Washington — Not too long ago, when oil supplies were tight and prices soared, it would have been unthinkable for Washington to cut off oil imports from Libya, no matter how much US officials disliked Col. Muammar Qaddafi.
In 1979, for example, Libya sold nearly 700,000 barrels of high grade crude oil daily to the United States, plus millions of dollars worth of refined products processed in the Caribbean.
To ban supplies from the third-ranking foreign oil supplier would have driven up US gasoline and home heating oil prices, boosted inflation, and inflicted economic misery on millions of low-income Americans. Not so today, amid reports that the Reagan administration -after consultations with allies and international oil firms - plans to ban imports of Libyan oil and deny sales of US oil and gas equipment to Colonel Qaddafi's regime.
The Libyan leader presumably was just as involved in fostering international terrorism in 1979 as he is alleged to be today. This involvement, including the reported sending of an assassination ''hit squad'' to the US with orders to kill American officials, sparks White House opposition to Qaddafi.
What has changed, dramatically, is the world oil picture, which now allows the administration to step up its campaign against the fiery Libyan leader.
Libya now furnishes less than 3 percent of all oil burned by Americans.
Crude imports from Libya in 1981, according to US government figures, averaged roughly 360,000 barrels per day - half the level of two years before. In December 1981, according the American Petroleum Institute, imports from Libya declined to 122,000 barrels daily, dropping it to 14th among foreign suppliers to the US.
The US could ban Libyan oil and easily make up the loss from other sources. ''Nigeria,'' said an oil analyst, ''would be happy to sell us every barrel of oil that we now get from Libya.''
Because of the world oil glut, the 13 members of the Organization of Petroleum Exporting Countries (OPEC) have cut back production sharply. Total cartel output is 19 to 20 million barrels daily, down from more than 31 million barrels three years ago.
''Libya's production,'' says an oil expert, ''has dropped by at least two-thirds - from more than 1.5 million barrels a day in 1980 to 500,000 or 600, 000 barrels now.''
Even at that low figure, Libya has trouble selling all its oil at the official $36.50 per barrel it charges - one of the highest price tags in the world. Libya offers a ''disguised discount,'' said a well-known analyst, by processing some of its crude in Europe and selling the refined products cheaply.
Currently Libya, along with other OPEC members, is pressuring Saudi Arabia to lower its huge output - officially 8.5 million barrels daily, but thought by experts to be a bit less - in order to drain oversupply from world markets and permit OPEC prices to firm up.
So far, for reasons of their own, the Saudis are resisting a sharp drop in production, although maintenance of the cartel's price structure is important to them.
Saudi King Khalid and Crown Prince Fahd reportedly oppose giving Iran a chance to step into the market with larger exports of oil, if the Saudis cut back. Increased oil exports from Iran, now fighting a border war with Iraq, would earn the country more money with which to foment revolution, in the Saudi view.
Libya would not be greatly hurt by a US embargo of its oil. Quaddafi's government could sell a bit more petroleum, says Lawrence Goldstein of the Petroleum Industry Research Foundation, Inc., ''if it were willing to pay the economic price'' - that is lower the cost of its oil.