One month ago, as Ferdinand Marcos fled the Philippines, his energy minister did the same -- in the state oil company's well-appointed executive Lear jet.

Now, Philippine government investigators say Geronimo Velasco, who also served as chairman of the Philippine National Oil Co., may have taken more than a plane ride. They say they are pursuing allegations that, in the decade since Marcos made him the country's virtual energy "czar," he used numerous tactics to siphon tens of millions of dollars out of the country's oil company and a complex web of other energy-related companies he headed.

Velasco, believed to be in the United States, has said that there were no improper payments.

Several Manila businessmen described Velasco this week in harsh terms and as someone to whom Marcos gave sometimes monopolistic powers in importing the country's vital crude oil supply.

Teofisto Guingona, who as chairman of the government's audit commission is supervising the investigation, refuses to comment on what his researchers are finding. "All I can say is that it is taking a long time to check the 28 government corporations and subsidiaries which were controlled by Velasco," Guingona said earlier this week.

Guingona said his commission is investigating allegations that, under Velasco, the company may have overpaid consistently for the purchases of massive oil tankers and other equipment, with the excess payment drained off by Velasco. Commission investigators also said they are checking information that Velasco forced shipowners to pay kickbacks for being hired to carry the Philippines' crude oil.

The commission is investigating other allegations that the national oil company, like other government corporations, was required to overspend on insurance, with payments going to a company owned by another close associate of Marcos.

The wealth of a man widely regarded as one of the Philippines' richest is left unexplained, investigators said, by Philippine National Oil Co. records on Velasco's salary, listed at about $4,700 per year, with another $37,000 in tax-free benefits.

Like many of the other state-owned corporations Guingona's commission is assigned to investigate, the Philippine National Oil Co. was never audited under Marcos' administration. One commission staff member estimated the company's weekly cash flow, however, at $70 million. That has made the firm a high-priority target in the new Philippine government's probe into how the Marcos family and some of their close associates allegedly spirited what the government has estimated at $5 billion to $10 billion out of the country.

The state oil company, which has imported about 40 percent of the Philippines' oil in recent years, "was one of the fattest cows the cronies milked," said Jaime Vicente, an audit commission researcher, in a telephone interview. The administration of the company was "systematic, institutionalized looting, no less," he said.

Velasco is widely criticized these days as one of Marcos' top "cronies." Reporters and editors of a respected daily, The Manila Times, staged a near-revolt this week when press reports said Velasco secretly owned part of the paper. The publisher printed a front-page article Wednesday announcing that Velasco had divested his interest under pressure.

Velasco's elevation to power in the Philippine petroleum industry began in 1973, when Exxon Corp. and Gulf, then major importers and refiners of oil for the Philippines, decided to pull out of the country. "We were all having problems because world oil prices shot up, but the government was keeping our selling price too low," recalled Amaury Gutierrez, president of Caltex (Philippines), an oil company in Manila.

"Marcos bought out Exxon and Gulf and created PNOC, with Velasco in charge, and two years later, made him minister of energy," Gutierrez said.

Philippine opponents of Velasco long have alleged that Velasco skimmed a percentage from all payments for oil imported by the Philippine National Oil Co.

But they have presented no evidence for this, and Filipino petroleum executives and government investigators working on the case said such a scheme would be difficult because the national oil company bought most of its oil on government-to-government contracts with countries such as Kuwait, Saudi Arabia, Indonesia and China.

Such contracts, they said, involve relatively stable oil prices and are generally governed by stricter regulations.

The commission is investigating, however, the large commissions paid by shipowners to shipping brokers who arranged charter voyages for the company, carrying crude oil to the Philippines.

Standard worldwide brokerage fees range from 1.25 percent to as high as 2.5 percent of the value of the charter, shipping industry sources said. But according to investigators and national oil company documents that include shipping contracts, shippers who carried oil for the Philippine National Oil Co. between 1976 and 1980 normally paid 7.5 percent. Investigators said they are looking into whether Velasco received kickbacks in the process.

Velasco has denied such kickback allegations, first reported last week in the San Jose Mercury News. In a telex message to the Mercury News, which was published in a local Philippine newspaper Thursday, Velasco said, "The commission is not my concern nor PNOC's . . . . No additional charges, commissions, fees, etc. are ever paid . . . to PNOC over and above the freight rate agreed upon."

Caltex President Gutierrez said the Philippine oil trade was "ripe" for abuse.

"There was a glut of tanker capacity , so that tanker owners were crying for business and were willing to pay," he said. He added that, from 1975 to 1978, Marcos effectively had helped Velasco "put shipowners over a barrel," by awarding him exclusive authority to charter vessels not only for the national oil company, but for all companies importing oil to the Philippines.

"Marcos issued a decree that required importers to use Philippine-owned tankers or to charter through PNOC," he said. "They dropped the rule in 1978, when the criticism and talk of corruption became too much."

Among the shipping contracts were about a dozen prepared by a U.S. shipping broker, Dietze Inc., then based in New York. They included four 1977 contracts between the Philippine National Oil Co. and Alpine Shipping Co. of Monrovia, Liberia, and specified that "commission of 7.5 percent is payable by ship owner to Dietze Inc. for division."

There were no attached documents to show what had happened to those payments, but an audit commission researcher said auditers had turned up records showing that some of the inflated commissions had been paid to numbered Swiss bank accounts. "I don't think that proves that Velasco was getting the money," he said. "But I also don't think that was normal industry practice."

David Dietze, chairman of the company, which is now based in Westport, Conn., said he had no records or direct knowledge of any contracts with the Philippine oil company, nor knowledge of any improper payments. He said the company had dealings with the Philippines in the late 1970s, before he became chairman, through a Manila shipping broker.

He said he understood that besides the standard broker's commission, "all charters to the Philippines for years and years (also) had an address commission," which he recalled was "about 5 percent."

A spokeswoman for the Association of Ship Brokers and Agents, a U.S. trade association based in New York, said address commissions are sometimes used in the industry and are paid by the shipowner to the customer chartering the ship as a rebate. She said the size of such commissions varies.

Investigators also are looking into allegations that the Philippine National Oil Co., like all other Philippine government corporations, was forced to overspend on insurance, with payments going to a company owned by another close associate of Marcos, Roberto Benedicto. In 1975, the Marcos government, which required all state corporations to buy insurance from a government insurance service, allowed Benedicto's Integral Factors Corp. to act as its insurance agent.

Audit commission chairman Guingona called Benedicto's corporation "a toll gate between the captive government clients and the government" insurance service that squeezed $35 million out of the government corporations. The arrangement, he said, never had been audited in the 10 years of its existence.

In tanker purchases, the commission is investigating whether Velasco's company overspent by tens of millions of dollars when it had five new oil tankers built in Japan in the mid-1970s. "They bought these at a time when there was already a world glut in tankers," said Caltex president Gutierrez.

"We even mentioned to them, 'Why not buy a five-year old tanker?' " he explained. "The going rate for older VLCCs very large crude carriers was $10 to $12 million, and they paid twice that amount."

In recent days, investigators say they have found records of financial transfers from the government oil company to a Hong Kong-registered corporation, Decision Research Management Ltd. That corporation has been identified in press reports as mostly owned by a corporation in Panama, but with symbolic stockholding by two of Velasco's sons.