The process of extracting "black gold" from rock below the ground is not easy and not as cheap as more conventional methods of oil extraction, but interest in such shale deposits is mounting.

President George W. Bush set a goal in his State of the Union speech earlier this year of reducing US imports of Middle East oil by 75 percent by 2025, and to some industry analysts western oil shale deposits could help achieve this goal.

Analysts estimate there are the equivalent of some 1.2 trillion barrels of oil waiting to be plucked from the western states' rich oil shale deposits.

At current demand levels, analysts say that is enough oil to satisfy the hunger of the world's largest oil consuming nation for the next 100 years.

However, in contrast to "conventional" oil deposits, oil shale cannot be easily extracted, especially at such low prices as oil is extracted from the deserts of the Middle East.

In order to extract oil shale, the underground rock must first be heated to extremely high temperatures in order to separate the precious hydrocarbons from the shale and then be pumped to the surface for processing.

During the 1970s, presidents Gerald Ford and Jimmy Carter had encouraged the development of the oil shale reserves, but the then Exxon company ceased its development project as oil prices tumbled.

Analysts say oil prices would have to remain between 55 and 70 dollars a barrel to make the complex extraction process economically viable today.

And although oil prices have fallen steeply in recent weeks from record highs struck in July, crude prices are hovering around 60 dollars a barrel and interest in such projects has been rekindled.

"There are a variety of technologies that various companies are working on, both 'in situ' and above ground retorting," said Anton Dammer, who is overseeing the development of a strategic fuels development program for the Department of Energy.

Royal/Dutch Shell is one company looking at ways to tap oil shale.

"For nearly a quarter of a century, Shell has been researching our innovative In situ Conversion Process (ICP) to determine if it is economically viable, environmentally responsible and socially sustainable," said Shell spokeswoman Jill Davis.

"Through those decades of research we have continued to improve upon our technology to a point where we now believe it has the potential to responsibly increase domestic oil supplies," Davis said.

However, US officials and energy groups are also watching developments across the country's northern border with Canada where technology is being used to mine oil sands in Alberta province.

Some experts have pegged Alberta's oil sands reserves at around 2.5 trillion barrels.

ConocoPhillips of the US and Canada's EnCana Corp said Thursday they will spend over 10 billion dollars over the next decade to boost Canada's oil output, part of which they said would be achieved by boosting extraction from Alberta's oil sands.

US government officials believe domestic oil production could rise to between three and four million barrels per day in the next decade from a current estimate of over one million barrels per day.

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