The Ivishak (sometimes referred to as the Sadlerochit) Reservoir is the largest producing horizon in the Prudhoe Bay Field. The 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. Subsequent burial, structuring and hydrocarbon emplacement resulted in a maximum accumulation 40 million years ago with a hydrocarbon column thickness of 2080 feet. Continuing burial resulted in flattening and hydrocarbon spillage resulting in the configuration at field discovery with a hydrocarbon column thickness of 1050 feet. 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 since 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.

Introduction and Geologic Setting

Prudhoe Bay, the largest oil field in North America, is located on the North Slope of Alaska along the coastline of the Beaufort Sea (see Figure 1) halfway between the National Petroleum Reserve Alaska on the west and the Arctic National Wildlife Range on the east.

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