xswtbcvfftestddtzaebtecvxyzvacd

West Africa Gamble Paying Off

Turbidite Trend is Reward

The handful of major international oil companies that negotiated concessions with the Angolan government in the early to mid-1990s seem like geniuses today.

But in reality, the world-class petroleum play that's exploded over the last five years was initially just an effort to leverage shallow water production.

"When we (Chevron and partners Sonangol, Totalfina-Elf, Agip and Petrogal) acquired Block 14 (in 1995), which covers about 1,560 square miles outboard of our shallow water Block 0 concession, we didn't anticipate the enormous potential of the Tertiary turbidite trend," said Tad Schirmer, exploration manager of Block 14 for Chevron Overseas Petroleum. "It simply fit into our company strategy."

That strategy involves minimizing risk as much as possible in the upstream "by working in areas with proven petroleum systems and leveraging our existing experience and infrastructure."

So, he added, this was a natural progression for Chevron in Angola.

"We felt it was likely that the petroleum system existed in deeper water," Schirmer said, "but the real question was, how much riskier was the traditional play going to be?

"We recognized that as we went into deeper water it was likely that the producing horizons on Block 0 could become too deep in terms of porosity retention and risk of gas cracking," he continued.

"There was quite a bit of risk pursuing an entirely separate play in this deeper water block, but we did realize that the Zaire-Congo River fan was a major subsea fan feature driven by long-term Tertiary deposition and drainage of the central African continent.

"It was possible," he added, "there could be a turbidite play similar to what had been recognized in the deep-water Gulf of Mexico."

Generating Some Heat

Several factors converged to make the mid-1990s the jumping-off point for what has become one of the hottest exploration plays in recent years.

  • Geoscientists began to recognize the geologic elements conducive to hydrocarbon occurrence in the new area.

    Such elements as an active petroleum system with multiple source rocks, widely distributed, excellent quality, deep-water turbidite sandstone reservoir rocks and complex tectonic history with salt dynamics, producing multiple structural trends and a myriad of traps across the basin.

  • Advances in technology that allowed exploration and development of oil fields in deep-water settings.

  • Support of the government for the petroleum industry in Angola.

"When you combine all those elements this was a compelling play when we began negotiating for Block 14," Schirmer said. "We give a lot of credit to the Angolan government and Sonangol, the state oil company, for providing a business environment that is conducive to this initially high-risk exploration."

Please log in to read the full article

The handful of major international oil companies that negotiated concessions with the Angolan government in the early to mid-1990s seem like geniuses today.

But in reality, the world-class petroleum play that's exploded over the last five years was initially just an effort to leverage shallow water production.

"When we (Chevron and partners Sonangol, Totalfina-Elf, Agip and Petrogal) acquired Block 14 (in 1995), which covers about 1,560 square miles outboard of our shallow water Block 0 concession, we didn't anticipate the enormous potential of the Tertiary turbidite trend," said Tad Schirmer, exploration manager of Block 14 for Chevron Overseas Petroleum. "It simply fit into our company strategy."

That strategy involves minimizing risk as much as possible in the upstream "by working in areas with proven petroleum systems and leveraging our existing experience and infrastructure."

So, he added, this was a natural progression for Chevron in Angola.

"We felt it was likely that the petroleum system existed in deeper water," Schirmer said, "but the real question was, how much riskier was the traditional play going to be?

"We recognized that as we went into deeper water it was likely that the producing horizons on Block 0 could become too deep in terms of porosity retention and risk of gas cracking," he continued.

"There was quite a bit of risk pursuing an entirely separate play in this deeper water block, but we did realize that the Zaire-Congo River fan was a major subsea fan feature driven by long-term Tertiary deposition and drainage of the central African continent.

"It was possible," he added, "there could be a turbidite play similar to what had been recognized in the deep-water Gulf of Mexico."

Generating Some Heat

Several factors converged to make the mid-1990s the jumping-off point for what has become one of the hottest exploration plays in recent years.

  • Geoscientists began to recognize the geologic elements conducive to hydrocarbon occurrence in the new area.

    Such elements as an active petroleum system with multiple source rocks, widely distributed, excellent quality, deep-water turbidite sandstone reservoir rocks and complex tectonic history with salt dynamics, producing multiple structural trends and a myriad of traps across the basin.

  • Advances in technology that allowed exploration and development of oil fields in deep-water settings.

  • Support of the government for the petroleum industry in Angola.

"When you combine all those elements this was a compelling play when we began negotiating for Block 14," Schirmer said. "We give a lot of credit to the Angolan government and Sonangol, the state oil company, for providing a business environment that is conducive to this initially high-risk exploration."

Of course, Chevron wasn't alone. Elf Aquitaine and Shell, along with Chevron, were the first players to stake a claim offshore Angola. That was a gamble that's paying off tremendously today.

Chevron and its partners drilled the first exploratory well, 14-1X, in 1996 to test the deeper Cretaceous play that is productive in shallow water, as well as some seismic amplitude anomalies in the Tertiary section, to calibrate whether they were sands and if there were hydrocarbons present.

The well did successfully encounter turbidite sands that were oil charged in the Miocene interval.

"We tested those sands, declared a discovery, and kicked off explosive growth in our exploration portfolio on Block 14," Schirmer said. "We now knew there were sands in the system and these seismic features could be oil charged."

From 1996 to 1998 the Chevron portfolio grew from a few prospects in the Pinda, which is the productive formation in shallow water and a primary focus when the block was acquired in 1995, to about 80 prospects in the Tertiary turbidites.

That first well indicated that the Pinda reservoirs were tight and that had a negative impact on the firm's perception of the deeper play. The success in the Tertiary more than offset that disappointment.

'A Tiger By the Tail'

Elf announced the first commercial discovery offshore Angola in 1996 as well, discovering the Girassol Field on Block 17.

While Chevron's first well was not a commercial success -- the terms of the Production Sharing Agreement and expected development costs called for flow rates of 5,000 barrels per day from a single reservoir, and the 14-1X tested 2,700 barrels of oil daily -- it was definitely a geologic success.

"Following that well we went on a very aggressive interpretation process to map up all these channels we were seeing on the 3-D data," Schirmer said. "Our original 3-D dataset was 1,100 square kilometers in the eastern portion of the block in less than 500 meters of water. We studied the imaged channels in that sector, and that's what drove the first seven wells in our program."

All those wells successfully encountered hydrocarbons.

"We had a tiger by the tail," he continued. "We recognized a new play existed, the hydrocarbon system worked and we had significant positive results in the turbidite sands with high quality reservoirs and robust trap geometries."

Today Chevron has shot over 2,500 square kilometers of 3-D seismic covering most of the block, and has an extensive portfolio of prospects.

First Success

Chevron drilled its first commercial discovery in 1997 with its second well, the 14-2X. The Kuito Field discovery well tested a robust amplitude anomaly in a flat spot in the northeast portion of block 14.

The well tested 7,500 barrels a day on a restricted flow rate from the Upper Miocene. The firm immediately drilled three appraisal wells, delineated the field and quickly moved to a phased development.

Chevron achieved first oil at Kuito in December 1999 -- just 30 months from declaration of discovery -- and today the field accounts for the only production from the deep-water turbidite play.

"We wanted to establish production in the block to generate cash flow to move the exploration program forward -- and to gain as much information about the reservoirs as possible," Schirmer said.

Today the Kuito Field is producing 65,000 barrels of oil a day from an FPSO, with ultimate production when all phases are complete to hit over 90,000 BOPD. Kuito is Chevron's first giant field in the play.

Chevron decided on the Floating Production Storage and Offloading System (FPSO) for several reasons:

  • The offshore environment of Angola is quite benign with no major storm tracks like you see in the Gulf of Mexico or the North Sea.
  • Deep-water settings are conducive to FPSO technology.
  • The FPSO allowed Chevron to get the field on-line quickly.

Ups, Downs and Ups

Chevron's third exploratory well (14-6X) commenced in 1997, 20 kilometers from Kuito, and the firm made its second commercial discovery in a new horizon. The well proved hydrocarbons in the Lower Miocene and was depositionally different from the Kuito accumulation.

"Upper Miocene features like the one at Kuito are turbidite fills in very deep canyon cuts -- similar to what we see in the present day Congo Canyon in the block," Schirmer observed. "These canyon cuts are eroded and depositionally backfilled and infilled with shale and turbidite sandstones."

The Lower Miocene, however, is generally deposited in more distributed turbidite channels with more subtle basal cuts and more laterally distributed channels, he added, as opposed to deeper eroded and vertically filled canyons.

Following the Landana Field discovery, Chevron drilled the 14-7X in 1998, 20 kilometers south of the Landana and 50 kilometers from Kuito. The new wildcat tested an Upper Miocene anomaly that appeared similar to Kuito.

The well was successful, but did not achieve commercial flow rates.

That same year the firm drilled two wells five to six kilometers south of Kuito, and both were commercial discoveries.

  • The 14-9X was a dual discovery from the Middle Miocene and Lower Miocene, once again proving up a new horizon in the Middle Miocene and confirming the Lower Miocene as a target. The well achieved a flow rate of 30,000 barrels of oil daily on a test.
  • The 14-10X well was a Middle Miocene discovery that flowed 10,000 barrels of oil per day on a test.

The Benguela and Belize fields will be developed together as a separate development from Kuito. Presently Chevron is in development planning for the fields with project authorization targeted for early 2002.

Coming Up Dry

Following 3-D seismic acquisition in the deeper portions of the block, Chevron was ready to step out with its exploration program in 1999. The company drilled two wells in deeper water in the block's western portion, and both were dry holes.

Factors of failure included trap risk and the need to understand the relationship of the seals within the section.

"Also, during this time we were trying to understand the relationship of seismic amplitudes -- especially amplitude vs. offset -- to the sands and hydrocarbons," Schirmer said. "We felt like we had a handle on that relationship based on the earlier wells, but when we stepped out into deeper water those seismic amplitude relationships had a subtle change.

"The information we gleaned from these two unsuccessful wells allowed us to fine tune our modeling efforts," he added. "We certainly weren't discouraged after all the success we have had."

The two dry holes forced Chevron to go back and analyze its portfolio based on the new information.

"We had a large number of prospects, and we went through a very rigorous process of maturing all the channels, making sure they were evaluated with a consistent risk and volumetric type approach," he said. "We racked up the entire inventory with our partners on a consistent common basis, and we incorporated lessons learned from all our previous drilling activity to better understand this relationship of traps and seismic amplitude to try and understand how to lower the risk on subsequent wells."

Modern-Day Pioneers

All that worked paid off last year, when Chevron discovered the Tomboco Field in the Middle and Lower Miocene about six kilometers west of the Benguela Field in 500 kilometers of water.

The well flowed 16,000 barrels of oil a day on test.

That discovery was followed by the Lobito discovery in the Lower Miocene, which flowed 10,200 barrels daily on test.

"These last two wells were significant breakthroughs for us," Schirmer said. "We developed a new methodology for looking at seismic amplitudes, and it broke some of the paradigms we had previously held. We've learned a great deal about what types of seismic attributes we need to see and how they relate to hydrocarbon sands within Block 14."

Chevron and partners recently announced a discovery at Tombua No. 1 that is one of the firm's key wildcats in 2001, and tested a new structure on the block. The Tombua 1 well flowed over 10,000 bopd from two zones in the lower Miocene.

Indeed, Chevron and its partners have drilled 12 exploratory wells in just four and a half years, gotten a field on-line and planned other major capital projects that will come on-stream between now and 2005.

Plus, Chevron has more primary exploration to do that has a similar risk profile to what has already been found.

"I've been working with Chevron for 15 years and started my career in the United States onshore working mature basins," Schirmer said. "I really felt like I'd missed out by a generation on the excitement of finding big fields and we were just mopping up.

"But here's a place where my generation has the opportunity to do the same kind of exciting wildcatting that generations before us enjoyed," he said. "It's incredible."

You may also be interested in ...