Presalt basins identified in Gabon
A new study indicates Brazilian analogs.Article By Jim Martin, Steve Toothill, and Jim Gulland, CGGVeritas; and Martial- Rufin Moussavou, Direction Generale des Hydrocarbures, Republic of Gabon
Published Jul 1, 2009
Gabon, along with other West African countries, has seen an increase in licensing and exploration activity over the last few years, driven by initiatives to reduce the fall in production and replace reserves. A six-month licensing round opening in May 2010 and closing in November will offer blocks from two Gabon deepwater basins: Zone Sud, where the targets are presalt, and Zone Nord, containing both pre- and postsalt targets.
In Zone Nord, the postsalt interest is in Tertiary and Upper Cretaceous La Ceiba analogs having the potential to contain large accumulations. Play concepts have been proven in Equatorial Guinea and shallow-water areas, and new data are being acquired to highlight the areas of interest.
Yet the biggest potential for Gabon is the prospectivity of the presalt section. For these targets, uncertainty remains as to how far offshore both presalt reservoir and source extend. In Gabon’s onshore and shelf blocks, the presalt section has been drilled and discoveries made, thus proving a presalt petroleum system is present.
Recent major discoveries in deepwater settings offshore Brazil have provided significant encouragement for making similar finds in the deepwater sediments offshore Gabon as, at the time of deposition of the presalt section, Africa and South America were co-joined as part of the Gondwana Supercontinent.
The key issue is imaging the presalt section on seismic data. The Ezanga salt deposited in late Aptian times has extensively deformed and now forms narrow, closely spaced diapirs, causing severe raypath distortion. Existing seismic data does not image the presalt section well, and this has discouraged many companies from exploring this deepwater area. However, a comprehensive work program, including a new long-offset test line acquired by CGGVeritas in 2008, has proved that the presalt section can be imaged much more clearly and reveals significant thickness of sediments in tilted fault blocks below the salt.
The formation of the Douala, Rio Muni, Gabon, Congo, and Kwanza basins along the west coast of Africa began in the Early Cretaceous during the breakup of Gondwana and subsequent opening of the South Atlantic Ocean. Early rift sediments include the Barremian-age Melania shale, a rich lacustrine source rock with average total oil content (TOC) of 6.1%. Later rift deposits include reservoir-quality sediments culminating in the fluviallacustrine deltaics of the early Aptian Dentale formation.
A transitional erosion and peneplanation phase followed rifting, culminating in a marine transgression during which the Gamba sandstone, the principal reservoir in the Gamba field, was deposited. This was followed by a period of salt deposition during the late Aptian, during which approximately 1,640 ft (500 m) of Ezanga salt accumulated.
During the drift-to-sag phase, marine carbonates were deposited over the Ezanga salt, and it was during this time that salt tectonism was initiated. The Upper Cretaceous saw a period of deposition of clastics followed by uplift and erosion so that uppermost Cretaceous and Paleocene sediments are missing from the section. Clastic deposition continued during the Eocene, but further uplift and erosion resulted in removal or non-deposition of Oligocene sediments in Gabon.
Hydrocarbon systems. A number of different petroleum systems are present and have been exploited onshore and in the shelf region of the offshore province but, as yet, there has been little exploration in the deepwater regions. Seismic data indicates the extension of the Ezanga salt province into the deeper water together with probable Dentale shale and Gamba sand sections in the South Gabon subbasin.
Source rocks and hydrocarbon generation. Much work has been done on source rocks and oil types from both margins of the South Atlantic Ocean, and published work indicates that direct comparisons can be made to pair basins such as the Sergipe Basin of Brazil with the North Gabon Basin and the Reconcavo Basin, also of Brazil, with the South Gabon and Congo basins.
Probably the most important source rocks are the Neocomian to Aptian shales of the Melania and Kissenda formations in the South Gabon subbasin with average TOCs of 6.1% and 1.5 to 2%, respectively. In the North Gabon subbasin, the Melania formation may be present but has not been penetrated by drilling, and the Kissenda formation has only been encountered in onshore wells.
Good Tertiary source rocks are also present along the margin but are generally immature.
Oil generation along the margin from early Cretaceous source rocks has been active since the late Cretaceous, with the maturity window moving westward over time with continued downwarping. The Madiela, Azile, and Cap Lopez formations did not mature until the Miocene.
Reservoir rocks. There are numerous potential reservoir rocks in the Lower Cretaceous of the North and South Gabon subbasins. The Basal Sandstone formation, which exhibits porosities up to 25% and permeabilities up to 100 mD, is extensive across the South Gabon subbasin but has not been penetrated by drilling in the North Gabon subbasin, although it is expected to be present from seismic interpretation. Also in the South Gabon subbasin, the Lucina and M’Bya sandstone members of the Melania formation exhibit porosities of 15 to 25% and permeabilities of 10 to 100 mD. The Barremian-age Dentale formation has porosities up to 29% and permeabilities up to 1 D, while the extensive Gamba formation has porosities from 20 to 30% and permeabilities from 100 mD to 5D.
Postsalt Cretaceous reservoirs are also present as platform sands of the Anguille, Pointe Clairette, and Weze formations or as sand-rich channels within the deepwater turbidite systems.
Traps and seals. Presalt traps comprise tilted fault blocks sealed by shales or salt and structural closures associated with salt movement. Structuring beneath the salt in the South Gabon subbasin is extensive and difficult to resolve on existing 2-D seismic data. Postsalt traps comprise drapes over salt domes sealed by shales or combination structural/ stratigraphic traps of sand-rich channels within turbidite systems.
The complexity of the geology requires a robust seismic acquisition and advanced imaging workflow to high-grade blocks. This has been accomplished through a three-phase approach designed to improve understanding of this complex basin at each stage.
Phase 1, the basin modeling phase, is completed and results are now available, providing a unique suite of data covering the 9,652.5-sq-mile (25,000-sq-km) Zone Sud area. An integrated workflow was undertaken combining new seismic acquisition with interpretation of vintage data and inversion of potential field data.
A test line acquired in 2008 used a deeptow, large-source, long-offset recording technique. Acquisition parameters were designed to maximize energy transmission through the salt and, in particular, low-frequency energy. Data processing was performed using a thorough sequence through to prestack time migration followed by prestack depth migration using controlled-beam and wave equation techniques. The final processing result demonstrates that significant improvements are achievable in presalt imaging compared to all existing seismic data.
Deep basement geometry is clearly visible and well-imaged. This provides important structural information on the deformational history during the rifting phase of the opening of the South Atlantic. The seismic data also images the presalt sediment section in tilted fault blocks.
Interpretation of this test line was combined with interpretation of existing pre-2008 data to provide key horizons. The seismic interpretation was then integrated with gravity and magnetic inversions to provide an estimate of the depth to basement and the presalt sediment isopach. Final 3-D integrated inversion results show clear regional trends with the identification of deep basement and thick, presalt sedimentary sections extending over large areas.
Phase 1 is now being followed by the acquisition of additional phases, including gravity gradiometry data and a major seismic program in high-graded areas.
Phase 2, gravity gradiometry, records high-resolution, multitensor gravity data providing lateral positioning information on the location of salt bodies. This can be incorporated into accurate depth models for prestack depth migration.
Phase 3 includes acquisition and processing of more than 6,700 miles (11,000 km) of new 2-D seismic data covering both zones. Data acquisition is complete and processing underway. In Zone Sud, the presalt requires acquisition parameters using longoffset, deep-tow acquisition, maximizing low-frequency energy penetration. For Zone Nord, the survey is complicated by pre- and postsalt plays. To image both targets successfully, the data is acquired using an over/under technique with two cables placed at different depths. These record the high frequency, high-resolution data for the Tertiary/Upper Cretaceous targets alongside the low-frequency, deep-tow data required for the presalt targets.
Additional recent-vintage 2-D and 3-D data is also being reprocessed.
The multiphase project being performed on the new tracts of Gabon acreage to be offered to the industry in 2010 demonstrates all the attributes and potential for these tracts to become world-class petroleum provinces of the future. The three-phase approach, using existing and new seismic data specifically tuned to unravel the complexity of structure below the salt, has helped to deliver this potential.
The presalt section of Gabon is revealed as having multiple indicators of significant prospectivity. Clearly there is considerable potential in the presalt of Gabon.