RMZ – MATERIALI IN GEOOKOLJE

Geophysical and sedimentological studies for reservoir characterization of ...415IntroductionAsphalt-impregnated sandstones otherwisereferred to as oil sands (tarsands) and active oil-seepages occurin southwestern Nigeria within themarginal pull-apart or margin-sag Dahomey(Benin) basin. The oil sandsoutcrop in an E-W belt, approximately140 km long and 4–6 km wide, extendingfrom Edo, Ondo and Ogun State insouthwest Nigeria. Bituminous sandsare composed of sand, heavy oil andclay that are rich in minerals and water.The heavy oil in the bituminous sand iscommonly called bitumen, which is avery dark coloured, sticky and viscoussubstance. Total reserve of the heavyoil is estimated to exceed 30 billionbarrels (Adegoke et al, 1980).The petroleum habitat is almost exclusivelythe Afowo Formation, a memberof the Abeokuta Group. This litho-unitis of Turonian - Maastrichtian age andconsists of interbeds of coarse-mediumgrained sandstones, siltstones andshale deposited in a transitional to marginalmarine environment (Omatsola& Adegoke, 1981). The oil is found inthe coarse grained clastics within theformation in two discrete bands (the Xand Y horizons), each 30–40 m thickand separated by 6–15 m carbonaceousshales with a thin band of lignite (Akoet al., 1983) and overburden thickness inexcess of 50 m at Agbabu, Ondo State,south western Nigeria (Enu, 1985).This research utilizes electrical resistivitystudies, sedimentological andpetrophysical studies to characterizethe reservoir sands of the tar sand depositsin Gbegude area; a continuum ofthe tar sand belt of North-Eastern Dahomeybasin, South Western Nigeria.The study is aimed at delineating thesubsurface distribution and occurrenceof the deposits as well as determiningthe reservoir characteristics of the tarsand deposits in the study area.Geological settingThe study area is situated in the easternpart of Dahomey basin and locatedwithin longitude 4°22 ’ E to 4° 30 ’ E andlatitude 6° 40 ’ N to 6° 43 ’ N (Figure 1).The Dahomey Basin constitutes partof the system of West African pericratonic(margin sag) basin (Klemme1975; Kingston et al 1983) developedduring the commencement of the riftingand associated with the opening ofthe Gulf of Guinea, in the Late Jurassicto Early Cretaceous (Burke et al,1972; Whiteman, 1982). The crustalseparation, typically preceded by crustalthinning, was accompanied by anextended period of thermally inducedbasin subsidence through the Middle– Upper Cretaceous to Tertiary timesas the South American and the Africanplates entered a drift phase to accommodatethe emerging Atlantic Ocean(Mpanda, 1997).RMZ-M&G 2012, 59