The 18 major U.S. oil companies allowed gasoline reserves on the East Coast to dip 21 percent below 1978 levels during a key period this spring, according to Department of Energy figures.

The 21 percent reduction was by far the greatest in country and provides the best explanation available to date for tne severity of the gasoline shortage here and in other parts of the East, some analysts said this week.

The disproportionate drop in East Coast reserves virtually guaranteed that the fuel shortage would strike here early, that it would be more intense and that it would last longer than shortages in other regions, one senior energy analyst said.

Oil industry officials said the depletion of the reserves stems in part from being at the end of Gulf Coast supply pipelines that are tapped at hundreds of points before they reach the Atlantic states. It is also explained, these officials say, by the fact that there is too little regional refining capacity in the East to cope with rising consumer demand.

The figures were compiled by the government in a special computer analysis based on confidential information provided by the oil companies. It supports the claim by local officials that the Washington area has been unfairly hit by the gasoline crisis, but does not explain why this area was hit before other metropolitan centers along the Atlantic Coast.

Federal energy officials have no plans to publicly release the computer analysis, which has circulated recently within the Energy Department and on Capitol Hill.

The Energy Department analysis compares the stored gasoline reserves of the 18 major companies during the last week in April this year with the same period last year. The 21 percent drop on the East Coast dwarfs the 1 percent decline in the Midwest for the same period and the less than 1 percent slip to the Gulf Coast.

In the Rocky Mountains states and on the West Coast, gasoline reserves dropped less than 15 percent during the periods compared.

The reserves were measured at a critical period because it falls about 30 days before the beginning of the peak summer driving season. It is during this period that refineries try to maintain maximum gasoline reserves because the companies cannot keep pace with gasoline consumption during summer months.

Together, the 18 largest oil companies control about 70 percent of the gasoline sold on the East Coast, During the months when the biggest companies were draining their reserves, smaller companies that serve the region managed to increase their reserves.

Numerically, the "drawndown" by the 18 major companies represented more than 440 million gallons of gasoline, the difference between 2.149 billion gallons that was on hand at the end of April 1978 and the 1.7 billion gallons available this year during the same period.

Normal gasoline reserves would supply the entire eastern seaboard for only 26 days, but the shortfall this year sliced that margin by nearly five days.

The role that regional gasoline reserves play in the availability of gasoline at the pump is similar to that played by a checking account balance: the trick is to keep enough money in the bank to ensure that the account is never overdrawn.

Senior energy officials said that this year's shortfall is not inconsistent with published reports that gasoline storage tanks in the Washington area were so full last month that new shipments were being diverted to New Jersey. Those storage tanks represent 1 percent or less of the total storage capacity in the East.

Indeed, by last month, overall gasoline reserves on the East Coast had surpassed June 1978 levels, according to "all oil companies" figures that are made public by the Energy Department. This explains why some storage tanks were brimming, officials said, and may foreshadow some improvement in gasoline supplies here.

But the troubling computer analysis of the 18 major oil companies provides a sharp contrast with Energy Department pronouncements that federal allocations rules ensure that the effects of the gasoline shortage are felt proportionately across the country.

Some oil company and federal energy officials said that even the best allocation system could be temporarily out of balance, leaving one region or another without adequate reserves. "it's like starting a 100-yard dash wearing a potato shack," said one analyst.

"it just takes away the safety factor," acknowledged Martin Sedlacek, dstribution manager for the Tulsa-based Cities Service oil company.

Others, however, disagreed. "It's just really not much of a problem," said Amoco's Ralph Stowe. "As far as Amoco is concerned, how much [reserve] is on hand at any given time is irrelevent since we can move stocks from one region to another in a matter of days."

Several of the major oil companies surveyed, however, said they have completely emptied bulk distribution tanks along the East Coast for three days or more at a time during the last two months. This causes corresponding delays in deliveries and outright shortages in hundreds of service stations served by those distribution centers, officials said.

Earlier this week, some Washington area service station operators said their initial July fuel shipments form suppliers had not arrived on time, forcing some of them to shut down over the busy Fourth of July holiday.

"if you've got somebody not getting their July obligation, then the inventory [reserve] problem is very, very real," said Ed Grigsby, allocations manager for Phillips Petroleum.

"the system has very little elasticity to it, so it doesn't take much to strain it," said Atlantic Richfield, spokesman Peter Zambelli. "we had some outages [at bulk storage terminals]. I think everybody did."

In normal times, regional reserves are carefully balanced by the oil companies to take up the slack between incoming shipments from the refineries and outgoing truckloads of gasoline headed for service stations from local storage terminals.

"As long as the gasoline keeps flowing fairly evenly, you can operate on low inventories [reserves]," said Chevron spokesman Frank Bradley. "But when something goes wrong, that's when low inventories can really bite you."

Under the Energy Department's beleaguered allocation program, the effects of the gasoline storage should be distributed proportionately across the nation -- though there are many exceptions for emergency, priority and growth-area customers.

During a general national shortage of crude oil, the rules require that a decrease in refinery production be passed along to service stations in equal proportions. However, low reserves on the East Coast apparently have created disruptions in gasoline deliveries from the refinery to intermediate storage terminals and, ultimately, to the gasoline pump.

The reason that gasoline reserves were short here is directly related to the nationwide shortage, but also to some peculiarities of supply and demand on the East Coast, oil company and federal energy officials say.

First, the East Coast -- from Maine to Florida -- consumes one-third of the gasoline produced nationwide and yet has barely one-tenth of the nation's refining capacity. Therefore, the region is heavily dependent on gasoline that must be shipped by pipelines or tankers and barges from the dense refining cresent along the Gulf Coast between Houston and New Orleans.

"there is more demand for shipment of product [gasoline] along the pipeline than there is space to accommodate it," said Cities Service's Sedlacek.

The normal delivery time for a quantity of gasoline along the two pipelines that serve the Washington area is about 10 days, officials say.

However, said, Sedlacek, "when you have a tight situation such as we have, that accelerates concern for withdrawals by your customers" from storage facilities located at hundreds of stops along the pipeline.

In such instances, Sedlacek said, gasoline middlemen "try to back up their own reserves and there's a run on the terminal to grab their supply. Hell, they may have that terminal cleaned out in five days," thereby creating shortages of up to a week for those customers further down the line.

Another explanation for lower reserves of the East Coast, according to Atlantic Richfield's Zambelli, is that in September 1978, "contrary to historic usuage, demand for gasoline did not fall off after Labor Day and it ran at a flat rate well into November.

"this was especially noticeable on the East Coast," Zambelli said.

Moreover, when the new year arrived, January and Febuary snow storms kept some refineries producing heating oil past the time when they usually change over to gasoline production, officials said.

Late winter is the traditional time for building gasoline stocks for the spring and summer driving season that begins around June 1.

At Phillips Petroleum, Ed Grigsby thinks that the East Coast's biggest problem is that it is at the "end of the system" in the geographic maze through which gasoline is distributed from Texas and Lousiana refineries.

The probability of under-delivery is greater at the end of the pipeline," Grigsby said. CAPTION: Map, The Energy Department uses five regions for the reporting of gasoline reserves. Region 1 fell short this spring. By Dave Cook -- The Washington Post