The Kuparuk oil field is located on the Alaskan Arctic plain in the Colville-Prudhoe Basin, 10 to 30 mi (16 to 48 km) west of the Prudhoe Bay Field. The 24 degrees API crude is similar to that in the Prudhoe Bay Field; however, it is from the Lower Cretaceous Kuparuk Formation. The origin of the oil is believed to be predominantly Lower Sequence formations with migration occurring possibly via the Prudhoe Bay Field. The dominant trapping mechanism is stratigraphic pinch-out and truncation of the reservoir at a local unconformity along the southern and western flanks of a southeast-plunging antiform. Structural dip closure exists along the northern and eastern flanks. The reservoir sandstones occur within sequences which become cleaner and coarser upward, and are thought to be shallow marine in origin with a provenance to the northeast. They are interpreted to be infrarift sediments on what is now a passive, atlantic-type continental margin. Two of the four major lithostratigraphic units mapped within the Kuparuk Formation exhibit good reservoir characteristics and extend over an area in excess of 200 mi 2 (518 km 2 ). The cumulative net pay in the Kuparuk Field ranges up to 90 ft (27 m), and the estimate of movable oil-in-place is 4.4 billion stock tank bbl. There is no gas cap. The field exhibits a variable oil-water contact ranging from -6,530 ft (-1,990 m) in the southeast to -6,700 ft (-2,042 m) in the north. After secondary waterflooding, the potential recoverable reserves are estimated to be about 1.0 to 1.5 billion stock tank bbl. Kuparuk Field, therefore, ranks as one of the largest oil fields in the United States.--Modified journal abstract.