A F R I C A
Think ahead and pass the salt
Some 20 years after its first licensing round, Namibia finally has become the oil explorers’ hot spot, writes Maria Kielmas.
O
ne of the most enduring myths surrounding oil exploration off- shore West Africa is about the
necessity of salt. The conventional wisdom stated that if you wanted to find a good quality source rock, and a decent seal to the reservoir, you needed to look in a salt province. Nigeria, where offshore structures are con- trolled by mud diapirs, was the obvious exception to this rule. The salt myth emerged as a result of Shell’s 1967 dis- covery of the Gamba oil field in Gabon. The West African salt basin, extending offshore from Gabon, through Congo to Angola, became the next focus for oil explorers in the region. Subsequent discoveries seemed to prove the new axiom. The thinking was that any exploration outside the salt basin would find just small accumulations of oil or gas. When newly-independent Namibia
launched its first licensing round in 1991, the salt myth still prevailed. The southern extent of theWest African salt basin is defined by theWalvis Ridge, an east-west trending zone of volcanic intrusions and faulting extending from the south-west African continental margin to approximately the island of Tristan da Cunha in the mid-Atlantic. It appears to have acted as a barrier to salt from the north and a dam for sedi- mentary flow from the south. The view was that as Namibian exploration acreage lay outside of the salt basin, its hydrocarbon potential was not spectac- ular. There was no shortage of explorers hoping to disprove this opinion. But Namibia’s legal and fiscal terms were viewed as too tough for poorly capitalised independent compa- nies to take the risk. Namibia held three exploration licensing rounds in the 1990s, offering
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acreage over its four offshore basins – (from north to south) Walvis, Namibe, Luderitz and Orange. The principal attraction at the time (and proof of hydrocarbon generation) was the Kudu gas field in the south, which was dis- covered in 1974 by Chevron (see box). Companies drilled nine exploration wells in the 1990s offshore northern Namibia, and a total of seven wells in the Kudu field over the period 1974 to 2007. Norsk Hydro was the only com- pany to register lacustrine oil shows in its well in the northernmost Walvis basin. The third exploration round in 1998, a time of low oil prices, attracted no bids at all. This persuaded the Namibian government to change its licensing procedure from bidding rounds to open licence. Companies could now apply for acreage at any time.
Persuasion from Brazil and Ghana Investors needed to be convinced fur- ther about Namibia’s merits. In the 1990s all of the exploration work had focused on the continental shelf areas
Namibia
where water depths were less than 350 metres. The targets were not massive structures but small pinch-out traps abutting the shelf. Deepwater explo- ration on either side of theAtlantic pro- vided a different perspective. The good news camewith oil discoveries between 2006 and 2007 in Brazil and Ghana respectively. In Brazil, it was the dis- covery of the Tupi oil field in a pre-salt formation of the offshore Santos basin. Tupi was followed by further pre-salt discoveries such as Carioca and Jupiter, some of the largest oil discoveries any- where in the world since the 1970s. In Ghana, it was the discovery of the off- shore Jubilee field in Tertiary age for- mations with no salt present. A reconstruction of the African and
Latin American land masses into the Gondwana proto-continent shows a common geological systemon each side of today’s Atlantic Ocean up until the continents
split from the Early
Cretaceous period onwards. Source rocks of the pre-salt oil discoveries in Brazil are present offshore Namibia. Two younger source rock sequences have also been identified in the country. Potential reservoirs in Namibia are the sandstones and marine sand- stones deposited as the Atlantic rift system was spreading. These were fol- lowed by widespread turbidite fan sys- tems deposited during the Upper Cretaceous and Tertiary periods. In parts, shale sequences are thick enough to provide efficient seals. Drilling in the 1990s showed that
Figure 1: Namibia’s licence map
Source: Energy and Mines Ministry
PETROLEUMREVIEW APRIL
2011
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