During the 80s several hydrocarbon research campaigns were held in Nyanga in view of the good results obtained in the north coast of Gabon. Although the results were satisfactory, the sociopolitical situation at that time in the region caused the projects were delayed. Thigs were worse in the early 90s and The Revolts of African Territories meant years of delays.
With the political situation normalized, the Hydrocarbons exploration projects in Nyanga began again in 2001. After 5 years of research blocks NC02 and NC04 were put into production with the construction of three platforms and 15 extraction wells that began to supply crude oil in 2009.
The formation of the Nyanga, Gabon, Douala and Rio Muni 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 which has an average TOC of 5-6% with local maximums of 20%. Later rift deposits include reservoir quality sediments accumulating in the fluvial-lacustrine 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 500m of Ezanga salt was accumulated.
The pre-salt section has all the ingredients for a successful petroleum system and this is confirmed by the number of discoveries made since the 1970s. However, seismic imaging of the pre-salt section has always been an issue in Gabon. Recent work has concentrated on Pre-Stack Depth Migration inan effort to improve the image.
Probably the most important source rocks are the Neocomian to Aptian shales of the Melania and Kissenda formations in the South Gabon sub-basin. Both are organic-rich, lacustrine deposits with average TOCs of 6.1% and 1.5-2% respectively. The Melania formation may be present in the North Gabon sub-basin but has not been penetrated by drilling. Tertiary source rocks are also present along the margin with the coaly Miocene Mandorove formation of the Nyanga Basin having average TOCs of 4-5%. It is, however, 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 (Edwards and Bignell, 1988). The Madiela, Azile and Cap Lopez formations did not mature until the Miocene (Teisserenc & Villemin, 1990).
The Basal Sandstone formation is extensive across the South Gabon sub-basin but has not been penetrated by drilling in the North Gabon sub-basin although it is expected to be present from seismic interpretation. The Basal Sandstone exhibits porosities up to 25% and permeabilities up to 100mD. Also in the South Gabon sub-basin, the Lucina and M'Bya sandstone members of the Melania formation exhibit porosities of 15-25% and permeabilities of 10-100mD.
The Barremian age Dentale formation has porosities up to 29% and permeabilities up to 1D, while the extensive Gamba formation has porosities from 20-30% and permeabilities from 100mD to 5D. Post-salt Cretaceous reservoirs are also present as platform sands of the Anguille, Pointe Clairette and Weze formations or as sand-rich channels within the deep water turbidite systems.
Pre-salt 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 sub-basin is extensive and difficult to resolve on existing 2D seismic data. Post-salt traps comprise drapes over salt domes sealed by shales or combination structural/stratigraphic traps of sand-rich channels within turbidite systems.
Oil & gas operations
This was the second block put into operation off the coast of Nyanga in late 2006. This is a block attached to the coast, irregular in shape and covers an area of about 700 square kilometers. The maximum depth in the block is at 103 meters.
Two platforms have been built on the block, each operating 8 wells of which 5 are for natural gas extraction and 3 for oil extraction. Exploration and operation permits of the block belong to the company Petronor that in 2010 obtained in the block a combined production of 111,000 barrels of oil per day and 19.53 million cubic meters of natural gas per day.
The NC04 was the first block which went into production in early 2006 and it has an offshore platform associated with 9 wells; 5 to produce natural gas and 4 to extract oil. This is a block about 300 square kilometers located off the coast in front of the city of Mayumba which is where the center of the oil industry in the region of Nyanga is located. The exploration and exploitation of the block is granted to Petronor.
A production of 60,000 daily barrels of oil and 13 million cubic meters of natural gas per day were extracted from the block in 2010. Maintaining current production level, there are proven reserves in the block for 8 years. Petronor is considering drilling two new wells associated with the existing platform that could increase reserves or production.
It is a block attached to the coast with an extension about 500 square kilometres. It is located on the far north coast of Nyanga, bordering waters of Gabon. The block is located in shallow waters that do not exceed 80 meters in any case.
Texan Universal Oil has the permits to explore and exploit the block and after a three year exploration campaign from 2008 to 2011 the company started the construction of two offshore platforms that will be operatives in the second half of 2012.
According to data provided by the Texan Universal Oil, both platforms will operate 8 wells each. They are expected to obtain a combined output of 90,000 barrels of oil per day and about 9 million cubic meters of natural gas per day. With these projected production levels, the company claims to have proven reserves for 10 years.
- GRYPHON: Pending to be awarded permits.
- ETAME: Pending to be awarded permits.
- NC06: Pending to be awarded permits.
- Diaba: Pending to be awarded permits.