Edet, James John; (1996) Neogene palynology and palynofacies of the eastern Niger delta, Nigeria. Doctoral thesis (Ph.D), UCL (Uniiversity College London).

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Abstract

A study of the paralic Agbada lithofacies has been undertaken to re-assess the Neogene palynological stratigraphy and sedimentation history of the eastern Niger Delta. The sediments examined are from five onshore boreholes including the Ama-1, Ibewa-3, Erema-3, Ihugbogo-1 and Yorla South-lx. The variations in sand/shale proportion determined by a combination of electric log (gamma-ray and spontaneous potential logs) and physical sample data, allows for an allostratigraphical subdivision of the Agbada Alloformation (= Agbada Formation) into three allomembers, each of which represents deposition during four distinctive phases of sedimentation. The palynofloral record based on integrated miospore-dinoflagellate cyst data shows that the studied sediments range in age from ?latest Oligocene-Early Miocene to ?earliest Pliocene. Based on the first downhole occurrence (FDO) of selected stratigraphically important species, fourteen palynological interval zones have been tentatively defined. The palynological interval zones, in addition to records of charred Gramineae cuticle peaks as well as previously known radiometric ages of volcanic activity around the adjacent Cameroon Volcanic Line, have been integrated to produce a preliminary palyno/biochrono-stratigraphical framework for onshore sediments of the eastern Niger Delta. Sedimentation was mainly controlled by a complex interaction of local basement epeirogeny, submergence/growth faulting and rate/volume of sediment supply during each episodic growth of the delta (i.e. the development of each depobelt). The resolution of palynofacies and depositional environments reveals that the three paralic allomembers of each depobelt are characterised by four depositional sequences each of which corresponds with a distinctive phase of sedimentation. The first phase, which is marked by upper shoreface-foreshore deposits, represents the initial paralic clastic wedge deposited in a relatively shallow basin. This is followed by a retrogradational phase during which mud-rich proximal offshore-lower shoreface sediments were deposited as the basin subsided rapidly. The third phase, which has been interpreted as a progradational to slightly aggradational phase, is typified by a more-or- less equal sand/mud-rich lithofacies developed predominantly under lower-upper shoreface conditions during gradual shallowing of the basin. A rapidly progradational late fourth phase of paralic sedimentation is marked by predominantly sand-rich lithofacies developed under upper shoreface-foreshore conditions and possibly with backshore deposits towards the more proximal parts of the depobelt. Deposits of the first and second phases constitute the transgressive systems tract while the third and fourth phases incorporate deposits of the highstand systems tract. These preliminary results suggest that sediments which are deposited during the first and third phases of sedimentation are, respectively, the most viable exploration and appraisal targets for hydrocarbons.

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