President Reagan's embargo on doing business with Libya left the handful of American companies still in that country uncertain of its impact on them.

The major companies in Libya all said they agreed to comply with the presidential order, but said they remained unsure what it means. Jack White, a spokesman for Marathon Petroleum Co., one of four U.S. oil companies still in Libya, said his firm received a 39-page booklet from the Treasury Department implementing the presidential order at about 5 p.m. yesterday.

"We're trying to figure out what it means," he added, calling the directions "complex" and "highly confusing."

"We are studying what we can do to protect our economic interests there," White said.

Four U.S. companies handle 75 percent of the 1.1 million barrels a day of oil produced in Libya. Besides Marathon, they are Occidental Petroleum Corp., Conoco Inc. and Amerada Hess Corp.

Occidental, which last year sold 25 percent of its interest in its Libyan operation to the Austrian state oil company, OMV A.G., said Libyan oil accounts for 1 percent of its $12 million in yearly sales.

A spokesman said the impact on Marathon, which holds a 16.3 percent interest in the Oasis Oil Co. of Libya, is unclear. Libyan oil accounts for 70,000 barrels of the company's total production of 280,000 barrels. The Libyan government owns 59 percent of that company.

Conoco, which also owns 16.3 percent of Oasis, said the Libyan operation is "insignificant" compared with other company oil sales. Amerada Hess, which owns 8.3 percent of Oasis, said it takes 70,000 barrels of crude a day from Libya.

All four oil companies said they have no U.S. employes in Libya, although between 1,000 and 1,500 Americans do work in that country's oil field. They all have been ordered home under the presidential directive.

Price Bros. Co. of Dayton, Ohio, which provided technology, machinery and equipment for a $20 million irrigation project, said its part of the job is finished and it has no plans to bid in the second phase of the contract. Under export licenses issued before the presidentially ordered embargo, Price Bros. already supplied the technology and equipment for the pressurized concrete pipe used in the project.

Brown and Root, the large Texas construction company, holds the main contract for the ambitious project, which Libyan leader Muammar Qaddafi has called "the eighth wonder of the world" and "a great miracle." Brown and Root is handling the operation through its British subsidiary, and officials of its parent company, Halliburton Co. of Dallas, said they were unsure yesterday whether the presidential order would affect a firm based overseas.

"I expect many of our competitors from West Germany, France, Great Britain and Italy will be delighted. They will be there with their hands out trying to get hold of that job" if Brown and Root is forced to give up the contract.

Britain, moreover, is likely to fight the United States if the presidential order is extended to companies based in that country.