* Angola could double reserves
* Brazil made huge finds in similar rock formations
* Drilling to rise in 2014, Congo plans auctions
* Pre-salt has tech risks, high costs
By Emma Farge and Shrikesh Laxmidas
DAKAR/LUANDA, Nov 29 (Reuters) - Investors are stepping up the hunt for hundreds of billions of dollars worth of oil beneath a deep submerged salt crust offshore West Africa, seeking to emulate Brazil's major discoveries across the Atlantic.
Geologists have long held that Africa's western seabed mirrors South America's. The continents were fused into a single plate nearly 200 million years ago.
Now, high oil prices consistently above $100 a barrel and cheaper technology make it possible for producers to explore thousands of feet below the surface.
The enthusiasm follows pre-salt finds by Total and Cobalt in Gabon and Angola, shifting focus to a region that has played second fiddle to east Africa's gas boom.
William Hayes, senior VP at explorer Kosmos Energy, told Reuters the firm expected a "suite of smaller, but still globally significant discoveries" in the region.
Jasper Peijs, BP's exploration director for sub-Saharan Africa, said he expected super giant discoveries off Angola.
"All the prospects there have the potential to be giant, which I would say is at least 250 million barrels and greater, or super giant of 500 million to a billion barrels and even greater than that," he said on the sidelines of an African oil and gas conference in Cape Town.
So far, most of the excitement has centred on Angola where prospecting is more advanced after a licensing round in 2011.
"The first year was one of acquiring seismic data, then evaluation of results and 2014 will be a year of drilling for most operators," Jean-Michel Lavergne, Total's General Director in Angola, said, adding that the company will drill two of the 10-15 pre-salt wells set to be explored in 2014.
In an interview with Reuters earlier this month, Angola's Oil Minister Jose Botelho de Vasconcelos said it could take operators eight to 10 years after discovery to start production.
Some expect the pre-salt zone to extend further north to the Republic of Congo and Equatorial Guinea or south to Namibia, stretching along a more than 2,000 kilometre coastline.
In addition to Angola, at least half a dozen pre-salt wells are either underway or have been announced in neighbouring Gabon and Equatorial Guinea.
Brazil has made several pre-salt finds since 2007 in the Santos Basin off its southeastern coast. It also has a host of new wells, such as the massive Libra field.
The International Energy Agency expects Brazil to become the sixth largest oil producer by 2035 and to meet a third of the net growth in global supply.
It said however that the pace of supply growth will hinge on the ability of its state oil company Petrobras to stick to its investment plans, which project $237 billion over five years, making it the world's largest corporate spending programme.
Wood Mackenzie's Martin Kelly, head of upstream research, estimated that total reserves in West Africa were between 10-15 billion barrels, or about a quarter and a third of Brazil's.
"The early signs are very encouraging that there will be commercial volumes of hydrocarbons," he said.
Angola, Africa's biggest producer after Nigeria, led the way, awarding 11 pre-salt licences to seven majors in 2011, including BP, ENI, Statoil and Total.
Amid the pre-salt buzz, the Republic of Congo also said earlier this month it had launched a new licensing round including offshore blocks.
This follows the completion of a well-attended licensing round by Gabon in late October which it says could nearly double its production to 500,000 barrels per day. Ophir Energy, Exxon Mobil and Repsol were among the winners.
"If all goes as well as expected, Angola can in a few years double its reserves, which stand at just under 13 billion barrels right now," said Jose de Oliveira, head of the Energy Nucleus at Luanda's Catholic University.
Risks for investors include Africa's well-known political instability. The technology is also formidably complex and drilling a single well can cost between $120-$200 million.
Paul McDade, COO at Africa-focused Tullow Oil was proceeding with pre-salt drilling in Gabon, but added that the scale and the location of reserves was "quite uncertain".
"The story is still evolving," he said.
The International Energy Agency expects output from non-OPEC African producers to trend lower in the long-term from 2.3 million bpd in 2012 to 2.1 million in 2035. Nigeria and Angola are the only two sub-Saharah African members of the Organization of the Petroleum Exporting Countries.
Angola's oil production is also set to decline to 1.4 million bpd over the same period from 1.9 million bpd in 2012 as some of its wells drilled over the past 15 years are maturing.
Still even the modest pre-salt discoveries in mid-sized producers could reverse declining output in a sector accounting for more than 50 percent of GDP.
"A pre-salt jackpot could turn fortunes," said Rolake Akinkugbe, head of oil and gas research at Ecobank.