The oil and gas fields under Utah, Wyoming, Colorado and Idaho are about one-third the size of the mammoth Prudhoe Bay finds in Alaska and among the largest on earth, a top government geologist said yesterday.

If additional estimates of undiscovered oil resources there are included, the field could be just as big as Prudhoe, according to Richard Powers of the U.S. Geological Survey office in Denver.

The resources are in addition to vast reserves of oil shale, coal and uranium that already have sparked major energy projects in the West. Powers' numbers, to be published by the USGS at the end of the month, are not new but represent the first comprehensive totaling of the oil finds, he said.

Powers was one of several scientists who warned the American Association for the Advancement of Science convention here that developing the huge energy resources of the West will create vast problems of sudden population growth, environmental pressures and economic impact. The stress already has begun with the expansion of oil shale and coal surface mining in the region.

Powers told a press conference that 3.3 billion barrels of oil and 16 trillion cubic feet of gas have been discovered along the so-called "overthrust belt" underground formation, which is a folded section of the earth's crust from one to four miles down. The average estimate of undiscovered recoverable reserves there is 6.7 billion barrels of oil and 58.4 trillion cubic feet of gas, he said, or 8 percent of all U.S. estimated undiscovered oil and 10 percent of all undiscovered gas, including fields offshore.

"The key thing is, we're finding large fields, major fields, giant fields" where the reserves are in a continuous formation, said Powers, head of the USGS's overthrust belt analysis project. There is at least one "supergiant" field of 100 million barrels of oil or more, he added.

It is too early to know how much of the oil can be gotten out of the ground, and at what price, he said. The soils are very hard, and wells now are costing $4 million to $6 million each.

Dr. William Freudenberg, assistant professor of sociology at Washington State University, said the western energy boom already is causing "dismal" statistics of mental illness, crime and other social impacts in the arid area, although the population flood there is just beginning.

The town of Rock Springs, Wyo., had a 100 percent population increase over three years in the 1970s and an 857 percent increase in the use of its mental health center, he said. Freudenberg said later that studies show local natives and old-timers, rather than the newcomers, are feeling the stress.

Although sudden population growth means that more sewers, police, schools, hospitals and other services are needed, the taxes the newcomers pay at first do not cover the needs, Freudenberg said. The government response is often to become larger, more bureaucratic and impersonal, changes "which may not be parts of the solution but parts of the problem," he added.