Conoco

President Reagan's order that all U.S. companies and citizens quit doing business with Libya will affect only a modest amount of trade with that country because it already is the object of a tough economic embargo imposed in 1981. Major companies that do business with Libya are the Amerada Hess, Occidental, Marathon, Conoco and Chevron oil companies, and Brown and Root, a construction firm. Occidental, Marathon and Conoco issued statements last night saying they would comply with the President's order.

About four years ago, Du Pont Co., normally one of the most cautious and deliberate of American corporations, stunned Wall Street by leaping headlong into a fierce three-way fight for Conoco Inc., the Stamford, Conn., oil company. Du Pont, of course, was victorious. It bested rivals Mobil Corp. and Seagram Co. Ltd. with a staggering bid of $7.7 billion and absorbed Conoco in what was, at the time, the largest corporate merger ever. But Du Pont paid another price as well. When the smoke had cleared, Seagram, the Canadian-based liquor distiller, was left holding 20 percent of Du Pont's common stock.