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Oil prospects in the Keta Basin.
The Devonian Miospores from Atiavi-1
The Keta basin is a southward open monocline controlled by basement flexures and faults (Akpati, 1975). It is characterized by two sub parallel faults, the Fenyi-Yakoe and Adina faults which are generally regarded as the north-eastern extensions of the Romanche fracture zone. The basin contains over 4400 m of Devonian to recent non-marine and marine sediments (Akpati 1978).
The basin continues offshore beneath the continental shelf, where the thickness of the sediments may exceed 5000 m and include Mesozoic and Cenozoic deposits (Blundell and Banson 1975)
The Keta basin is one of the chains of Mesozoic and Tertiary sedimentary basins along the West African
Gulf of Guinea. These basins lie parallel to the present coastline and are concentrated along mobile belts that were established in the Late Precambrian (Kennedy, 1965)
The occurrence of these basins are attributed to the subsidence along the line of the Late Precambrian orogenic belt, that later marked the separation of the Africa and South America (Sutton, 1968) Miospore assemblages from the Atiavi-1 well in the Keta basin have been described to corroborate the upper Lower Devonian (Emsian) to lower Upper Devonian (Frasnian) age assigned to the lower section of the well based on chitinozoan and acritacrch assemblage by Anan-Yorke (1974).
The occurrence of the Devonian sediments in the Keta basin could be reconciled with the reconstruction of the continents by Bullard et al. (1965) in that the sediments were deposited in an inland basin that later became inundated by the sea between Gedinnian and Givetian time (Saul et al., 1963). The thirty seven samples processed contain a rich assemblage of miospores, chitinozoans and acritarchs.
The study also assigns Uppermost Devonian (Strunian) age to the upper part of well with a diverse, well preserved and abundant assemblage of pteridophytic spores and phytoclasts which hitherto has not been dated. Observations from palynomorph distribution reflect a change from marine environment of deposition for the lower section to that of fluvio-deltaic or nearshore environment for the upper part in a humid climate. The palynomorophs are well preserved and thermal maturity is within the oil floor and wet-gas zone.
Studied the palynology of Atiavi-1 well in the Keta basin which penetrated Paleozoic sediments. He recognized a lower marine interval based on chitinozoan, acritarchs, spores and occasionally scoloecodonts and dated the sediments upper Lower Devonian (Emisian) -lower Upper Devonian (Frasnian) and an upper continental unit with predominant spores which was not studied.
This study describes well preserved spore assemblages recovered from the Devonian strata in the Atiavi-1 (Fig. 1) to corroborate the age assigned to the lower section based on chitinozoan and acritacrch assemblage (Anan-Yorke, 1974).
The study of Devonian miospores from the Atiavi-1 well in the Keta basin allows the following conclusions:
• Miospores from the well indicate an age of upper Lower Devonian (Emsian) to uppermost Devonian (Strunian)
• The miospore zonation in the upper Devonian can be compared with the spore zonation scheme of Higgs et al. (1988) and Higgs and Streel (1984) recognized in Western Europe and British Isles
• From a palaeo environmental viewpoint, the distribution of palynomorphs (spores, chitinozoans and acritarchs) in the well indicate a lower marine interval (1524-1224 m) and an upper near shore or fluvio-deltaic interval (1192-936 m) in a humid climate
• Thermal maturity from spore colour suggests a Thermal Alteration Index (TAI) within the oil floor and wet gas zone.
Research Journal of Environmental and Earth Sciences 4(12): 1052-1059, 2012
© Maxwell Scientific Organization, 2012
Oil seepage was sighted in the onshore Keta area as far back as the last decade of the 19th century, and in 1965 the Romanian government provided technical assistance for the exploratory drills. Oil wells were drilled at Anloga and Atiavi without any significant find.