It has taken $46 billion over 12 years to bring Kashagan, the biggest world oilfield discovery since 1968, to the brink of production.

But some in the seven-member development consortium are wondering whether they will be able to recoup their investment in the western Kazakh oil field before the current Production Sharing Agreement expires in 2041.

They hope the departure of one partner, ConocoPhillips, will lead to a reshuffle that will give them greater operating control and extend the PSA beyond 2041, long enough to guarantee returns from the larger second phase of development.

"We've developed only a small fraction of this gigantic field. This is a lifetime development," said Alain Guenot, planning director for North Caspian Operating, or NCOC, the consortium that unites the seven investors.

Up to 12 billion barrels of oil, enough to supply the world for four months, lie in wait beneath Kazakhstan's portion of the Caspian Sea, to be extracted by the consortium that includes ExxonMobil, Shell and Eni.

ConocoPhillips, which is disposing of billions of dollars' worth of overseas assets to reduce debt and increase its exploration and dividend budgets, wants to sell its 8.4% stake, Oil and Gas Minister Sauat Mynbayev has said.

The Houston-based company has not confirmed this.

State option

If ConocoPhillips does sell up the state oil and gas company, KazMunaiGas, would have first refusal on a stake some analysts estimate to be worth more than $5 billion.

Kazakhstan, the largest economy in central Asia, has grown in confidence and is pushing to redraw contracts agreed in the lean years after being cast adrift by the Soviet Union's collapse in 1991.

The state entered the project as a shareholder in 2005, after locking horns with the developers over ballooning costs. KazMunaiGas doubled its stake three years later to its current 16.8%.

This is the same as Eni, ExxonMobil, Shell and Total. If peak production of 1.5 million barrels per day is ever achieved, these five partners would each be entitled to 252,000 barrels per da.

Home to 3% of the world's recoverable oil reserves, Kazakhstan is the largest former Soviet oil producer after Russia. Oil is the cornerstone of the economy and production is forecast to rise 60% by the end of the decade.

Increasing the KazMunaiGas stake would allow the country greater access to the profits from its offshore fields as its reserves on land dwindle, due to extraction that in some parts has been going for more than a century.

"There once was a saying: 'What's good for General Motors is good for the United States'," Daniyar Berlibayev, deputy chief executive of state oil and gas company KazMunaiGas, said during an oil conference on Oct. 4.

"I could also say: What's good for KazMunaiGas is good for Kazakhstan."

But it is not a foregone conclusion that KazMunaiGas, stretched by a $6 billion refinery overhaul and the social costs of rehiring oil workers laid off elsewhere, will buy Conoco's stake.

The Kazakhs are also aware that they need the financial and technical muscle of the oil majors who want to make sure they get access to the second phase of development.

Long life

Oil minister Mynbayev expects first oil in March 2013. Daily output is forecast to reach 370,000 barrels by 2016. The ultimate Phase One target, requiring the addition of extra gas compressors, is 450,000 barrels per day.

But with up to 12 billion barrels of recoverable reserves within the Kashagan concession area - about the same as OPEC member Algeria's total proven reserves - the real value lies in a second phase that will begin beyond 2020.

"Everybody would like to extend the PSA. There have been discussions," said Guenot. "We know that the life of this field is much longer than 2041."

An official in the capital Astana, who spoke on condition of anonymity, said ExxonMobil and Shell had proposed a 20-year extension of the PSA and requested operating control of Phase Two.

Asked about this at recent conferences in Astana and London, senior executives from both companies declined to comment. The oil ministry's executive secretary, Kanatbek Safinov said he was not aware of any request to extend the PSA.

He estimates Phase Two investment costs at anywhere between $10 billion and $20 billion. Mynbayev, citing prohibitive costs, sent an earlier Phase Two development plan back to the drawing board in 2008.

"Ultimately, further development needs to be profitable for the majors," said Dominic Lewenz, oil and gas research director at Almaty-based investment bank Visor Capital.

"If they feel the overall development not sufficiently attractive, the government may need to put some sort of sweetener on the table."

Shallow and icy

Kazakhstan needs the foreign companies to be involved because it does not have the first-hand experience of developing a deep and exceptionally complex offshore field involving a man-made archipelago stretching across the northern Caspian Sea.

"At the right levels of government in Kazakhstan, they know that they also need support in developing the deeper and offshore fields - which are, after all, still regarded as some of the world's most formidable engineering challenges," said Lewenz of Visor Capital.

The sea is less than five meters deep in places, too shallow for large ships. It freezes for five months a year, heaving ice floes against islands made from local limestone. Once, they even carried a stranded wolf ashore.

And though the water is shallow, the seabed reservoir itself is more than four kilometers deep. It's also high-pressure and rich in combustible hydrogen sulphide. Safety measures, Guenot said, are part of the reason that costs have overshot estimates.

Twenty wells on two islands - there will be 40 by 2016 - will be controlled remotely from D Island, the offshore hub.

From a peak of 6,500 during construction, staff numbers will drop to 240 at a time when production begins. Another 200 per shift will be onshore at the Bolashak plant which lies at the other end of a 130-kilometer pipeline from the sea. Eighty percent will be Kazakh citizens.

Control room operator Sagyndyk Taniyev from east Kazakhstan acknowledges the role the consortium has played in his training: "We have many foreign specialists here. We're always learning from them."