JIDDAH, SAUDI ARABIA, OCT. 14 -- Saudi Arabia, already the world's biggest oil exporter, has discovered extensive new reserves of high-quality crude oil in areas of the country never previously explored.

Saudi officials have been cautious about the overall size of the new finds, but there are indications that the discoveries represent a major new resource -- one that could boost Saudi Arabia's official reserves by 20 percent -- and possibly a still larger bonanza.

The Saudi exploration program that discovered the new oil was begun more than a year before Iraq's Aug. 2 invasion of Kuwait. Saudi officials were responsive today to questions about the new finds -- in the midst of the Persian Gulf crisis -- perhaps because they could signal the jittery world oil market that long-term supplies of oil are plentiful.

Unless all the early indications are wrong, the new discoveries will enable Saudi Arabia to pump oil at its present rate or more well into the 22nd century -- long after some of the other oil states have gone dry.

At a minimum, "it's more oil than the whole reserves of some other members" of the Organization of Petroleum Exporting Countries, one government official said.

"We have a significant finding of high-quality crude," said Nasr Ajmi, executive vice president of Saudi Aramco, the state-owned oil company.

He said that in addition to the newly discovered fields in the central part of the country, Saudi Aramco has drilled successful wells in the trackless "empty quarter" of the southeast that are not being tapped because they are too far from any pipeline. Within a year or so, he said, exploration crews will begin work in the Red Sea, also virgin territory considered promising by Saudi Aramco geologists.

In an interview today, Petroleum Minister Hisham Nazer said Saudi Aramco has not yet developed a firm estimate of the total size of the new fields.

"We can only say we have discovered oil. The extent we will talk about later," he said. "It's part of the credibility of Saudi Arabia" to make certain of the size of the new reserves before quantifying them publicly. But he said that "we haven't withdrawn" an estimate issued by Saudi Aramco more than a year ago putting the country's potential reserves from all areas at 315 billion barrels. That would be almost 60 billion barrels more than the current official reserve figure -- an amount equal to the total proven reserves of Venezuela, one of the world's leading oil producers.

Oil economists in the United States have minimized the importance of the new finds because Saudi Arabia already has 257.5 billion barrels of proven reserves, 2.5 times as much as the next biggest oil country, Iraq, and 10 times the reserves of the United States.

"It's like finding more water in the ocean," an economist said.

But officials here take a different view. To them, the new fields provide a guarantee of prosperity and international influence for Saudi Arabia far into the future.

The country's long-term planning is based on developing a relationship of mutual dependence with its customers. Saudi Arabia says it offers assured supplies at reasonable prices in exchange for guaranteed markets for its only important natural resource.

The more oil Saudi Arabia has, the more it can influence the market and the longer it can keep its customers from getting nervous about their supplies.

In addition, the new wells contain low-sulfur, extremely light oil of the kind prized by refiners. Light oil commands a premium price. By substituting oil from the new fields for heavier crudes currently produced, Saudi Arabia could increase its oil income without increasing its total exports or its base price.

"It's not just 'more water,' it's different water," Nazer said. "We're getting to a point where we think the industry will have to improve the quality of its products. It's easier to work with cleaner oil in the first place" than to refine out impurities.

Between June 1989 and July of this year, five test wells drilled in the central desert -- between 47 miles and 127 miles southeast of Riyadh -- found commercial quantities of oil, according to announcements by the Ministry of Petroleum and Saudi Aramco. Another struck large deposits of natural gas, the primary fuel of the country's fast-growing petrochemical industry.

The new fields are located hundreds of miles west of the fields along the Persian Gulf coast that have produced all Saudi oil up to now, and in a different geologic zone.

According to local lore, exploration in the area began after a farmer drilling a water well found traces of oil in the water and complained to local authorities.

In fact, Saudi Aramco geologists had long regarded the area as promising, but it had not been drilled because it was outside the concession zone held by the company when it was still American-owned. The biggest question now is whether the drillers have found separate fields, useful but not breathtaking, or one connected monster pool of oil similar to the Gawhar field in the Eastern Province, the world's largest.