A Primer for Global Success Science, Alliance Yields Agbami

The Value of Exploration

ChevronTexaco and partners will develop the Agbami Field using subsea wells tied back to a floating production storage and offloading vessel, and a dry tree unit with drilling capabilities located in about 4,400 feet of water. Production will flow from the dry tree unit to the FPSO for processing. The firm expects a total of 37 wells will be necessary to fully exploit the field.

Appelbaum said the most critical element in the Agbami Field is the continuity of the reservoir.

"If you have to drill many extremely expensive delineation wells, the commerciality of the field is impacted adversely," he said.

The first wells drilled at Agbami cost from $60 to $90 million -- but, those costs are coming down considerably as ChevronTexaco and its partners' geoscientists learn more about the pressure regimes in the reservoirs and the best drilling practices.

"Development wells will likely cost about one-third of those first wells," he said.

Just how big is Agbami? It is one of the 10 biggest discoveries of the 1990s, and Appelbaum said he witnessed a phenomenon at the field he had only previously heard about.

"During the deep production tests we could actually see the pressure results of tidal effects on the oil flow," he said. "That was a first for me, although I understand this effect has been seen in some of the largest Middle Eastern fields.

"That was certainly an indication we had a sizable discovery," he laughed.

Appelbaum knows the reorganization that brought all Texaco's exploration professionals under the same umbrella was a major factor in the Agbami discovery.

"Because the team was centralized they were able to feed off each other," he said. "All the various disciplines could talk and learn from each other -- we weren't reinventing the wheel in every area of the process. When people can communicate and interface with each other it creates a more productive environment and fosters a learning/collaborative situation for exploration teams.

"As super majors grow, it becomes more difficult to replace the monumental amount of reserves they produce yearly," he continued. "The whole reason for the large acquisitions we've witnessed in the last several years is the difficulty of finding new reserves.

"But, at the end of the day, we do still have to find new reserves -- and people who can find oil and gas are going to become more and more valuable to companies."

http://www.aapg.org/explorer/2002/02feb/agbami_geology.cfm

Agbami a 20-Mile Long Thrust-Faulted Anticline

According to ChevronTexaco officials, Agbami itself is a northwest/southeast trending doubly plunging thrust-faulted, detachment-fold anticline that's 20 miles long. The structure is characterized by crestal extensional faulting of the Pleistocene to upper Miocene sequences over middle Miocene to Paleocene strata that were thrust-faulted and later uplifted by a shale-cored detachment fold.

Oligocene and earliest Miocene sediments deposited over the Akata shale Formation were middle to lower bathyal shales and basin floor fans with broad areal distribution. Isochron and isopach mapping indicates a major lower Miocene sediment fairway across the crest of the Agbami structure, with the presence of basin floor fan sands likely.

Regional paleoenvironmental and palinspastic reconstructions of the Agbami area support unconfined sheet-like basin floor fan deposition throughout the lower and part of the middle Miocene.

Biostratigraphy and facies analyses of the Agbami 1 and 2 wells have confirmed bathyal environments within the middle Miocene slope channel, slope fan and basin floor fan facies. As the shelf prograded outward, depositional loading induced structural folding, beginning about 12.5 million years ago. This has resulted in bathymetric features that channelized subsequent sedimentation, leading to channel-levee type deposition.

-- KATHY SHIRLEY

http://www.hgs.org/en/articles/printview.asp?209

HGS Int'l Dinner June 21, 2004; Agbami Discovery, Nigeria

Westchase Hilton, 9999 Westheimer, Houston Social Hour 5:30 PM, Dinner 6:30 PM

David Grimes (speaker), Elliott Ginger, and John Spokes

Agbami Field was discovered in late 1998, approximately 105 km offshore Nigeria in the Gulf of Guinea. The field is located in Blocks OPL216 and 217 in approximately 1500 meters of water. The structure is a northwest to southeast trending detachment fold anticline covering an area of 180 km2 at spill point. The discovery well, the Agbami No. 1, was drilled by Star Deep, a wholly owned subsidiary of Texaco, Inc. acting as technical advisor to FAMFA, an indigenous Nigerian oil company. Star Deep brought Petrobras in as a partner and followed the discovery with three appraisal wells and one sidetrack in Block OPL216, plus Statoil drilled the Ekoli 1 well into the same structure in adjacent Block OPL217. In late 2003 and early 2004, drilling resumed with ChevronTexaco, also through Star Deep, adding the first 2 wells from the development plan. After a short drilling pause to acquire seismic data, these wells will be followed by further development drilling later in 2004. All wells to date have penetrated oil bearing sands. The field is a world class development opportunity with significant resources.

Seismic Data Issues

The seismic reflections from the shallower, secondary reservoirs occur above the water bottom multiple and are of relatively good quality. Amplitude extractions of both near and far angle data match predicted AVA models. We can interpret stratigraphic and fluid changes from the seismic data at this level.

However, the seismic data from the deeper, main pay intervals in the 17MY interval suffer from several data limitations. Principal among these is significant multiple energy contamination. Multiple generating surfaces exist not only at the water bottom, but also at other shallow horizons below mudline. The energy from these multiples occurs at the same time as the main pay interval primary reflections over much of the field, seriously degrading the data, especially in the pre-stack domain. Earlier processing of the data during the exploration and appraisal phases of the field, while attenuating the multiples, did not adequately resolve the problem. Consequently, extracted amplitudes from the 3D seismic data did not follow expected Class II AVA behavior. Recent reprocessing efforts by ChevronTexaco using a Gaussian beam method for attenuation of the multiples have given encouraging results that should allow better characterization of the reservoirs from seismic in future reservoir models.

Additionally, wavelet estimations from the seismic data indicate that the frequency content is relatively low, limiting the ability of the seismic to resolve pay sands. Multi-sand intervals tend to image as low frequency, high amplitude far-angle reflectors with typically two or three sands imaged by one peak-trough-peak seismic event.  The minimum sand thickness detectable at the main pay intervals in these data is about 30 meters.

Also, reflectors from the inboard limb of the fold at the northwest end of the structure have diminished stacked amplitude responses and in general, the inboard limb is less well imaged than the outboard limb. Shallow toe thrusts north of the field appear to be masking far offset traces which would normally contribute to the expected high amplitude far angle reflections of oil bearing sands. To record higher angle traces in this area, and hence better map the sand distribution, we are proposing to re-acquire data parallel to this thrust front.