Late last year Exxon Corp. began to circulate a draft of its annual analysis of the nation's energy future and, for the first time, the oil giant stretched its horizon into the next century.

The new numbers included a bombshell: To meet the likely need for liquid fuels three decades hence, the United States will have to develop a synthetic fuels industry capable of producing 15 million barrels a day.

In a manner of speaking, the bombshell exploded over the northwestern corner of Colorado, which would be the center of an 8-million-barrel-a-day oil shale industry if the Exxon vision comes to pass.

About the time the analysis was published last spring, Exxon Chairman C. C. Garvin went so far as to remark at a lunch with Washington Post reporters that perhaps that corner of Colorado would have to be declared a "national energy zone" in which the "normal rules" would not apply. The nation's security and continued economic growth could require it, he said.

Until the Exxon paper -- it does not describe a "plan" Exxon intends to put into effect, the company stresses -- hit the scene, the debate about oil shale development on the area of Colorado known as the Western Slope had been proceeding at a leisurely pace, with most participants still talking about an industry that probably never would produce more than several hundred thousand barrels of liquid fuel a day.

"That paper they sent scared the bejesus out of everybody," one Colorado energy expert says. "Exxon changed the game overnight."

The paper galvanized many of the groups that will be affected by development of the thousands of square miles of Colorado oil shale deposits, some of which are as much as 1,500 feet thick. (Smaller but still enormous quantities of oil shale also are located in nearby Utah, with lesser quantities in Wyoming.)

As a result of meetings, hearings, discussions and just sheer publicity over the last few months, the groups -- among them ranchers, businessmen, state, county and municipal officials, environmental organizations, ski resort operators, water district representatives and civic clubs -- generally now realize that large-scale production is coming, with the first major impacts to be felt soon.

According to Sen. Gary Hart (D-Colo.), a consensus has formed that 400,000 to 500,000 barrels of liquid fuel will be produced daily in the area by 1990 and double that a few years later.

Even a level of 400,000 barrels a day would require construction of at least eight plants, each with a capacity of 50,000 barrels daily and a cost of between $2 billion and $4 billion. Construction and operating crews and concomitant development of everything from new housing, roads, sewers, water supplies, power plants, schools and retail stores likely would add at least 75,000 persons to the sparsely populated Western Slope.

But no one in the area is reconciled to the mammoth development depicted by Exxon, or to changing environmental rules to accomodate it. "If I'm still in the Senate, they could do that only over my dead body," Sen. Hart exclaimed when told of Garvin's remark.

The first factor limiting shale development is apt to be water. Even Exxon says the upper limit is a 1.5-million-barrel-a-day industry in Colorado without the importation of water from other states in the Missouri and Mississippi river basins. Exxon says that, costly as such water plans would be, they are feasible in an engineering sense. Political feasibility is another question.

For now, however, there is enough water available for the first several plants to be built, and most of the operators have options to buy the necessary water. Later, as more plants are built, air quality standards could become a limiting factor.

Oil shale has been on the verge of "going commercial" so many times that some people still aren't convinced it's going to happen this time. The last close call was in 1974 when Atlantic Richfield Co., and Tosco Corp. -- its name is an acronym for The Oil Shale Co. -- reluctantly postponed plans to build a 10,000-barrel-a-day plant at their joint Colony Project site on a mesa high above the Middle Fork of Parachute Creek about 15 miles west of here. However, no government backing was available, oil prices were under controls, and the risks -- particularly the difficulty of complying with new environmental regulations except after long delays -- looked just too great.

But now those barriers have been breached and commercial development seems assured. Several major projects are underway, and others are in various stages of planning or experimentation. Again, Exxon is a factor because Aug. 1 it brought out Atlantic Richfield's 60 percent share of the Colony Project for $400 million.

In recent weeks construction has begun on a road from the creek up past the planned mine opening and with several switchbacks, on to the top of the mesa more than 1,000 feet above where the plant will be located. If the project stays on schedule, the first oil from shale should flow late in 1985. Construction will cost about $2 billion, figured in "as-spent" dollars, according to Bob Larkins, manager of Exxon U.S.A.'s synthetic fuels division.

Meanwhile, to the north, in the middle of the Piceance Creek Basin, on a 5,100-acre federal lease, Rio Blanco Oil Shale Co., a partnership of Gulf Oil Co. and Standard Oil Co. (Indiana), in a few days will conduct its first experimental effort to extract oil from shale by heating the rock underground. If this and subsequent experiments go well, Rio Blanco hopes to have a 50,000-barrel-a-day operation by 1987.

At the moment, there are three possible ways to get the oil out of the greyish shale. Two of them differ only in the way the rock is mined. Colony, which has a relatively thin layer of rich shale on its property, plans to mine it underground using what is called the room-and-pillar approach, in which Rio Blanco with its much thicker deposit eventually might turn, is open pit mining.

In both these cases, the excavated shale is crushed and heated in a retort, with the oil being driven off in the form of a vapor. The vapor condenses into a thick, heavy oil that generally will require some on-site refining to make it less toxic and thin enough to transport easily.

Rio Blanco's current experiment, termed modified in situ, or MIS, involves retorting the shale underground. Occidental Petroleum Corp. for years has been running similar experiments a few miles away on another federal lease, and also is planning to move to commercial MIS operations.

Actually, MIS requires normal retorting on the surface of more than one-third of the shale. That much has to be removed to create a space into which shae above it can drop when blasted into chunks. The column of rubble formed is set afire from the top. As the shale begins to burn, the oil vapor and water vapor, flow downward between the chunks of rock. They condense and flow out the bottom ot the rubble pile into tunnels which channel them to a separator room and then through pumps to the surface.

According to Blaine Miller, the Gulf man who heads Rio Blanco, there are 9 billion barrels under his company's lease, nearly as much as was found in the original Prudhoe Bay oil field in Alaska, the largest ever found in the United States. With room-and-pillar mining, about one billion of it could be recovered. With the MIS approach, perhaps 5 billion could be. With a surface mine, all 9 billion could be.

Miller, and state officials, stress the need to recover as much of the oil as possible, which in the center of the Piceance Basin means open-pit mining. Environmentalists are unhappy at the prospect of a mine perhaps two miles by three miles in size.

On the other hand, Miller points out, there is enough oil on the Rio Blanco lease alone to support a 300,000-barrel-a-day operation for at least half a century.

Thus far, Gulf and Standard of Indiana have no intention of seeking any government backing for their project. Nor does Exxon for its share of the Colony project. After all, it is the world's largest oil company, and in the first six months of this year had revenues of $53.7 billion and after-tax profits of $2.95 billion.

On the other hand, Exxon's partner Osco, which in the last dozen years has built itself from a small reseach company into one of the country's larger independent oil refiners, had six-month revenues totaling only $1.13 billion, far less than half of Exxon's profits.

"Alongside Exxon we fit comfortably with the shadow," Tosco's president, Morton M. Winston, said wryly in an interview in his Los Angeles office. "We can afford the plant but not the risks."

Winston has two primary concerns. First, he noted, "We have cleared all the conventional titantic environmental roadblocks . . . (but) almost anyone can challenge any industrial project in court." A suit with no merit might cause delay costing $150 million, he estimated. Second, the Tosco chief executive said, "The federal and state governments have had a habit of revising laws and environmental regualtions with a retroactive impact."

Colony has in hand every environmental and other type of permit it needs, save one to operate a commercial mine, which it experts to receive after a hearing early next month.(And to help handle part of the impact on already strained local public facilities and the housing market, it has brought 3,010 acres of land just south of the Colorado River at the foot of Battlement Mesa for a new town in which homes, schools and other facilities should be rising within two years.)

But to protect itself, Tosco will ask for some type of government guarantee to cover the risks. Such guarantees are avalable now under two new laws, including the recently passed Energy Security Act.

Another company tooking at the possibilities of government loan, price or purchase guarantees is Chevron Shale Oil Co., a subsidiary of Standard Oil Co. of California which owns land along Parachute Creek a short distance south of the Colony project site.

Chevron, like Atlantic Richfield, has decided it will get into the shale oil production business with a second-generation plant. Roger Loper, president of the Chrevron subsidiary, says his company is going slowly because "economically it is not the rosiest prospect" the parent company has in sight. It would take at least eight years before Chevron could get its first commercial production and likely and mid 1990's before output could hit 100,000 barrels a day.

Such a plant probably would cost between $5 billion and $6 billion in "money of the day" that is, including inflation over the eight years, Loper calculates. That would make it one of Chevron's most costly investments and still yield only about 10 percent of its daily crude needs.

While the investment would keep paying off year after year because production would not decline rapidly the way most oil wells do, there would be a very long lag between the big cash payouts and the time profits began to roll in. "If liquid fuels stay at their present value, we don't have a project," Loper says flatly.

That conclusion is completely at odds with those of Exxon and Tosco, and probably those of Gulf and Standard of Indiana as well.

The Chevron numbers are another reason Exxon's jump into the oil shale scene has so changed the outlook for northwestern Colorado. Exxon has the engineering skills and the money to complete Colony.

Atlantic Richfield, on the other hand, sold its interest, says William F. Kieschnick, vece chariman of the board. That was because, big as it is, his company cannot afford to be committed to more than one multi-billion-dollar project at the same time. The other one will be the Alaska Natural Gas Pipeline because the company already has 8.5 trillion cubic feet of gas waiting in the ground at Prudhoe Bay for transportation. Other sources say Arco's share of the cost will be between $2 billion and $3 billion.

Arco told Exxon that Colony "is ready to go," and backed that claim with a proviso that if the announced schedule for the project is not met, Arco will get only the $300 million Exxon paid on Aug. 1, not the full $400 million purchase price.

Exxons Bob Larkins expects Colony to meet its target date, or at least come very close. "we think that's possible, and we are working to make that come about," he says. The access roads and mine bench come first, then development of the mine itself.