The easternmost point on the Golden Triangle of the world’s biggest deepwater play — West Africa, Gulf of Mexico and Brazil — draws majors and independents alike to the region’s promise and its challenges.

Offshore West Africa, big production is generated in a variety of plays as sediments follow canyons from the shelf to the deepwater sea floor. (Diagram courtesy of Anadarko Petroleum Corp.)
Wafik Beydoun, with Total, outlined the promise offered by the whole of Africa during presentations titled “Africa on the Horizon” at the Global Theater at the annual conference of the Society of Exploration Geophysicists in New Orleans.

Africa, with 14% of the world’s population, furnishes 12% of its hydrocarbons, and 85% of that production comes from the “big five,” Nigeria, Libya, Algeria, Egypt and Angola. The nation holds 12% of the world’s petroleum reserves and will account for 30% of hydrocarbon growth by 2010.

The focal point for much of that production lies in the huge, prolific deepwater fields of West Africa. The shelf edge is the staging area for the deepwater deposition, said Henry Posamentier, chief geologist with Anadarko Petroleum Corp.

At the prolific plays, sediments typically flow from the shelf down a deep slope channel to form stacked or amalgamated sands on a frontal splay at the base of the shelf.
The sediments follow a deep scour down the slope and flatten out at the bottom to a maximum channel width about 2.5 miles (4 km) wide. In the final stages of deposition, the channel is much smaller and the floor sections aren’t as thick, but they still can be prolific, he said. The frontal splays feature turbidites that are “extraordinarily prolific and relatively thick,” he added, while the smaller splays may be large, but they’re small compared to the main frontal splay. A sediment wave occurs when the sediments overflow the banks of the splay channel and widen the splay.

Most of the production from West Africa comes from that slope environment offshore Ghana, Benin, Nigeria, Gabon and the Congo.

Nigeria

The slope channels show up clearly on seismic offshore Nigeria, Posamentier said. When geophysicists integrate a section view with a plan view, it’s easy to see the channel threads. The channel belt ranges from 656 to 984 ft (200 to 300 m) wide with a meandering loop migration. That meandering channel bed provides a fairway about 1.9 miles (3 km) wide.
As analysts go deeper into the seismic slice, the channel fairway narrows to only 0.6 to 1.2 miles (1 to 2 km) wide.

The sediments start with a fairly wide sand at the base with subsequently lower energy flows, similar to the Gulf of Mexico slope, he said.

Ghana

Offshore Ghana has a different deposition. It shows subtle erosion at the base that may be 9.3 to 10 miles (15 to 16 km) wide. That should be a warning flag to explorationists, he said. Analysts could find a lot of small channels at the top of the system, but they couldn’t find the big, meandering channel like the channels offshore Nigeria.

Apparently this was a mass transit deposit and smaller channels are harder to image, Posamentier added. This section looks like the slump scar in the eastern Gulf of Mexico. At the shelf edge in the Gulf of Mexico in Pleistocene sediments, seismic shows numerous small channels. He said he believed that was the same case in Ghana.

Congo

The deposition into deepwater offshore Congo goes back to the narrower system seen in Nigeria, he said. It has the same amalgamated fill and a 1.9-mile- (3-km-) wide belt with a visible channel.

It has a more complex fill later in the system and a fairway 0.6 to 1.2 miles (1 to 2 km) wide and deep, as much as 328 to 656 ft (100 to 200 m) thick. “It should be an excellent reservoir,” he said.

Gabon

Operations haven’t drilled into the deep water offshore Gabon yet, but surveys show basin-floor turbidites with shelf slide.

One big difference here is the direction of the channels. Instead of flowing off the shelf into deeper water, they parallel the slope. That means the originating slide area is either to the north or the south, or both. Slope-hugging currents acted as a magnet to pile the sediments along the slope, Posamentier said.

That deposition suggest a better chance

of sediment waves and crevasse plays toward the bottom of the sedimentary column
and channels along the slope higher up.

In the higher section, seismic identifies a nice channel, he said. That should offer a good stratigraphic trap along the slope with more small channels on the basin floor deposited as a frontal splay, a classic sheet-bedded turbidite pattern.

It appears that the sedimentary input from the north flowed earlier in geologic time, and some inputs from the south are shallower. If it turns out to be similar to the Gulf of Mexico, the channels could be 328 ft (100 m) wide and the whole system could be 9.3 to 10 miles (15 to 16 km) wide.

Models show a massive geobody, he said, and as analysis moves in close it’s possible to see the lineaments that invariably underlie a mass transport deposit.

Benin, he added, has a shelf-edge potential play that offers a staging area for a slope channel.

Angola

In the same program, Henri Houllevigue, manager of geophysical operations and technology with Total, the company that operates the pioneering Girassol field on Block 17 offshore Angola, described results from time-lapse seismic at the field. Girassol produces from a channel levee system, and Total surveyed the field for a baseline in 1999. It ran a monitoring seismic survey in 2002 and another in 2004.

The 2004 survey, he said, shows more differentiation in the reservoir. It shows the water injection and flooded area and the movement of the flood. It also confirmed for the company that the channel levee acts as a barrier to communication with other zones. A fault could be the northern limit of the field.

In Block 17, the company placed geophones and hydrophones on the seabed for its measurement, and it will use the same system in Nigeria and in other areas of Angola for its time-lapse surveys.
Challenges
The area is geophysically challenging and the challenges grow as Total will try to explore and appraise deeper targets that include toe thrusts and pinchouts.

For that detail work, it uses 26,248-ft (8,000-m) towed streamers with two lower streamers. The over-under streamer design gives the company better resolution.

Block 32 offshore Angola and seaward of Block 17 is structurally complex. In its pre-stack depth migration work, it starts with low-frequency processing for a better overall image. Detail and complexity increases as it raises the frequency of the survey, but this is a good tool for tuning structural models, Houllevigue said.

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