Abstract:A 42 m thick outcropping portion of the Mamfe Formation is subdivided into Manyu (31 m thick) and Kesham (11m thick) Members on the basis of textural, mineralogical and structural differences. The Manyu Member (Albian) consists of folded and indurated, medium to coarse grained arkosic sandstones and thickly laminated organic-rich shales deposited in a lacustrine environment. The Kesham Member (Cenomanian) consists of subarkoses intercalated with massive green shale and mudstone deposited in a fluvial environment. The change in depositional environment was tectonically controlled. The mid Cretaceous paleogeography of the embayment was governed by the NE-SW trending "Ikom ridge" which prevented marine incursion from adjacent the Benue Sea. Evaporites found within the basin were precipitated from ocean water that was periodically spilled by strong tides and storms across the ridge into the embayment. The filling-up of the embayment to base level in the Cenomanian resulted in a shift in the depositional center downstream to adjoining lower Benue Trough. Similarity in heavy mineral composition and maturity of the Cenomanian sandstones with recent clastics in the embayment indicates their derivation from the same source terrain and relatively stable tectonic and climatic conditions at the source area since the Cenomanian time.
The palynological study of the outcrop Formations in parts of Ini Local Government Area of Akwa Ibom State, Southeastern Niger Delta Basin, Nigeria has contributed to unraveling the palynostratigraphy and paleoenvironments of the outcrops through the use of diagnostic palynomorphs. The section is made up of carbonaceous shales, sandstones, mudstones and claystones. The conventional maceration technique for recovering acid-insoluble organic-walled microfossils from sediments was used to prepare twenty outcrop samples for palynological studies. The samples produced poor to fair diversity assemblages of palynomorphs. A total of 176 palynomorphs species were recorded, with 48% pollen grains, 33% spores, 12% dinoflagellate cysts, 5% fungal spores, and 2% microforam test wall linings. Based on stratigraphically selected index taxa recovered,age determination and correlation wereestablished. The samples from Imo Formation were assigned Late Paleocene-Eocene based on Psilatricolporites Crassus, Retitricolporites irregularies, Psilatricolporites sp. and Leiotriletes adriennis with the presence of Paleocene dinoflagellate cysts such as Lejeunacysta beninensis and Selenopemphix nephroides. Ameki samples were dated Early-Middle Eocene based on Proxapertites operculatus, Retistephanocolpites williamsi, Mauritidites crassiexinius, Monocolpites marginatus and Longapertites marginatus. Ogwashi-Asaba shale samples were assigned Late Eocene-Early Oligocene due to the abundant presence of Verrucatosporites usmensis, Laevigatosporites discordatus and Retitricolporites irregularis while Eze-Aku samples were tentatively assigned Paleocene because of the presence of two pollen index fossils Proxapertites operculatus and Pachydermites diederixi. Inferred depositional conditions suggest that Imo Formation was deposited in a shallow marine or near shore brackish water estuarine environment. The Ameki Formation was accumulated in a marginal marine or near shore, brackish water-estuarine environment in the upper deltaic setting. The Ogwashi-Asaba Formation was deposited in a progradational shoreline moving between the brackish water and fresh water deltaic plain. The Eze-Aku Formation was deposited in an upper foreshore fresh water environment. The palynological study of the samples are Paleogene Formations while the paleoenvironment is in a costal deltaic to shallow-marine environment and are appropriate for hydrocarbon accumulations and exploration.
An integrated approach to Seismic interpretation that combines techniques of sequence stratigraphic analysis, seismic facies analysis and attribute analysis is one of the most effective approaches for hydrocarbon exploration in growth-faulted deltaic strata of offshore eastern Niger Delta. These strata are generally thick paralic/marine units deposited along an unstable progradational continental margin. Here, shale ridges, toe thrusts and diapirism are common features. Thus, system tracts along this margin differ significantly from those described for classic stable progradational continental margins. Development of good reservoir sands, on the shales of the upper transgressive systems tract form barriers which are good, particularly on the outer shelf where high stand systems tract sediments accumulate. Alternation of the high stand systems tract sands and transgressive systems tract shales provides a bridge linking reservoir facies with the shales or seals of TST which is essential for hydrocarbon accumulation and its stratigraphic trapping in the study field. This work has provided an analytical model for a detailed and cost-effective hydrocarbon prospecting through the use of sequence stratigraphic framework in tandem with seismic data interpretation.
Total organic carbon and Rock – Eval pyrolysis studies were conducted on seven (7) shale samples outcropping in parts of Ini Local Government Area of Akwa Ibom State, Southeastern Niger Delta, Nigeria. The studies were done to determine the quantity and quality of organic matter in the shale source rock, and their hydrocarbon generation potential. Total organic carbon (TOC) values indicate poor to excellent organic richness with values ranging from 0.46 wt.% to 5.98 wt.%. The free oil content (S1) values range from 0.02-0.21 mg HC/g rock, (mean = 0.07 mg HC/g rock), while the source rock potential (S2) ranges from 0.08-1.09 mg HC/g rock with an average value of 0.45 mg HC/g rock both indicating poor source rock generative potential. The Hydrogen Index (HI) and the Oxygen Index (OI) range from 11 mg HC/g TOC to 59 mg HC/g TOC and 41 mg CO/g TOC to 74 mg CO/g TOC with an average value of 27.28 mg HC/g TOC and 55.71 mg CO/g TOC respectively. HI versus OI and S2 versus TOC cross plot results indicate that shale in parts of Ini Local Government Area contains organic matter capable of generating kerogen type III to type 1V which is gas prone.
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