The Ivishak (sometimes referred to as the Sadlerochit) reservoir is the largest producing horizon in the Prudhoe Bay field. ?he field, located on the coastline of the Beaufort Sea, has been divided into eight zones with lower sections being deposited in low-energy, marine and delta-front environments and the remaining zones consisting of sandstones and conglomeratic units deposited by braided streams.(1) Subsequent burial, structuring, and hydrocarbon emplacement resulted in a maximum accumulation 40 million years ago, with a hydrocarbon-column thickness of 2,080 ft. Continuing burial resulted in flattening and hydrocarbon spillage, resulting in the configuration at field discovery with a hydrocarbon-column thickness of 1,050 ft. Understanding the structural and hydrocarbon histories is key to understanding and calculating the gas-, oil-, and water-saturation distributions at the time of field discovery because some areas of the reservoir appear to have been in capillary equilibrium with the discovery oil/water contact, while structurally high portions of the reservoir appear to be in equilibrium with conditions at the time of maximum hydrocarbon accumulation.