Exxon Co. U.S.A. has agreed to buy for $400 million Atlantic Richfield Co.'s 60 percent interest in the Colony Oil Shale project near Grand Valley, Colo., the companies announced yesterday.

The project is supposed to produce 46,000 barrels of oil a day from shale beginning in 1985. The remaining 40 percent of Colony is owned by Tosco Corp. of Los Angeles, which has been developing oil-shale-processing technology for many years.

Exxon U.S.A. President Randall Meyer said "the project will use the Tosco retorting process that Exxon and Arco believe is ready for commercial application." Exxon, which has scattered oil shale properties elsewhere in northwestern Colorado and is trying to consolidate them through exchanges with the federal government, hasn't experimented with oil shale technology in the past, a spokesman said.

Arco Vice Chairman William F. Kieschnick said the shale "should increase the likelihood that the project would proceed expeditiously and at the same time enable Arco to carry out other energy projects of its own in the near future."

Other Arco top executives have been skeptical in the past about oil shale development, saying total production of oil from it will be limited severely both by environmental factors and the availability of water in the area. Arco still has other oil shale property in Colorado, however.

Tosco Corp. President Morton Wiston welcomed Exxon's entry. "The alliance of Tosco's oil-shale technology with Exxon's engineering and project construction capabilities promises an acceleration of the date for production of the oil shale reserves," Winston said.

The Colony approach to oil production involves heating the oil-bearing shale in a retort, or furnace, until the oil in effect is cooked out of the rock and then upgraded through limited refining.

The purchase price for Arco's share is contingent upon completing the project on schedule. If it is late, the price will be reduced, possibly by as much as $100 million. The agreement is subject to confirmation by Exxon that the project can be advanced in a satisfactory manner and on a timely schedule, Meyer said.