The relatively recent, very large discoveries offshore Brazil have put pre-salt exploration in the South Atlantic back in the foreground.
Pre-salt exploration history, however, is far more ancient and marked with some major successes – but also many failures.
Here’s a brief overview of this exploration in West Africa until the M’Boundi discovery, which will be described in detail as it brings some lessons that readers can discover for themselves.
Problems for Pre-Salt Exploration
It may be useful to explain, with our present knowledge, what makes pre-salt exploration particularly difficult.
Schematically two main plays can be described:
♦ First we look at the the clastic Chela Congo)-Gamba (Gabon) formation, which is linked to the salt formation, acting as a seal, and may function by structural trapping. These sandstones have generally good characteristics and the reserves may be increased by the sands of the Dentale formation, found only in Gabon and deep offshore Congo.
The first major discovery was made onshore Gabon by Shell (1963) with the Gamba field – 50 years and 230 million barrels later it is still in production.
Immediately below the salt we encounter, although less frequently, the Toca formation, a good carbonate reservoir, associated with local preservation following peneplanation of the pre-Gamba formations.
Part of the Takula field in Cabinda, discovered by Gulf in 1971 with the 44-1-X well (about 150 million barrels reserves), is associated with this play.
♦ Second, there are the older formations, some related to the rift, which have generally poor reservoir characteristics. The traps are either structural (mainly fault related) or stratigraphic.
Within these series only small or sub-commercial fields have been discovered, such as Lucina, M’Bya and Tchibala, offshore Gabon; Mengo, a significant accumulation (1979); and Kundji (1980) onshore Congo, which, before production stopped in 1993, produced fewer than one million barrels.
Exploration of these plays faces many challenges:
- The salt can be a screen for penetration of the seismic waves as well as generating multiples (although these can be removed more easily today) and the varied components of the salt have different velocities that represent complexity for time-depth conversion.
- The seismic quality also is damaged by carbonate turtle back features and dissolution basins.
- In addition, seismic acquisition in rain forest and swamp areas is very expensive.
- The existence of excellent quality source rocks, which partially compensates for the rather poor quality reservoirs and sparse trapping, results in effective petroleum systems.
The Rabi-Kounga Discovery
In 1985, Shell-Elf JV, encouraged by the Echira discovery (1984) and oil shows on the M’Bari 1 well (1978), drilled the Rabi 1 well. After some appraisal wells the giant Rabi-Kounga field was confirmed onshore Gabon with ultimate reserves of about one billion barrels within the Gamba-Dentale formations.
This discovery was followed by a rush of oil companies, with huge commitments mainly on onshore permits in Gabon as well as in Congo. This activity resulted in dusters or minor discoveries such as Etame (1998) offshore Gabon, when the Rabi-Kounga trend was still a prospective-but-difficult play; Coucal (1987, 70 million barrels); Avocette (1989, 150 million barrels); and Tsiengui (2004).
Pre-Salt Exploration in Congo
The conclusions of a 1982 Elf pre-salt study group on Congo and the South Gabon offshore, which I was leading, were not very optimistic.
Later, Jean-Marie Masset, Elf exploration manager in Congo, proposed the hypothesis that the Grès de base (Vandji) may have better characteristics closer to the basement and started a seismic survey over this area.
This CGG survey (1989) was stopped before its completion as crew members were taken hostage by Cabinda rebels.
Thankfully they were released after several days and a long march in the forest.
Nevertheless, with the few lines recorded, it was possible to define a prospect: Kouakouala, which René Vernet, who succeeded J.M. Masset, decided to drill in 1993. This prospect appeared better defined than the M’Boundi prospect located about twenty kilometers westward.
As anticipated the Grès de base were encountered with fair characteristics – however it remains a modest discovery (15 million barrels reserves).
This success was an incentive to carry on exploration, and new stratigraphic studies run on the Holle 1 well indicated that the poor reservoir quality encountered in this well could be attributed to the Djeno formation – and proved that the Grès de base had not been reached.
The M’Boundi prospect, although not very well defined, started to look interesting.
Nevertheless, Elf – despite the unrisked 80 million barrels stakes – was disappointed by the modest size of Kouakouala and the amount of exploration spendings made on this permit, and the company proposed the relinquishment of the La Loeme permit.
At that time Arco, interested in M’Boundi, started discussions with Elf.
I had moved from Africa exploration vice president (1987) to vice president-new ventures and negotiations, and I received a phone call from Marlan Downey. I knew Marlan since his time as president of Pecten (the international branch of Shell U.S.), which was a partner of Elf in Cameroon, where I had been previously exploration manager (1983).
Marlan was now Arco E&P president. He proposed that M’Boundi could be drilled without additional seismic that would have been equally as expensive as the well.
But he did not succeed in convincing Elf, which relinquished the permit in 1995.
The Maurel & Prom Period
Jean-François Hénin, CEO of Maurel et Prom, wished to transform this shipping and trading company into an oil company.
After the Elf relinquishment, Kouakouala and the Kouilou permit (including the former La Loeme permit) were granted to an African businessman, who later sold some interests to Heritage and Tacoma, two junior companies. In 1999 Maurel & Prom joined the J.V. and became the operator. Maurel & Prom had a budget for only three wells, two on Kouakouala, as they wanted to start early production by truck.
M’Boundi 1 was spudded in May 2001. The rig Maurel et Prom contracted was in a poor condition; the drilling difficult and longer than forecast. Maurel & Prom being short of cash, was tempted to stop the operations in what, some thought, to be conglomerates close to the basement.
Peter Mey, the Maurel & Prom geologist who had taken the permit with Philippe Labat, operations manager, realized that on Kouakoula 1, a carbonate layer was present above the Grés de base, and this layer had not yet been reached in the well.
The technical team managed to convince Jean-François Hénin that drilling should continue, and after crossing the prognosed carbonate layer the well encountered the Grès de base with a 130-meter oil column! The reservoirs were better developed and better still than on Kouakouala. The reservoir could not be logged to TD, but the well was tested (1,400 bopd).
After the discovery, Daniel Pélerin, who had just joined Maurel & Prom, managed to acquire a 3-D survey on the field in 2002, which enabled accurate field definition. The relatively smooth structure mapped by Elf in 1995, modified in 2002 after the first three wells (101 to 103), transformed into a larger structure consisting of tilted blocks inherited from the Barremian rift.
The reserves were estimated at one billion barrels OIP – far larger than the initial Elf estimations. However, the complexity of the field with various compartments made production and water injection difficult.
After an initial production without water injection, Maurel & Prom decided to sell the field to ENI, which took over operatorship in 2007. This is another story …
To date about 130 million barrels have been produced (and only five million barrels on Kouakouala).
At the same time Maurel & Prom was positioning itself along the basement in Congo and Gabon. This strategy led to the Onal discovery and some satellite fields in Gabon, south of Lambarene in the Grès de base, as well as Omoc, within the Kissenda formation (Djéno equivalent), and the Loufika discovery close to Kouakouala in Congo, in the Pointe Indienne formation.
All these successes were modest, as well as the others drilled by the Industry both onshore and offshore Gabon.
Conclusions
Pre-salt exploration in West Africa has had euphoric periods followed by disappointments.
The post M’Boundi period has not yet been brilliant. However the large pre-salt discoveries in the deep offshore Brazil have encouraged similar exploration in West Africa, with some successes already in Angola (Cameia), Gabon (Diaman) and in Congo, with the Néné discovery in 50-meter water depth.