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.