Reservoir Description of Endicott Field, Prudhoe Bay, Alaska: ABSTRACT

Abstract

Located about 2 mi offshore and several miles east of Prudhoe Bay, the Endicott field contains about 1.4 billion reservoir bbl of oil and 0.5 billion reservoir bbl of gas. Hydrocarbons occur within Mississippian fluvial sandstones of the Kekiktuk formation, which unconformably overlies the Neruokpuk Formation and grades upward into the Kayak and Itkilyariak formations. Stratigraphy is subdivided into three lithofacies that, from the base upward, reflect deposition in a swamp/lacustrine/flood plain environment (zone 1), a braided stream system (zone 2), and a meandering stream system (zone 3). Sediment dispersal was from a northerly source. Endicott field structure defines a southwesterly plunging antiform that is bounded to the north, northeast, and southwest by major normal faults and is truncated to the northeast by the Lower Cretaceous Unconformity (LCU). Shales overlying the LCU and shales of the Kayak and Itkilyariak formations form the reservoir cap. Reservoir properties within the hydrocarbon column vary by zone with zones 3 and 2 typified by an average net/gross-porosity-water saturation-permeability of 37%-18%-22%-500 md and 88%-22%-13%-1,100 md, respectively. In contrast, zone 1 quality is very poor. Reservoir sands are compositionally very mature and exhibit an enhanced pore network. Diagenetic minerals include quartz along with lesser kaolinite and carbonate. Gas is present from about 9,500 ft (2,850 m) to 9,855 ft (2,958 m), oil is down to 10,180-10,200 ft (3,054-3,060 m), and tar accumulations are down to 10,400 ft (3,120 m) subsea. Average oil gravity is 23° API. Geochemical data indicate that the tar accumulations originated through a physical deasphalting process. Cenozoic imbibition resulted in water overriding tar. End_of_Article - Last_Page 656------------