--> Abstract: Seismic Stratigraphic Analysis in the Niger Delta: a Case Study of The Benin River 3-D Seismic Cube, by E. Ibie; #90933 (1998).

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Abstract: Seismic Stratigraphic Analysis in the Niger Delta: a Case Study of The Benin River 3-D Seismic Cube

Ibie, Elliot - Chevron Nigeria

This study was carried out in an attempt to place the subsurface over the Benin River field area within a structural as well as chronostratigraphic framework. The Benin River 3-D seismic dataset was acquired in the southwestern flank of the Niger Delta within Chevron Nigeria's OML-49 concession. The study area covers the Benin River, Gbokoda, Dibi, Fragbene, Olure and Utonebu fields. The present-day Benin River drainage system runs NNE-SSW of the study area. Relatively intense river channelization in this area is evident from the Miocene through to the Pliocene, although the general axis of orientation of these channels has progressively rotated in an anti-clockwise direction over time, probably due to channel switching and river rejuvenation.

Analyses within the survey area involved an initial development of a structural framework that identified 58 synthetic and 21 antithetic faults. A major sedimentary depocenter, bounded to the north by the OLUGBOKO regional synthetic fault trending almost in an east-west direction with throws in excess of 600 milliseconds (1,750 feet), was identified, with the DIBI fault - a splay off the OLUGBOKO - as its southern limit. The NW-SE trending ESCRABENIN and UTONANA faults are to the north and south of this depocenter respectively.

The accompanying seismic stratigraphic analysis attempted to identify genetic reflection packages that were defined by stratigraphic discontinuities on the basis of reflection termination, seismic stratal onlap, downlap or toplap. Six sequence boundaries were observed with six associated sequences within the studied 3500 milliseconds of vertical seismic section. Sequences C to F are related to submarine canyons and formed by slumping of delta front sands. Sequences A to B are related to submarine fans. The study was aided by well log analysis which, in addition to biostratigraphy, helped in validating and/or uniquely identifying some of these regional unconformities. Age determination limited the studied section from the Oligocene to the Miocene. Paleo-environmental analysis indicated that the sediments contained within these sequences were deposited in an inner neritic to upper bathyal environment. Furthermore, these sequences were found be largely of third-order cyclicity.

It was necessary to determine from well logs what the relative resistivity response was from sequence to sequence. Four of the sequences were found to contain substantial accumulations of oil and gas. It is possible though that in addition to booked reserves, additional reserves abound as a result of the presence of stratigraphic traps occasioned by the erosional truncations identified within the survey area.

Preliminary regional correlation with adjacent seismic cubes has been very insightful. Sequence Boundary_F seems to correspond to the base of one of the shallowest depositional units within the main Opuama Canyon complex (Opuekeba 3-D survey: courtesy, Chevron Nigeria). It is therefore advocated that a thorough sequence stratigraphic analysis be carried out to unravel the intricacies, as well as evolution, of this complex system.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil