Geoscientists plumbing the depths of the
south Atlantic Ocean offshore West Africa must feel like the wildcatters
of yesteryear. The deep-water play centered off the coasts of Angola
and the Republic of Congo is for this new generation of explorers
what the prolific fields of Texas, Saudi Arabia and Venezuela were
for earlier legions of elephant hunters.
The successes tallied so far in this play rival any
of those earlier prolific provinces as well.
In just eight years, 62 exploratory wells have been
drilled in the Lower Congo Basin Tertiary deep-water trend (water
depths range from 200 to 1,500 meters) and 42 of those wells were
Of that total, up to 31 may be commercial accumulations.
The 68 percent success rate is astounding for a rank exploration
In April, ExxonMobil announced its 10th oil discovery
in three years in Angolan waters.
The key? Spokesmen for all the major oil companies
working the play agree that 3-D seismic technology has been the
key to unlocking the secrets of this new petroleum system and keeping
technical risks very low.
To date, several fields announced offshore West Africa
are giant accumulations with recoverable reserves of 500 million
barrels of oil or more. The mean size of the field discoveries:
at least 200 million barrels of recoverable reserves.
Several of the fields also are vertically stacked
and grouped geographically around structural traps -- characteristics
that will enable production facilities to tap multiple accumulations,
which will add value to the infrastructure.
Plus, fields in this deep-water trend have and will
continue to benefit from recent technical advances in deep-water
development technology that makes production in 1,000 meters of
water feasible, according to the paper "Lower Congo Basin, Deepwater
Exploration Province, Offshore West Africa."
That paper, authored by J. Laite Da Costa and J.
Leite of Sonangol, Angola's state oil company, and Tad Schirmer
and B.R. Laws with Chevron Overseas Petroleum, was presented at
the Petroleum Provinces of the 21st Century Pratt Conference last
year in San Diego.
The authors expect this new prolific deep water province
to be the impetus for future advances in deep-water technology to
tap the prospective areas in water depths up to 3,500 meters.
They say the benign ocean environment typical of
this part of Africa's western coast will allow technology to push
into greater water depths than is practical in more extreme environments.
Indeed, the future seems just as bright as the recent
past for this exploration province, because:
- Significant areas with numerous channels and traps remain undrilled.
- About 60,000 square kilometers of highly prospective acreage
with play elements consistent with the successful Tertiary prospects
in the present trend are completely unexplored.
This unexplored area lies outboard of blocks 14 through
18 in Angola and Haute Mer concession in the Republic of Congo,
in water depths from 1,500 to 3,500 meters.
Exploration in the Lower Congo Basin, which covers
115,000 square kilometers out to water depths exceeding 3,500 meters,
began in 1966 when Gulf Oil was granted the Cabinda concession.
That early round of exploration uncovered the Cretaceous
pre- and post-salt trend that today stretches 350 kilometers along
the coastline -- in water depths of less than 200 meters -- from
the Republic of Congo to central Angola.
Currently, the shallow-water Cretaceous reservoirs
account for 500,000 barrels of oil daily from Block 0 Cabinda concession,
which Chevron inherited as part of its acquisition of Gulf Oil.
Additional production comes from along trend in the
Republic of Congo to the northwest and the Democratic Republic of
Congo and Angola to the southeast. A large number of fields have
been discovered here, with cumulative produced and proven reserves
of 14 billion barrels of oil equivalent.
Interest in the Lower Congo Basin deep water began
to build in the early 1990s as companies picked up blocks in the
200- to 1,500-meter water depths. That interest intensified in 1996,
when Elf Aquitaine and partners announced the Girassol discovery
on Block 17 -- reportedly a giant accumulation.
In following year activity increased again, with
Chevron's Kuito and Landana discoveries on Block 14, and the Dahlia
and Rosa successes on Block 17.
An expansion of exploration in the basin in 1998
netted four new discoveries on Block 15 by Exxon and partners, as
well as the discovery of the Benguela and Belize fields in Block
14. That set up 1999 activity, when there were two additional discoveries
on Block 15, and one in Block 17 and Block 18.
The Trap Zone
Today the deep-water Tertiary turbidite trend stretches
300 kilometers, from the Haute Mer block in the Republic of Congo
to Block 18 in central Angola.
The fields discovered to date have combined structural
and stratigraphic trapping mechanisms. The most obvious features
- Large traps associated with drape over deeper structures.
- Rollover associated with extensional faults.
- Fault truncation of channels.
- Salt-related structures.
- Faults appear to be an important trapping mechanism, and may
cause compartmentalization of fields.
Stratigraphic components of trap are commonly caused
by lateral pinchout of sand facies at the margin of channel deposition.
Since the bulk of the rock column in the Congo submarine
fan is mud/shale, with sands deposited mostly within channel systems,
the lateral seals to the system should be good. In areas where channel
density is high, the lateral seals to the system may be compromised
due to lateral overbank sands and crosscutting channel systems.
The lowest risk for lateral seal are within isolated
channel features cut into shale.
In the near term, exploration efforts will continue
to focus on the combined structural/stratigraphic traps -- but sometime
in the future the subtle and pure stratigraphic traps likely will
be better understood, and will contribute to the basin's prospectivity.
In addition, there are significant areas of subsalt
that are prospective -- but the lower quality of seismic imaging
and the prospect's spatial location beneath the salt overhangs will
raise the geologic risk in these areas.
The extra effort likely will be worth it, officials
say, since the petroleum system and turbidite channels bordering
these subsalt areas to the east make the subsalt play highly prospective.
They also believe that as geologic understanding
of the Lower Congo Basin grows with each deep-water well, the future
of this new elephant hunting ground shines brighter still.
For them, Africa's West Coast and the ultimate potential
seems as limitless as the vast plains of Texas must have been for
wildcatters watching the gusher at Spindletop.