Back in the 1920s, a Union Oil geologist told his company he was onto a major oil discovery in central California. Impressed, Union drilled a string of wells and hit -- black goo.

The geologist was fired.

Today that black goo is known as heavy crude and -- thanks to some new developments in extraction technology -- several oil companies are betting a lot of money that they can get it out of the ground and sell it at a tidy profit.

Getty Oil, for one, is opening a $21 million operation outside Bakersfield, Calif., not far from the Union find, to tap a reservoir Getty believes contains 400 million barrels of crude. Other companies are contemplating similar efforts in New Mexico, Utah and other oil-producting states.

In fact, Shell Oil's $3.6 billion purchase of California's Belridge Oil Co. earlier this year may have been predicated on Shell's ability to squeeze a lot more out of Belridge's holdings than could be obtained through conventional drilling.

The key to all this is oil mining, a term that encompasses several processes. In one, the oil-bearing rock is simply mined out of the ground and the crude "cooked" out of it. In others, huge pits are dug down to the oil formation and chemicals applied to loosen the oil. In still others, shafts are drilled underneath the reservoir and holes cut upward so the oil drips out, like sap from a maple tree.

These processes are attractive because they are applicable not only to heavy crude, but also to tar sands, a hydrocarbon-bearing soil called diatomite, and, perhaps most importantly, to old fields of lighter crude where conventional wells have run dry.

Studies for the Interior Department's Bureau of Mines conclude that oil mining could increase America's economically exploitable oil reserves tenfold, adding hundreds of billions of barrels to the nations's current 30 billion barrels of proven reserves.

John Hutchins of Energy Development Consultants, who worked on one of the studies, says: "It's quicker and probably a lot cheaper than oil shale and coal liquefaction. The only thing left is just going out and trying it." And that is what Getty and the others are doing.

The idea of mining for oil is not new. A 1932 Bureau of Mines study by George S. Rice concluded, "Where conditions are favorable, mining methods in depleted oilfields may bring large financial returns and recover oil that might otherwise be lost."

But until recently an important factor has been lacking: price.

In the development of any mineral resource, the first question that must be answered is whether the deposit is "economic" -- that is, can the mineral be mined and processed and sold for a profit at the prevailing price?

Oil is no different, and when crude was selling for $2 and $3, a barrel, only the cheapest extraction process could be employed profitably.

Now all that has changed.

Bureau of Mines consultants say that surface-mined oil can be produced at a cost ranging from $12 to $21 a barrel, and that the cost for oil from underground mining the operations ranges from as little as $10 a barrel to $60 a barrel.

World oil prices have risen more than 70 percent this year. The Organization of Petroleum Exporting Countries is charging "official" prices averaging $22 a barrel, and also sells much of its oil on a one-time, or spot, basis at prices of up to $40 a barrel.

Richard Dick of the Bureau of Mines' Twin Cities Research Center in Minneapolis says: "A couple of million barrels a day of production from oil mining is possible, by 1990, no doubt about it."

Dick oversaw the studies prepared by Golder Associates and Energy Development Consultants and released to the public earlier this year.

"Under today's economics, many of the oil deposits in this country can be mined economically," he adds.

Sheldon Wimpfen, the bureau's chief mining engineer, also is optimistic.

"From a mining standpoint, all of this is proven technology in use worldwide," Wimpfen says.

Wimpfen became interested in oil mininy years ago when he noticed that mining engineers continued to make advances in ore recovery processes, but that oilmen still left 40 percent to 60 percent of the oil they discovered in the ground, even with so-called "enhanced oil recovery" operations.

"We have some mineral operations that typically recover up to 90 percent of the ore, but the oil boys have settled for a lot less," Wimpfen continues.

In the last century, more than 450 billion barrels of oil have been discovered in the United States. But just 115 billion barrels have been produced. Current conventional production technology will allow the oil companies to produce about another 30 billion barrels, leaving some 305 billion barrels out of reach.

Another 26 billion barrels of oil are locked in Utah's tar sands, and billions more elsewhere. Then there are an estimated 30 billin barrels of "heavy" viscous oil in California, and billions more in shallow diatomite formations.

The one million to two million barrels a day of new production from oil mining that supporters say is possible, is equivalent to President Carter's most optimistic forecast of production from synthetic fuels by 1990.

Not everyone familiar with the oil mining concept is quick to embrace it, however, or agrees with the Bureau of Mines studies.

Lee Marchant of the Energy Department's Laramie Energy Research Center is one of the skeptics. He says the optimistic conclusions of the Golder Associates and Energy Development consultants studies "have to be considered speculative." Further, Marchant says, the firms have a "vestedinterest" in generating more studies through their encouraging reports.

Until an oil or mining company actucally mines oil on a commercial scale, Marchant says, it will be too soon to accept unequivocally the bureau's economic analysis.

As for the priority the Department of Energy assigns to oil mining, Marchant says: "We don't see spending a large portion of our money on this technology . . . . We feel mining is only applicable to a small percentage of our total resource."

Conoco, a major oil company that has tried underground oil mining methods on a limited basis on its Lakota field near Casper, Wyo., is skeptical.

"If reservoir conditions are favorable, we might try this again," says Aurelio Madrazo, Conoco's head of North American production.

Conoco has been operating a 50-barrel-a-day underground mining plant for the last three years, draining oil into a 2,000-foot-long horizontal shaft, 180 feet underground, beneath a shallow oil field.

"It's not something we see as solving the energy crisis," Madrazo says. "It is still a very small contribution."

Getty Oil Co., however, is moving ahead with its $21 million pilot plant at its maKittrick field outside Bakersfield.

Construction will begin early next year, Getty spokeman George Schwarz says, and the company expects to be producing 20,000 barrels a day by the late 1980s.

The McKittrick operation, if it works, is an illustration of oil mining's potential. Discovered in 1896, the McKittrick field produced 15,900 barrels a day at its peak. But by June of this year, production had dropped to 6,000 barrels a day.

Schwarz says Getty is confident that the company will be able to extract nearly 400 million barrels before the field is mined out -- largely through digging and processing hydrocarbon-rich diatomite overlying the field. The 400 million barrels Getty hopes to get amount to nearly twice the total production from the field during the 80 years it has been worked.

Most of the oil-soaked diatomite laced through and around the mcKittrick field easily can be surface-mined. A few miles away, another company has a surface mining operation to extract diatomite that is free of oil, for use as cat litter.

Getty's pilot plant will produce 150 barrels of oil daily, from 240 tons of suface-mined ore processed at one of two facilities.

The purpose of the test is to determine which of the two methods of separating the oil from the ore is the most profitable. One method will employ a variation of a process devised by the Germans to convert coal to oil. eThe other will use a solvent from Dravo, a company that is experienced in extracting vegetable oil from soybeans.

"With conventional methods you can't get the oil out, but mining should work," Schwarz says.

Similar plans are under way in Utah to mine and process billions of barrels of oil locked in tar sands deposits.

Dr. Francis Hansen, of the University of Utah, says that maybe 25 percent of the state's tar sands can be surface-mined. While no major oil company has announced plans to go ahead, several are exploring it, Hansen says.

Hansen and other researchers believe it is feasible to construct units that could produce from 50,000 to 150,000 barrels a day by mining the tar sands. They believe the process could yield quality oil that could be sold profitably at $25 a barrel.

"I'm bullish on oil mining," Hansen says, adding, "It is only a year or two away."

The nation's largest gasoline retailer, Shell Oil Co., according to oil industry executives, also has plans for mining-style operations to recover billion of barrels of oil in the 33,000 acres of Kern Country, Calif fields it bought from Belridge Oil Co.

"There is a widespread belief that Shell has the capability to squeeze oil out of those formations," says Bruce Wilson, an energy analyst with the brokerage firm of Smith Barney Harris Upham Co. Inc.

"If you have a process with a higher recovery rate, then you have a larger exploitable resource base," Wilson points out.

This could explain why Shell's purchase of Belridge -- the largest merger in U.S. history -- called for paying almost $9 a barrel for the little-known California producer's known reserves, compared with the $6 a barrel that industry analysts normally figure in transactions of this type.

Yet another oil mining project is taking shape near Santa Rosa, N.M. There, James Young, president of American Mining and Exploration Co., has obtained the rights to 11,000 acres of tar sands deposits.

Young says his plan to establish a $25 million oil mining operation at the site is "strictly a private venture, not requiring state or federal money."

Young anticipates the tar sands should yield some 250 million barrels of oil that will be mined and processed with solvents. He expects a recovery factor of "about 95 percent."

He is confident that his oil mining project will prove competitive with oil selling for $18 a barrel, once his plant is in operation.

"It sounds simple, and it is," Young insists. "We're combining oil technology with mining technology. When you stand in the quarry and see a face of rock 30 feet high, with oil bleeding out in the summer sun, you can't deny that there is oil in that rock."