'Very Far Offset AVO' Indicates Pay Dirt

Technology Helps in Congo Play

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 geologic successes.

Of that total, up to 31 may be commercial accumulations. The 68 percent success rate is astounding for a rank exploration play.

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.

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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 geologic successes.

Of that total, up to 31 may be commercial accumulations. The 68 percent success rate is astounding for a rank exploration play.

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.

Building Blocks

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 include:

  • 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.

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