The Nigerian Benin Basin extends southward from the basement rock outcrop to offshore area. The northern limit could range from gravity and aeromag data confirms depth to basement of 1,700 - 3,000m. It should be noted that there is anomalous sediments pile- up around the southern part of the acreage towards the offshore which could translate to better set up for petroleum accumulation.
- Stratigraphically, the Basin is dominated by a series of eroded or non-depositiona1 Haiti viz. the Lower Cretaceous Valanqinian to Aptian erosional hiatus representing about 7my of the geological time, two Upper Cretaceous Haiti, the 6my Ceriomanian and the 14my Coniacian to Campanian, and finally, a relatively short (3my) Paleocene hiatus. These are associated with regional orogenic events associated with Rh valley formation, translation continental plate movements, subsequent inland tectonic events and finally, the continental drift of South America from Africa. The Basement is overlain by pre-Albian shales as old as l80my or younger continental fan deposits (l35my). These in turn are overlain by either continental Albian sands or la- custrine sands and shales of the “Ise formation”, above which are the Turonian Abeokuta formation. Another continental facies, the Maastrichtian Sands, typically overlie the Abeokuta Araroml shales. Palocene deposits are the Ewe koro formation comprising of shelfal limestones and shales, grading offshore into deltaic Imo/Akata shales. Although shelfal conditions obtained through the Middle Miocene, resulting in shallow water/coastal shales of the Ilaro and Afowo formations, deltaic influences are more noticeable during this 4Omy interval. A deltaic environment has obtained in the Basin forever l0my as represented by the continental Benin formation topmost sequence.
The presence of hydrocarbons in the Benin Basin is shown b9 Seme Field (over
100MM barrels of oil-in-place) and the Bituminous Sand Belt (1000MM barrels of
recover- able oil) located respectively southwest and east of OPL-304.
The source rocks are Cretaceous shales, both presently at significant depths in
the south and encountered at shallow depths in the onshore through removal of
over-burden. It is generally accepted that the Bituminous Sand.
Belt resulted from a lack of adequate traps to the south thereby permitting up dip
migration of generated hydrocarbons to the surface. To the west of the Belt, and north of
OPL-304, significant hydrocarbon seeps are absent, thereby suggesting good trapping
mechanisms in these areas. Four types of hydrocarbon-traps can be expected from the
structural/stratigraphic systems outlined:
- Graben structures enclosing reservoir and source rocks of lacustrine origin and scaled by the Araroml shale or the younger lmo/Akata and/or Ewekoro shales. These lacustrine sediments termed the “Ise formation" should be similar to the petroliferous Cabinda, Congo, and Campos (Brazil) Basins deposits.
- Enclosed reservoir sands with up dip truncations; a possibility confirmed by the lack of surface outcrops of both Neocomian and other Lower Cretaceous deposits.
- Structural highs such as Basement drapes which would trap hydrocarbons migrating up dip from the fetch/source areas as well as those dis- placed from down dip accumulations by regional tecnotic adjustments
- Carbonate build-ups associated with shelfal environments.
- Available data suggests the presence of one or more graben structure in parts of OPL 304. On the basis of the existence of only this form of trap, potential hydrocarbons in the concession are very roughly estimated at between 500MM Barrels and over 2000MM barrels.