Considering the long productive life of the Gulf of Guinea off West Africa, the area still seems primed for many more years of exploration and exploitation.
Its oil production is at high levels, only a few projects have tapped its gas bounty, and its geology still is attracting new players.
An article in OGJ's Gulf of Guinea Geology Report, starting on p. 30, describes the Golden Rectangle. More than 5 billion bbl of recoverable oil has been found in this area—off Nigeria, Equatorial Guinea, and Cameroon—which is producing more than 1 million b/d. Author Joe Bruso, president of Sovereign Oil & Gas Co., Houston, said largely undrilled petroleum plays still exist in the Middle Miocene through Oligocene sedimentary section along the Niger Delta's southern margin.
Few wells have probed the deeper formations, and close local knowledge and local business associations are important, Bruso noted.
Probing deeper zones
The deepest known penetration in the 1.2 million acre Golden Rectangle is 13,862 ft at the East Luba-1 gas-condensate discovery on Marathon Oil Co.'s Alba Block off Equatorial Guinea.
Maximum drill depths in the Golden Rectangle have been determined mainly by the onset of advanced formation pressure, necessitating 16 lb/gal mud weights. Deeper drilling is minimal because the pressure onset can occur as shallow as 7,000 ft, Bruso said.
For its part, Nigeria is limited by its Organization of Petroleum Exporting Countries quota and has plenty of production capacity in its normally geopressured section. Drilling deeper into higher geopressure is expensive, and all exploration wells drilled deeper than 10,000 ft have discovered gas or gas-condensate accumulations.
"Temperature vs. pressure relationships obtained from Upper Miocene Isongo sandstones in Alba field and Middle Miocene sandstones in East Luba-1 suggest that petroleum accumulations in these older, more deeply buried sediments will be primarily comprised of gas with varying amounts of condensate, according to the bubble point of each reservoir," Bruso said.
Plays and potential
There is a good possibility that several large if not giant hydrocarbon accumulations will be found in Biafra and Isongo-aged deepwater sandstones in the translational and compressional areas of the Golden Rectangle in the next 2 decades, Bruso said.
"The gas and condensates tested from the Middle and Upper portion of the Miocene stratigraphic section drilled at Tourmalina-1 and East Luba-1 could be heralding a future play fairway.
"This fairway would encompass highly commercial Miocene petroleum reserves pooled in duplex hanging-wall anticlines at depths between 10,000 [ft] and 15,000 ft along the apex and frontal zones of the toe-thrust prism," Bruso said.
Commercial gas-condensate accumulations also could be discovered in duplexed anticlines that contain folded Oligocene or Eocene deepwater sandstone reservoirs in the same two areas.
Marathon, Bruso noted, is rapidly commercializing gas in Equatorial Guinea, and numerous LNG, gas-to-liquids, and other gas commercialization projects are proposed in Nigeria, which is already shipping LNG at Bonny.
Marginal opportunities
Along with remaining large finds, the gulf has numerous marginal discoveries and fields.
Okwok field off Nigeria, on OML 115 held by ExxonMobil Corp. and Nigerian National Petroleum Corp., is the subject of farmout negotiations with Oriental Energy Resources Ltd. and a Nexen Inc. unit.
Okwok is being treated as a marginal field, but Sovereign mapped OML 115's existing 2D and 3D seismic data 2 years ago. The study revealed large-potential traps in Qua Iboe in 150 ft of water with the potential of several hundred million barrels of oil reserves, Bruso said.
In Equatorial Guinean waters lie the Tourmalina, Ambar, and Tsavorita discoveries. Tourmalina is too distant to be a Zafiro field satellite and too small at present for stand-alone development. Ambar, found in 1998, could yield 25-80 million bbl of oil and is expected eventually see more drilling. Two-well Tsavorita holds 25 million bbl but is close to an undefined boundary between Cameroon and Equatorial Guinea.
Operating tips
Bruso said Sovereign has observed two important factors critical to commercial success in the Gulf of Guinea region.
Key staff, he said, must have a strong local knowledge base, including technical, commercial, and legal. These managers must be comfortable in the local environment. This generally means a minimum of 10 years of West Africa experience and applies whether the manager is a geophysicist, driller, lawyer, or negotiator.
Second, every operator must have a first-rate local (national) partner or advisors.
"There is an absolute correlation between commercial success and having the correct relationships in the host country. You can avoid corruption and undue delays with the correct indigenous partner-representative," he said.
