California's Monterey Shale formation is estimated to hold as much as two-thirds of the recoverable onshore shale-oil reserves in the U.S.'s lower 48 states, but there's a catch: It is proving very hard to get.

Formed by upheaval of the earth, the Monterey holds an estimated 15.4 billion barrels of recoverable shale oil, or as much as five times the amount in North Dakota's booming Bakken Field, according to 2011 estimates by the Department of Energy. The problem is, the same forces that helped stockpile the oil have tucked it into layers of rock seemingly as impenetrable as another limiting factor: California's famously rigid regulatory climate.

California has become one of the U.S.'s top oil-producing states over the past century, largely by tapping into the easier-to-get oil that has seeped out of the Monterey beneath places like Bakersfield and Los Angeles County. But with production in general decline since the 1980s, producers are trying a smorgasbord of techniques -- called enhanced oil recovery in industry parlance -- in an effort to tap into the mother lode.

So far, there have been no production breakthroughs.

Venoco Inc. of Los Angeles, for example, said in a report that after drilling 29 wells in the Monterey Shale from 2010 to 2012, no 'material levels of production or reserves' resulted. A Venoco spokeswoman declined to comment further.

But even many skeptics believe the Monterey eventually will have its day.

'My view is we do the easy oil first, like the Bakken,' says Bob Brackett, senior oil and gas analyst at AllianceBernstein LP, who in 2012 wrote a report critical of the Monterey's current potential. 'But time will bring us back to the Monterey.'

Here are some of the technologies being used by oil companies to tap that hard-to-get oil:

Few techniques have garnered more scrutiny in California than hydraulic fracturing, or fracking, which entails injecting water, often mixed with chemicals, into a well to fracture rock formations and unlock trapped oil and natural gas. Widely used in North Dakota and other big fields, fracking is less common in California, where only 560 of 50,000 producing wells were fracked in 2012, according to the Western States Petroleum Association.

Fracking is more difficult to do in the Monterey because the formation is so jumbled, says Amy Myers Jaffe, executivedirector of energy and sustainability at the University of California, Davis. That makes it hard to find large amounts of shale to frack, industry officials say.

'The technical challenges are such that it makes it more expensive to frack in California,' Ms. Jaffe says.

Despite its limited use in the state, fracking is drawing fire from environmentalists and other critics for potentially causing harm, such as lowering water quality. On Friday Gov. Jerry Brown signed a bill requiring more disclosures on fracking, which producers have been reporting to the state on a voluntary basis. The law takes effect at the start of 2014.

Fracking in the Monterey has been 'sort of like the Wild West, because nobody has been watching or paying attention,' says Kathryn Phillips, director of the Sierra Club's lobbying arm in Sacramento.

Industry officials say that the existing rules have been adequate, and that critics mainly want to constrain or stop increased oil production. 'They believe the technology is dangerous,' says Tupper Hull, spokesman for the Western States association, based in Sacramento. 'We certainly don't believe the science or experience suggests that.'

There are two main steam-injection techniques that companies are using to help coax trapped oil to the surface: steam drive and cyclic steaming.

With steam drive, large amounts of water are heated and the resulting steam is injected down separate drilling holes to heat up a large area of oil deposits. The heat loosens the oil, allowing it to flow toward wells that have been drilled for production, says Scott L. Montgomery, a consulting geologist in Seattle.

One drawback of steam drive, Mr. Montgomery says, is that the process requires a large amount of water.

The cyclic method, on the other hand, uses much less water because the steam is injected down only one well. With this technique, which is also known as the 'huff and puff' method, the steam is left underground for a few days to soak the shale, freeing up oil that is then pumped back out the same well.

But this technique has drawn criticism, too.

In Santa Barbara County, Santa Maria Energy LLC has proposed a total of 136 wells that would use cyclic steaming, including 26 that have already been drilled on a pilot basis. But the county's planning commission earlier this year declined its staff's recommendation to approve the project, calling for more study on concerns raised by environmentalists about greenhouse-gas emissions.

Santa Maria officials declined to comment. But on the company's website, they say emissions could be reduced by, among other things, building a pipeline to transport the oil instead of hauling it in trucks.

The commission is expected to take up the matter again by October.

This recovery method involves injecting liquefied carbondioxide into the rock so that it displaces the trapped oil, allowing it to flow more freely to wells. A newer technique, it has been widely adopted in recent years in Texas and New Mexico.

But tests in the Monterey -- including a joint study around 2000 by Chevron Corp. and the Department of Energy -- have produced mixed results, says Mr. Montgomery. He says the main problem was that the carbondioxide didn't increase production as much as hoped.

That could be because the rock formation is so jumbled up, it's hard to find the right spot in which to inject the carbon dioxide, he says: 'Finding sweet spots is very difficult.'

Officials of Chevron, based in San Ramon, Calif., confirmed the study but declined to comment on its details.

But at a shareholders' meeting in May, Chief Executive Officer John Watson said the Monterey's potential remains in question. 'I think the jury's out a little bit on the Monterey Shale,' Mr. Watson told reporters at the time. 'I don't think we've completed -- the industry has completed -- the assessment enough to reach a conclusion.'