On any given night, tourists in Aruba's five casinos crowd around roulette tables, play 21 at a $10 minimum, or bet on the outcome of NBA basketball and college and professional football games. But not even the most hardened gambler is taking any bets on this Caribbean Dutch island's future after March 31, when Exxon closes its refinery, the mainstay of the island's economy for the past 60 years.

"The tiger has roared," said one high-ranking Venezuelan oil man of Exxon's abruptly announced decision to close Lago Oil and Transport Co. after failing to win preferential treatment on crude oil prices. Lago is expected to post losses of $55 million this year.

The tiger's roar has highlighted the economic crisis faced by Caribbean refineries, most of which are suffering losses of a similar magnitude. It also has underscored the danger that political instability may disturb the already troubled waters of the Caribbean.

The refinery, once the largest in the world, provides Aruba with 30 percent of its real income and its government with more than 50 percent of its revenue. Announcement of the closing sent shivers of apprehension through the island's 68,000 residents and set off a scramble for solutions by the United States, the Netherlands and nearby Venezuela, all of which have last year's U.S.-led invasion of Grenada fresh on their minds.

"It's an economic disaster," said Chris van Krimpen, the Dutch director of the Aruba Ports Authority. "Local people just don't realize it. In U.S. terms, imagine if 30 percent of the real income of a town disappears. In a small island, the whole industrial sector will disappear." The closing will wipe out some 950 refinery jobs and also will spell the end for another 500 independent contractors and suppliers. Unemployment in Aruba already stands at 19 percent. For many of these workers -- and in particular for skilled workers -- immigration to the Netherlands may be the only way out.

Although hope is still alive on the island that the refinery may not close, and rumors about potential buyers circulate in nervous conversations, managers already have begun to brace for the worst. Van Krimpen is considering firing half of the 17 employes of a ship-radio repair company who used to do most of their work for tankers docking at the refinery. "I can meet the payroll for five months, then we are out of money," he said.

Another casualty of the Exxon pullout may very well be Aruba's long struggle to achieve a modified independent status within the Kingdom of the Netherlands.

Aruba, which with five other islands forms part of the Dutch Antilles, long has chafed under the domination of the central government in the bigger island of Curacao, some 30 miles away. It is scheduled to separate from the other islands -- status aparte -- on Jan. 1, 1986, as the first step toward eventual independence, a further 10 years away.

The economic disaster that Lago's closing would mean to Aruba will make independence "very difficult," acknowledged Betico Croes, the leader of the majority Movimiento Electoral del Pueblo party, who has spearheaded the independence movement on the island. Croes has been a constant visitor to Caracas, Venezuela, and apparently hopes that his close ties with the ruling Accion Democratica party will win him Venezuelan aid in keeping the refinery open.

He insists that independence is in the cards and that status aparte will be proclaimed as scheduled. But with no money in the till, status aparte looks very different now than it once did, and some islanders question whether it would be wiser to delay it.

"If Betico Croes keeps the refinery open for two or three years, he will be crowned emperor of Aruba," said van Krimpen. "If not, he has a problem." For the Netherlands Antilles, whose population of 260,000 boasts the highest standard of living in the Caribbean, the threatened closing is the third in a flurry of body blows that have sent the economy reeling.

Two years ago, a sharp devaluation in Venezuela dried up the flood of free-spending tourists who used the two main islands of Aruba and Curacao as shopping-center annexes to Caracas. Since then, dollar-conscious U.S. tourists on cheaper packaged tours have largely replaced the Venezuelans, who are sorely missed, especially by island merchants.

Meanwhile, Congress' decision earlier this year to eliminate the 30 percent withholding tax on foreign earned dividends destroyed the main incentive for U.S. corporations to set up financial subsidiaries here. The decision spells the end for the island's lucrative offshore banking business, which provides the Antilles government with 17 percent of its revenue.

The effort to keep the refinery afloat even has involved President Reagan. Shortly after Exxon announced the closing, Venezuelan President Jaime Lusinchi fired off a letter to Reagan requesting U.S. intercession in the matter. The subject reportedly was brought up when the two men met in Washington during Lusinchi's state visit earlier this month.

However, given Reagan's aversion to government intervention in private business, Dutch Antilles officials do not expect an answer to their problems to emerge from Washington. "The reaction we have had so far from the State Department is, 'We don't interfere with private enterprise,' " said Don Mansur, the Dutch Antilles minister for economic affairs. "The answer has to come from Venezuela."

But if the answer is to be based on business considerations alone, there is scant hope there will be a positive response from Caracas. "I'm not betting on a solution with Venezuela," said Mickey McGuire, the casino manager for the Aruba Holiday Inn and a betting man. "It's not on the board." Venezuela has troubles of its own. Burdened by an estimated $34 billion foreign debt, it has had to slash oil production to 1.55 million barrels a day from a high of 2.4 million barrels a day in 1979.

For Venezuela to return to the "netback" price system under which Exxon got a discount of as much as $3.00 a barrel under government-posted prices means a yearly subsidy to the U.S. company of about $50 million at a time when the state oil company, Petroleos de Venezuela (PDVSA), has been unable to scrape up the crude oil necessary to supply its other clients.

Venezuelan oil men estimate that they could make another $25 million a year by running some of the oil now going to Exxon through Venezuela's own underused refineries and by selling the rest on the market at posted prices.

"The Aruba refinery is not viable," the PDVSA executive said flatly. "It has survived only because the Venezuelan government subsidized it -- be it for market reasons or whatever." The unanimous opinion of the Venezuelan oil industry is that it should be closed, he added.

Island officials acknowledge the soundness of Exxon's economic reasoning and understand Venezuela's reluctance to shoulder the burden. But they hold out hope that Aruba's strategic importance for Venezuela will outweigh the purely financial considerations of the case and that somehow an agreement will be reached that will allow Exxon to keep the refinery open.

"With an abrupt closure of the refinery, we will have social problems that can cause political instability in Aruba," Croes said. The solution "can't be a commercial one. It must be political," he added.

A geopolitical solution is more like it. Aruba lies just 125 miles from the giant oil fields of Lake Maracaibo. With Curacao, it sits astride tanker routes to the area. On a clear day, one can see the coast of Venezuela's Paraguana Peninsula, Aruba's oil affairs minister, Nelson Oduber, points out. "Political stability has no price," he said, and he hopes that Venezuela will buy his argument and come up with some cheap oil.

The refinery's raison d'etre has always been Venezuelan crude. Built during the long reign of Gen. Juan Vicente Gomez, the strong man who dominated Venezuelan political life during the first third of the century, the Aruba plant and the Shell Oil Co. plant in nearby Curacao gave the oil companies safe haven to profitably process Venezuelan crude away from Venezuelan political storms.

Even after the nationalization of the Venezuelan oil industry in 1976, Lago continued to make handsome profits thanks to the netback pricing formula worked out with PDVSA.

But the profits ended abruptly when PDVSA notified Exxon that it would not renew its 1984 contract on the old terms. Exxon nevertheless accepted a one-year contract paying posted prices, but warned it would not do so in 1985.

Lago was built to produce heating crude for the U.S. East Coast market from Venezuelan crude. Despite some modernization, it remained a simple "coffeepot" operation unable to produce quantities of higher-priced "white" products such as gasoline that would have improved the plant's refining margin -- the difference between what a company pays for its crude, refining costs, and the price it obtains in the market for the products it makes.

Far away from its markets, and caught between high crude prices and low prices for its heating oil, for which demand has tumbled in the last few years -- between the hammer and the anvil, as Lago President Antonio Cavallaro puts it -- Lago has nowhere to go but to a refining elephant's graveyard.

For the refinery, the numbers just don't add up. Even island officials agree that the refinery has no long-term future, and they know the best they can hope for is a gradual shutdown spaced over two or three years, which would buy the island time to retool its economy.

Ironically, the closing of the refinery could help keep open the Shell refinery in Curacao. In October, Shell presented the Antilles government with a four-point proposal for continuing operations in Curacao.

The main problem for Shell Curacao, which has a more modern refinery than Lago, is that it has not been able to get enough of the cheap Venzuelan crude it needs to survive. Conceivably, some of the 180,000 barrels a day now going to Lago could be diverted to Shell once Lago closes. Such a partial solution may indeed be reached. During a recent visit to Caracas, Dutch Antilles Prime Minister Maria Libria Peters said she was "99 percent" certain Shell's request for an additional 50,000 barrels a day of heavy crude would be approved by PDVSA.

"We think this refinery can have a future," said Van Mourik Broeckman, the general manager of Shell Curacao. "It is important we don't see it as a phased closure."