Stuctural and Depositional History of Gulf of Guinea

8/13/2019 Stuctural and Depositional History of Gulf of Guinea

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STUCTURAL AND DEPOSITIONAL HISTORY OF GULF OF GUINEADiapiric shale structures began forming by late Miocene time in response to lateral shale

withdrawal from the beneath the advancing deltaic load, combine with compressional uplift andfolding of prodeltaic strata. During the Pliocene and Pleistocene time, these structures were

buried by the prograding delta and extensional growth fault commenced. Subsidence within the

depobelts ceased episodically, at which time alluvial sands

advanced rapidly across the delta top, concurrent with a basin ward shift in deposition and

thereby creating seaward- stepping depocentres.

Figure 3a: An aerial distribution of the Structural and Depositional Setting of the Gulf of

Guinea.

Extensive gravity tectonism has deformed sediments over the continental slope and the resulting

folding, faulting and diapirism have created intraslope basins 10 to 25km wide filled with thick

sequences of ponded sediments that represent a wide range of depositional processes. Submarine

canyons cut across these deformed zones and give rise to aggradational channel/levee systemswhich are distributaries for large sea fans. Transport and deposition of terrigenous sediments

beyond the shelf have been accomplished mainly by turbidites and mass transport deposits

(slumps, debris flows).

During periods of low sea-level, deltas migrated seaward to the shelf edge and large amount of

sediments were transported to the slope, rise and deep-sea fans by turbidity currents and related


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mass flows via submarine canyons. Turbidity flows were confined within deep leveed channels

on the upper and middle fan, but spread laterally out wards as sheet flows on the lower fan

dispersing large amounts of coarse sediments across broad areas (see Figure 3a).

Figure 3b: Cross sectional picture of the Structural and Depositional Setting of the Gulf ofGuinea.

STRUCTURAL ELEMENTSZonal of imbricate thrusts occur in the deep offshore Niger Delta. These are well represented in

the multi-client seismic data. These compressive structures are typically situated seaward of thezone of shale structure on the upper slope separated by relatively undeformed zone which

exhibits minor thrusting and shale swelling.

The deep water Niger Delta hydrocarbon province encompasses a wide range of syn-depositionalstructural styles and deep-water sedimentary facies. The region can be broadly separated into

four domains. These extend from the zone of shale diapirism on the present-day upper slope,

basin ward to a zone of relatively minor structuration, a frontal toe-thrust zone, and a zone offrontal deformation on the present-day lower slope and continental rise (see Figure 3b).

REGIONAL PROSPECTIVITY


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The principal trapping scenarios of each domain are conceptualized on the schematic section.

The shale diapirism zone, on the present-day upper slope, has proven prospectively (Bonga,

Erha, Aparo and Usan fields). Basinward, the zone of relatively minor structuration, is highlyprospective (the Agbada and Akpo fields are just two examples) and extends into the northern

blocks of the JDZ. The frontal toe-thrust zone has much potential as evidenced by DHIs and flat

spots, and the zone of frontal deformation on the present-day lower slope and continental rise isas yet untested.

SOURCE ROCKS AND MATURITYThe source rock in the Gulf of Guinea is considered to be the Akata shale. Additional source

rock potential exists in lacustrine deposits of the pre-delta rift sequences. Thermal modeling

indicates that sediments situated in the northern sector of the Gulf of Guinea are within thematurity window.

RESERVOIRReservoir rocks in the area are sourced from clastic input from the Niger Delta. These are

essentially deep sea turbidities consisting of channel sands and basin floor fans. Porosities in this

reservoir can be as high as 20% and permeabilities are generally good.

TRAPPING

Possible hydrocarbon accumulations in the Gulf of Guineaare essentially structural in nature.

However, there is significant potential for stratigraphic traps, especially in the zone of minorstructuration. Two distinct domains have been identified in the toe thrust zone. These are:

(i) The simple thrust domain(ii) The imbricate domain made up of

(a)A proximal imbricate sub-domain(b)A distal imbricate sub-domain

These structures within the thrust domain are compressional in origin and consist of simple thrustsheets. The average width of the individual thrust sheets is of the order of 5 km. This domain

presents large structural closures which are likely to be less fractured due to exposure to lesser

compressional forces.

The proximal imbricate sub-domain is situated to the southeast of the simple thrust domain.

Structural dips in this area are high and the average width of the individual thrust sheets are of

the order of 1 to 2 km. The distal imbricate zone is situated to the southwest of the simple thrust

domain with individual thrust sheets of the order of 2 to 3 km.

PROSPECTIVITY

Approximately 30 leads and prospects, essentially structural in nature have currently beenidentified in the Gulf of Guinea. Seismic sections show excellent reflection amplitudes with


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several prospects presenting stacked objectives. Flat spots indicative of fluid contacts have been

identified on several of the prospects.

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Stuctural and Depositional History of Gulf of Guinea