palynological results revealed abundant and diverse palynomorph occurrences. Although land-derived palynomorphs represented by angiosperm pollen, gymnosperm pollen, pteridophyte/bryophyte spores, fungal spores and freshwater algae dominated the observed palynomorph assemblages, the forms of marine origin comprising dinoflagellate cysts, microforaminiferal wall-linings, silicoflagellates, diatom frustules and scolecodonts, were rarely represented. Angiosperm pollen comprised 79-85 % of the recovered palynomorphs in the wells. They are dominated by Zonocostites ramonae, Z. duquei, Monoporites annulatus, Arecipites spp., Cyperaceaepollis spp., Psilastephanocolporites spp., Psilatricolporites crassus, Chenopodipollis spp., Retistephanocolpites gracilis, Nymphaeapollis clarus, Peregrinipollis nigericus, Canthiumidites spp., Gemmamonoporites sp., Retibrevitricolporites obodoensis, Retitricolporites irregularis, Pachydermites diederixi, Echitricolporites spinosus and Elaeis guineensis. The pteridophyte/bryophyte spores comprised 9-10% of the total palynomorph assemblages and among these, Verrucatosporites spp., Laevigatosporites spp., Stereisporites sp. and Acrostichum aureum were prominent. Based on these taxa the strata penetrated by these wells ranged in age from Late Miocene (P860 Subzone) to Pleistocene (P900 Subzone). The contrasting relative abundances of mangrove and Poaceae pollen during the Late Miocene to Pleistocene indicated fluctuations in the climatic conditions in this area during these epochs. These unstable climatic conditions are interpreted to have resulted from rapid changes in the sea level. The palynofloral assemblages revealed that the sediments were deposited in three main iv

depositional environments: nearshore, shallow and marine. They are further subdivided into deltaic distributary channel, tidal channel, submarine channel, regressive marine and prograding delta sub-environments. The sand units of these sub-environments have good reservoir quality, while the shales provide good quality source and sealing rocks in the Niger Delta. Integration of the palynostratigraphy, lithofacies and well log data reflects some cycles of sequence systems tracts and sedimentary surfaces- with similar patterns and occur within almost the same depth interval ranges. The major contribution of this research has been to show that there is some degree of correlation between microfloral zones and depositional sequences based on the various methods employed. These relationships imply that climatic conditions, coupled with other factors like localized subsidence, played an important role in the sea level changes in the Niger Delta area. Four major depositional sequences of Late Miocene-Pleistocene age have been delineated. The sequences are bounded chronologically by type-1 sequence boundaries (SBs) 5.5 Ma SB, 4.1 Ma SB and 3.0 Ma SB. These depositional sequences are interpreted to have experienced four major flooding events marked by four maximum flooding surfaces (MFS), namely the 5.8 Ma MFS, 5.0 Ma MFS, 3.9 Ma and 2.0 Ma MFS in ascending order. This suggests that the combined analyses of palynostratigraphy, lithofacies and well log data can be used as a means to predict the location of the system tracts and sedimentary surfaces in order to raise the confidence in the correlation and interpretation of depositional sequences.