In June 1968 ARCO-Exxon completed the Prudhoe Bay State no. 1 well, discovering the largest oil accumulation in the United States. This discovery was the result of a [proposed] three-year seismic program begun in 1963. The three predecessor companies of Atlantic Richfield were involved in separate geophysical programs and by 1964 each program had delineated two major structures on the North Slope coastal plain: Colville and Prudhoe. During the first state sale on the North Slope (late 1964), Sinclair, in partnership with British Petroleum, leased the entire Colville structure. The critical state lease sale covering Prudhoe Bay was held in July, 1965. This sale determined the eventual ownership of the Prudhoe Bay field. ARCO-Exxon acquired the top tracts, with British Petroleum acquiring flank acreage. In January, 1967 ARCO-Exxon acquired additional offshore tracts and began drilling the Prudhoe Bay State No. 1 in April.