Abstract The Prudhoe Bay field is the largest petroleum accumulation found to date in North America. Recoverable reserves are expected to be 9.6 billion bbl of oil and 26 trillion cu ft of gas. The field is located on the Barrow arch, a major subsurface feature of the Arctic Slope province. The Prudhoe Bay structure is a west-plunging anticlinal nose, faulted on the north and truncated by an unconformity on the east. The area of closure mapped on the top of the Sadlerochit reservoir is about 125,000 acres. The excellent reservoir properties of the upper part of the Sadlerochit Formation are the result of its alluvial deposition by a braided-stream complex. Mississippian through Jurassic rocks, which contain the field's important reservoirs, were derived from northern uplifts. The unconformable Cretaceous clastic rocks, which contain the probable source rocks of the Prudhoe Bay oils, were derived from the Brooks Range uplifts on the south. The unconformity at the base of the Cretaceous source rocks truncates the pre-Cretaceous reservoirs far down the plunge of the Barrow arch, and the resultant reservoir-source rock relation at Prudhoe Bay has produced a major subunconformity accumulation. The earliest attempts by private industry to develop the Arctic Slope petroleum potential were halted in 1923 by establishment of Naval Petroleum Reserve No. 4. The U.S. Navy carried out a 10-year exploration program beginning in 1944. Drilling was conducted on the Barrow high and on the foothills anticlines. The largest oil discovery was at Umiat, which has an estimated reserve of 70 million bbl. Private-industry exploration resumed in 1963, and several unsuccessful wildcats were drilled east of NPR-4. In 1964, Humble joined with Richfield Oil Corporation (now Atlantic Richfield Company) in joint exploration of federal acreage south of Prudhoe Bay. Regional seismic data and the exploration incentive provided by state competitive lease policy resulted in the shifting of the exploratory effort to the eastern Arctic coastal area. Abundant seismic reflectors allowed definition of the Prudhoe Bay structure by conventional analog single-fold data. Most of the structure was leased jointly by Humble-Richfield and by British Petroleum Alaska Inc. in July 1965. The discovery well, ARCO-Humble No. 1 Prudhoe Bay State, was completed in June 1968.