Regional Geology and Petroleum Prospectivity

REGIONAL GEOLOGY
The Gulf of Guinea is one of the most prolific hydrocarbon provinces of the world. Intensive exploration efforts over the last 35 years in and around the Niger Delta in particular has led to a succession of significant discoveries, notably the Bonga, Agbami/Ekoli and Akpo discoveries in Nigeria and Zafiro and Alba in Equatorial Guinea. However, the full potential of the continental slope and rise seaward of the shelf break is only recently becoming apparent, with a number of exploration programs having resulted in world-class discoveries being made in recent years.

The Niger Delta is a Paleogene to Recent, wave-dominated delta situated in the Gulf of Guinea and extending into the northern JDZ. Following the Mesozoic rifting of the Atlantic, sedimentation began with Albian drift deposits. Sediments filled the Benue Trough and by Late Eocene time began to prograde across the existing continental slope into the deep sea. Continued seaward progradation since the Eocene has extended the continental margin to its present position.

Modern seismic data and improved models of sand distribution indicate that in places prospective acreage can extend up to 300 km from the present-day coastline of Nigeria. Extensive regional 2D and 3D multi-client seismic data shot by a number of seismic contractors provide a high quality regional dataset that has enabled unprecedented insight into the tectono-stratigraphic evolution of the Niger Delta and especially the deep-water province. The total sedimentary prism, an area of some 140,000 km2, has a maximum stratigraphic thickness of about 12 km.

STRATIGRAPHY
The stratigraphy of the Niger Delta is divided into three diachronous units of Eocene to Recent age that form a major regressive cycle. The uppermost unit, the Benin Fm, comprises continental/fluviatile and backswamp deposits up to 2500m thick. These are underlain by the Agbada Fm of paralic, brackish to marine, coastal and fluvio-marine deposits, organized into coarsening upwards ‘offlap’ cycles. The underlying Akata Fm, comprises up to 6500m of marine pro-delta clays. Shales of the Akata Fm are overpressured and have deformed in response to delta progradation. These shales facilitate regional decollement for updip extension and downdip compression. Shales of the Akata Fm constitute a world-class source rock. Deepwater turbidite sands also exist within this formation.

STRUCTURAL AND DEPOSITIONAL HISTORY
Diapiric shale structures began forming by Late Miocene time in response to lateral shale withdrawal from beneath the advancing deltaic load, combined with compressional uplift and folding of pro-delta strata. During the Pliocene and Pleistocene time, these structures were buried by the prograding delta and extensional growth faulting commenced. Subsidence within the depobelts ceased episodically, at which time alluvial sands advanced rapidly across the delta top, concurrent with a basinward shift in deposition and thereby creating seaward-stepping depocentres.

Extensive gravity tectonism has deformed sediments over the continental slope and the resulting folding, faulting and diapirism have created intraslope basins 10 to 25 km 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 systems which are distributaries for large deep-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 of low sea-level, deltas migrated seaward to the shelf edge and large amounts of sediments were transported to the slope, rise and deep-sea fans by turbidity currents and related mass flows via submarine canyons. Turbidity flows were confined within deep leveed channels on the upper and middle fan, but spread laterally outwards as sheet flows on the lower fan dispersing large amounts of coarse sediments across broad areas.

STRUCTURAL ELEMENTS
Zones 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 the zone of shale structures on the upper slope separated by a relatively undeformed zone which exhibits minor thrusting and shale swelling.

The deep water Niger Delta hydrocarbon province encompasses a wide range of syn-depositional structural 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, basinward to a zone of relatively minor structuration, a frontal toe-thrust zone, and a zone of frontal deformation on the present-day lower slope and continental rise.

REGIONAL PROSPECTIVITY
The principal trapping scenarios for each domain are conceptualised on the schematic section. The shale diapirism zone, on the present-day upper slope, has proven prospectivity (Bonga, Aparo and Usan fields). Basinward, the zone of relatively minor structuration, is highly prospective (the Agbami 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 DHI’s and flat spots, and the zone of frontal deformation on the present-day lower slope and continental rise is as yet untested.

Technical Review of Areas on Offer

The Joint Development Zone is between Latitudes 1 and 3 degrees north and longitudes 4 and 8 degrees east in the Gulf of Guinea. It covers an area of 34,548 km2 with water depths ranging from about 1500m in the northern part of the JDZ to over 4000m at its southwestern sector

A total of 9 blocks situated in the northern sector of the JDZ have been put up for tender in the 2003 JDZ Licensing Round. These blocks cover a total area of 8429 km2 and range in size from 588 km2 to 1723 km2. Water depths for the blocks on offer range from 1500 to 2500m, well within the reaches of today’s technology.
Two major structural regions are present in the Joint Development Zone. The first is a toe thrust zone situated in the northern sector of the area corresponding to the distal parts of the Niger Delta. To the south of this is a second zone of minor structuration underlain by a rift system of Cretaceous age.

SOURCE ROCKS AND MATURITY
The source rock in the JDZ 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 JDZ are within the maturity window.

RESERVOIRS
Reservoir rocks in the area are sourced from clastic input from the Niger Delta. These are essentially deep sea turbidites consisting of channel sands and basin floor fans. Porosities in these reservoir can be as high as 20% and permeabilities are generally good.

TRAPPING
Possible hydrocarbon accumulations in the JDZ are essentially structural in nature. However, there is significant potential for stratigraphic traps, especially in the zone of minor structuration. 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

The structures within the thrust domain are compressional in origin and consist of simple thrust sheets. 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 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 been identified in the in the JDZ. Seismic sections show excellent reflection amplitudes with several prospects presenting stacked objectives. Flat spots indicative of fluid contacts have been identified on several of the prospects.