When Chevron Corp. abandoned its federal oil shale lease in Colorado's Piceance Basin in February, environmentalists declared that the decision was proof that oil shale development is too expensive and too difficult. Rob Dubuc, staff attorney at Western Resource Advocates, described Chevron's move as "another in a long line of examples proving that nobody knows how to develop oil shale on a commercial scale."

Chevron officials said the oil shale project had been "productive" but explained that the company was dropping out of the oil shale race to concentrate on "other priorities and projects in North America and around the globe." Chevron hopes to transfer its lease to another company.

But although the oil giant has bowed out of the oil shale race, several other companies continue to invest millions of dollars to develop technologies that they hope will give them access to America's massive oil shale reserves.

James Bartis, a senior policy researcher at RAND Corp., predicted that large-scale production from U.S. oil shale reserves may not be technically possible for another two decades.

"We don't know what the fundamental parameters are of the most attractive technologies for getting oil shale out of the ground," Bartis said. "We won't know that until the first pioneer plants are up and running. That would probably take 12 years at minimum. In 20 years, with a really rapid program, we might get to 1 million barrels" of oil per day from the oil shale reserves.

The following are five of the companies that are focused on extracting oil and gas from the Colorado and Utah oil shale reserves.

Red Leaf Resources expects to break ground this year on an oil shale project aimed at extracting oil and natural gas by sealing crushed rock into a clay-lined pit and heating it to at least 650 degrees Fahrenheit, a process known as retorting. The resulting hydrocarbons will be collected from the bottom of the pit, and the oil vapor will be condensed back into oil.

Operating on private lands, the company will work on football-field-sized areas on its 17,000-acre lease. Once the oil is extracted from the plot, the company will reclaim the area by reseeding it with vegetation.

Red Leaf officials say the process, known as EcoShale In-Capsule Technology, is designed to produce transportation-grade fuels from oil shale, oil sands, coal, lignite and biomass.

Enefit American Oil is the U.S. branch of the Estonian national energy company Eesti Energia. The Estonian firm created the U.S. subsidiary last year after it acquired Oil Shale Exploration Co. (OSEC), along with the company's large tracts of private lands in Utah. Enefit estimates that its state, federal and private leases hold 2.6 billion barrels of oil equivalent.

The company is seeking state and federal permits to use an advanced oil shale retorting technology that Eesti developed during 30 years of oil shale development in Estonia.

Under the process, the company will crush the mined rock and heat it to produce a hydrocarbon vapor. As the vapor is cooled, it's separated into oil and gas. The resulting hot ash is used to drive the heating process on the next batch of oil shale rock.

"We don't use any external energy," explained Rikki Hrenko, CEO of Enefit. "The process is self-sufficient once started up. The oil shale itself is able to deliver all the heat that's needed for the process."

Royal Dutch Shell has been researching oil shale extraction technologies for more than 30 years, and has been actively operating in Colorado since 1996. Seven years ago, the company produced 1,700 barrels of oil from a 30-by-40-foot test area, called the Mahogany Demonstration Project South.

The Mahogany technology entailed building an underground freeze wall around the oil shale test site, pumping any subsurface water out of that plot, and heating the oil shale rock vein for four years to 650 to 700 degrees Fahrenheit. Although Shell owns three federal oil shale leases in the Piceance Basin, the project was conducted on privately leased lands.

The company is currently closing down that project and plans to conduct further tests on the technology in Jordan.

Meanwhile, Shell will field-test a new multimineral extraction technology, this time on one of the company's federal leases. That study will focus on extracting hydrocarbons from the underground oil shale rock, while also mining the nahcolite -- commonly known as baking soda -- located near the shale.

The researchers will dissolve and extract the nahcolite before inserting an underground heating system into the cavity left by the mineral. Oil created by heating the oil shale kerogen will be pumped to the surface. No freeze wall is needed for the multimineral test because it's located well below the water table, according to Carolyn Tucker, a Shell spokeswoman.

The company will begin extracting the nahcolite this summer and expects to see preliminary oil production in one to two years, Tucker said.

American Shale Oil LLC, known as AMSO, is a partnership of Genie Energy and Total, a French energy company. Operating on a federal lease in Colorado, the company is testing a new technology on an oil shale formation located under a clay-rich layer of earth. AMSO experts say the clay will provide an impermeable barrier between the oil shale operations and local aquifers.

AMSO plans to drill vertical and horizontal wells, which will be used to heat the formation as well as to transfer heat by refluxing the oil generated by the process. The wells also will provide an avenue to collect the produced oil. The company expects to heat the rock for three to 12 months to produce oil.

AMSO is planning a six-month pilot test on the technology, which scientists expect will produce 1,000 barrels of oil. Eventually, the firm hopes to produce 100,000 barrels of oil each day.

Exxon Mobil is testing a process known as "Electrofrac," which company researchers sometimes refer to as the giant toaster.

Under the technology, the company will drill and hydraulically fracture a series of wells into an oil shale formation. Rather than use sand to prop open the fractured rock, the company plans to use a proppant that can readily conduct electricity. Once inserted into the fractured rock, the material would help spread heat through the oil shale formation. The oil produced in the process would be extracted through separate wells.

Exxon Mobil has been involved in the oil shale industry since the 1960s and is remembered in Colorado for closing down a massive oil shale facility in 1982.

The company won mineral rights to a tract of land in Colorado's Piceance Basin in 2010 and has been testing the Electrofrac process on private lands.