Forty-five years after its accidental discovery deep under the swamps of West Siberia, the race is now on to develop the world's largest shale oil resource, Russia's Bazhenov.
A drilling push begins in earnest this spring, spurred by President Vladimir Putin's promise to loosen the Kremlin's grip on proceeds from its natural resources in hopes of sparking a shale revolution aimed as much at remaking Russia's oil industry as at raising output.
ExxonMobil and Russian state oil company Rosneft are headlining the new phase of exploration to see whether Bazhenov lives up to its billing.
Russian producers have already reported 500 million metric tons (551.16 million tons), or 3.5 billion barrels, of recoverable crude oil reserves in Bazhenov to the Russian government.
Much studied but largely untapped, it lies in impenetrable black clay beneath existing oilfields covering most of West Siberia, whose westernmost hub is the oil town of Tyumen, 1,700 kilometers (1,060 miles) northeast of Moscow.
"Back when I worked in exploration, one of our geologists called it the wild black stone," said Tatyana Smagina, head of reserves management at the Tyumen Oil Research Centre, part of Rosneft.
The centre holds core samples from 30 years of drilling in West Siberia that are studied to help determine where Russia's new oil will come from.
The government now estimates that the wild black stone could yield 1-2 million barrels per day by the end of the decade.
Yet the full scale of its riches remains a mystery. Estimates range from a conservative three billion metric tons, or over 20 billion barrels, to as much as 143 billion metric tons, according to a survey of Russian research by oil consultants IHS Cera.
The upper estimate would mean an extraordinary one trillion barrels, nearly four times the size of Saudi Arabia's oil reserves or 30 years of world supply at current rates of consumption.
The International Energy Agency describes Bazhenov as the world's largest source rock, a bed of ancient organic matter dating back to the Jurassic period which has given rise to most of the crude oil pumped from the fields of West Siberia.
NO BAKKEN SURGE, YET
Those vast reserves do not necessarily add up to a U.S.-style production surge in the making.
The U.S. state of North Dakota has estimated oil production from its Bakken and other formations could reach 1.2 million barrels per day by 2015.
Yet Russia already pumps more conventional oil supply than Saudi Arabia at around 10.3 million barrels per day and may look to shale to maintain rather than revolutionize production.
Its oil-dependent leaders, rather than fearing the surge in U.S. output from shale, have looked on with curiosity and some envy.
For them, Russia's rediscovery of its shale resources is aimed more at offsetting declines in the fields that produce most of Russia's crude - the Soviet giants of West Siberia, like the Salym group fields where Bazhenov was discovered in 1968.
Output there is now declining at an average of about 2 percent a year.
The U.S. experience has produced a new generation of high-tech, low-cost drillers and producers with advances in technology, including horizontal drilling to tap reservoirs more efficiently, and hydraulic fracturing to release hydrocarbons from non-porous "tight" rock.
In contrast to the entrepreneurs who predominate there, Russia's oil industry has been concentrated in the hands of a few powerful players over the last decade, mostly controlled by the state, raising questions about its efficiency.
It faces the task of moving away from standardized bulk drilling and top-down design decisions toward shale and other new deposits where new technology must be developed and honed along the way.
The challenge for the crude oil market posed by the U.S. shale surge "would not hurt us so much if we were more competitive," said Tatiana Mitrova, head of the oil and gas department at the Russian Academy of Sciences Energy Research Institute.
The drilling now under way should determine whether Russia can frack its way to a more modern, competitive oil industry with all the technological savvy of the upstarts in North Dakota, and whether Bazhenov is the key.
"Bazhenov is a huge formation. It covers half of western Siberia. I have seen maps that claim it is 22 times the size of the Bakken in North Dakota," said Richard Andersen, chief financial officer of Eurasia Drilling, Russia's biggest driller.
But Andersen said it was "not homogenous" across western Siberia. "It may work very well in some areas and not in others. I would not say it will be condemned or proven in the next year. But we should get a view as to whether it will work well or not."
Recent drilling by producers such as Salym Petroleum, a joint venture between Shell and Gazprom Neft, which operates in the group of fields where the Bazhenov was discovered, have yielded healthy flows of light, low-sulphur crude. It has also indicated the presence of microfractures in some areas which could help the oil flow out of the tight rock.
The challenges to Russia's tight-oil revolution so far, however, have had little to do with geology.
Efforts to establish joint drilling ventures have run up against post-Soviet legislation which does not recognize project operators, while cumbersome planning requirements have forced some producers to scale back their plans for this year.
The biggest question of all is posed by the tax system. With lifting costs for a barrel of Bazhenov crude estimated at up to $40, the tax regime has been the main obstacle to its development as producers worked their way through cheaper conventional reserves.
The Kremlin's willingness to loosen the state's grip on oil taxes - which skim 90 percent of the revenue from each barrel of export crude - underscore its interest in exploiting shale.
But even with the prospect of a boost to the oil revenues that amount to 40 percent of the Russian budget, that willingness only goes so far.
The government has offered a package of tax breaks which includes a sliding scale of breaks on Russia's revenue-based mineral extraction tax and targeted breaks on the development of Bazhenov and other shale deposits which are part of Rosneft's drilling ventures with ExxonMobil and Statoil.
But the package - which was due to be approved on October 1 - has yet to reach parliament. Finance Ministry officials have assured investors that it will pass in time to come into effect next year after undergoing tweaks.
Producers also face the expensive prospect of re-drilling their West Siberian fields to access the Bazhenov below, even though it could be tapped through existing wells.
A boon to drillers such as Eurasia and Schlumberger but a drag on the economics of developing Russian shale, this is essentially an accounting measure to prevent fraudulent reporting of conventionally produced oil as new oil to claim tax breaks.
"They are worried that suddenly 70 or 80 percent of Russia's oil is going to be tight oil," a source close to one of the exploration projects said.
The proferred tax breaks may be enough to accelerate exploration of Bazhenov and other tight formations and allow Rosneft's drilling partnerships to proceed, but commercial production will probably require another round of tax reforms, the source added.
The very low levels of rock permeability required to qualify for tax breaks also may rule out profitable development of some tight sandstone deposits on the edges of what the Energy Ministry has called the "grey area" between true shale and West Siberia's current, conventional plays.
Of Russia's 26 billion metric tons of recoverable reserves, two-thirds or more are thought to be "tight", a share that is rising as Russia's easily tapped conventional reserves are depleted.
"That is semantics," said Oleg Mikhailov, who once ran pilot projects in tight oil at TNK-BP and now works at mid-sized producer Bashneft.
"What is easy today was tight yesterday. What is tight today with technology and a good tax regime will be easy tomorrow."
(Editing Richard Mably and Jason Neely)