West Africa Basins Share Secrets

Ancient River Systems Provide Targets

Activity and success in the prolific West African margin is no longer confined to high-profile plays in the Niger Delta of Nigeria and the Congo Basin of Angola. Companies of all sizes are probing the virtually untouched basins all along the western coast, from Morocco to South Africa.

And in doing so, they are unlocking many of the geologic secrets critical to finding potentially massive oil and gas deposits.

"The industry's geologic knowledge of the West African margin began in the Congo fan and Niger delta," said Paul Dailly, staff geologist for West Africa for Amerada Hess Corp., "and over the last several years we have extrapolated that knowledge to other basins."

Dailly was part of the team with Triton Energy that made the basin-opening Ceiba discovery in Equatorial Guinea's Rio Muni Basin.

"In West Africa there are a number of key elements to hydrocarbon accumulations," he said. "Obviously, there is the source rock, which is developed over large areas of the region, but more locally specific are large river systems and deltas that provide reservoirs and the sedimentary thickness to mature the source rock."

In addition, the major successes in West Africa have been found in basins with mobile substrates — either salt or shale, he commented. In Angola it's salt and in Nigeria it's typically shale.

"For years everyone believed you had to be close to one of these big river systems like the Niger, the Congo or the Ogooue in Gabon to find the right combination of mature source rocks, reservoirs and traps" he said. "Consequently, for the last 20 to 30 years these three areas have been the focus of exploration."

As the shallow water portions of these deltas were played out, exploration efforts began moving to the deepwater — but major international oil companies quickly dominated these plays, leaving little opportunity for independents.

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Activity and success in the prolific West African margin is no longer confined to high-profile plays in the Niger Delta of Nigeria and the Congo Basin of Angola. Companies of all sizes are probing the virtually untouched basins all along the western coast, from Morocco to South Africa.

And in doing so, they are unlocking many of the geologic secrets critical to finding potentially massive oil and gas deposits.

"The industry's geologic knowledge of the West African margin began in the Congo fan and Niger delta," said Paul Dailly, staff geologist for West Africa for Amerada Hess Corp., "and over the last several years we have extrapolated that knowledge to other basins."

Dailly was part of the team with Triton Energy that made the basin-opening Ceiba discovery in Equatorial Guinea's Rio Muni Basin.

"In West Africa there are a number of key elements to hydrocarbon accumulations," he said. "Obviously, there is the source rock, which is developed over large areas of the region, but more locally specific are large river systems and deltas that provide reservoirs and the sedimentary thickness to mature the source rock."

In addition, the major successes in West Africa have been found in basins with mobile substrates — either salt or shale, he commented. In Angola it's salt and in Nigeria it's typically shale.

"For years everyone believed you had to be close to one of these big river systems like the Niger, the Congo or the Ogooue in Gabon to find the right combination of mature source rocks, reservoirs and traps" he said. "Consequently, for the last 20 to 30 years these three areas have been the focus of exploration."

As the shallow water portions of these deltas were played out, exploration efforts began moving to the deepwater — but major international oil companies quickly dominated these plays, leaving little opportunity for independents.

Smaller companies, in turn, had to go in search of new geologic concepts and plays along the West African margin.

"In the last five to six years there has been a growing realization that these deltas and fan systems have been in their present position since the middle Tertiary, when there was a large uplift of the continental margin that reset the drainage patterns for the whole continent," he said.

Through time the areas where these river systems have been dumping sediment has changed, and further analysis led to the conclusion that other places along the margin also have all the key elements necessary for hydrocarbon accumulations.

"There are regions all along West Africa where ancient river systems created the same type of situation that's seen today in the Niger and Ogooue deltas and the Congo fan," he said.

These ancient river systems account for major recent discoveries in the Rio Muni Basin in Equatorial Guinea and offshore Mauritania.

"Today we know there are a number of basins that have been overlooked but have the same elements," he added. "They just aren't as obvious as the Congo and Niger systems."

Stratigraphic Focus

When Triton first began exploring in the Rio Muni Basin it sought areas downdip of older wells that didn't find commercial accumulations but did provide evidence of an oil prone source rock — this had been proven in wells drilled by Elf Aquitaine and Total in the late 80s early 1990s when they drilled several wells in the Rio Muni's shallow waters.

"Due to Late Cretaceous and Tertiary uplift, turbidite reservoirs were only preserved in the deepwater and structurally low areas — these wells simply missed the reservoir further downdip," Dailly said. "These reservoirs are turbidites, similar to the deepwater fields in Angola, but they are Late Cretaceous age rather than Tertiary."

The Ogooue delta just south of the Rio Muni Basin has proven reserves of over two billion barrels — and Triton saw quite a few similarities between Rio Muni and the Ogooue delta.

"We thought there was a good case to be made that the Late Cretaceous sands of the Ogooue delta would also be present in the Rio Muni Basin," he said. "That turned out to be the case. We found similar Campanian age reservoirs. Everyone had been so focused on Tertiary turbidites that the Cretaceous reservoirs were not tested.

"We saw no reason to expect that the Cretaceous turbidites wouldn't be just as prospective."

Triton saw one conventional structure on its acreage, and naturally that was the first prospect the firm tested, resulting in the Ceiba Field.

"This was another reason companies stayed away from the Rio Muni Basin — they could only see one structure and didn't think there was much else to drill," Dailly said. "The three dry holes we drilled after Ceiba told us a great deal about the charge and reservoir systems and about the rock physics. This allowed us to focus in on a stratigraphic play."

Since then the firm has been drilling incised channel systems with updip pinchouts that create stratigraphic trapping geometries — and has made five additional oil discoveries.

"Drilling in the Rio Muni Basin has demonstrated that the Aptian salt basin found in Angola and Gabon continues further north than previously thought," Dailly said, "and likely extends into Cameroon as well."

Areas of Interest

Other salt basins around the margin are essentially completely unexplored. The Senegal Basin, for example, has good oil shows updip, evidenced by a heavy oil accumulation, but downdip in deepwater is virgin territory.

"We know going in that this area at least has a source rock," Dailly said. "Whether that source is mature in our block is a different issue, but at least we have some comfort that it was deposited."

Mauritania is another example. Last year Woodside Petroleum drilled the Chinguetti 1 discovery, which preliminary reserve estimates total 180 million barrels.

"This basin is another example of a paleo delta system that was deposited on top of salt with a thick enough section to mature the source rock and cause the salt to move, which in turn created structuring," Dailly said.

Another area that has people excited, he added, is offshore Morocco, where the same elements of an old river system deposited over salt come together.

A mobile substrate, either salt or shale, is a key element all along the West African margin because it provides both the trapping geometry and focus for hydrocarbon charge. The age of the salt or shale isn't critical, according to Dailly; the salts off northwest Africa are Jurassic compared to Aptian in the Gulf of Guinea and offshore Angola.

An Analogous Approach

"You can match basins fairly well between offshore West Africa and offshore Brazil using plate reconstructions," he said. "For example, we were fairly confident that the Rio Muni Basin was the conjugate basin to the Sergipe-Alagoas Basin in Brazil, which has produced a considerable amount of oil. Early on that was one of the reasons we felt Rio Muni may contain a working hydrocarbon system."

Hence, the company did field work in Sergipe-Alagoas to help understand the geology of Rio Muni.

Understanding the tectonic setting offshore West Africa is critical to understanding the distribution of hydrocarbon play elements. Triton/Amerada Hess also used a combination of satellite gravity data and large-scale regional seismic data to reconstruct the structural evolution of the margin. Onto that structure map were overlaid the play elements of reservoir system, source system and traps.

"This has allowed us to focus on the areas that might have been overlooked in the past," he said.