Joint Geologic/Engineering Analysis of the Sadlerochit Reservoir, Prudhoe Bay Field
Authors: Wadman D.H. , Lamprecht D.E. , Mrosovsky Ivan 
Published: December 2007

This paper demonstrates the importance of coordinated geological and engineering effort to develop an accurate reservoir description. The techniques discussed and applied when developing the description of the Sadlerochit reservoir of Prudhoe Bay Field have particular application where sophisticated numerical simulation models are used to evaluate operating alternatives for a field.

Introduction
The Prudhoe Bay Field on the north coast of Alaska is the northernmost oil field in the world, situated about 250 miles (402 km) north of the Arctic Circle. The field was discovered in 1968, and subsequent drilling confirmed that it is the largest oil and gas field in North America. Shortly after the discovery, the major working-interest owners and the State of Alaska recognized the benefits of operating the field as a unit; unitization negotiations began in 1969. The Prudhoe Bay Unit, which is operated by ARCO Oil and Gas Co. and Sohio Petroleum Co., became effective April 1, 1977, and is Petroleum Co., became effective April 1, 1977, and is one of the few units in the U.S. formed before production began. After completion of the Trans-Alaska production began. After completion of the Trans-Alaska Pipeline System, the field began producing June 19, Pipeline System, the field began producing June 19, 1977, almost 9 years after its discovery.During the past 8 years, the Prudhoe Bay Field has been the subject of intensive geologic study and engineering analysis, both to determine in-place volumes and to develop an operating plan for the field. This paper describes the methods used by geologists and engineers to develop reservoir descriptions of Prudhoe Bay for use in numerical simulation studies. Similar studies were conducted by Exxon Co. U.S.A., ARCO Oil and Gas Co., and Sohio Petroleum Co. Since these studies were conducted independently, they naturally led to somewhat different descriptions. Thus, data given here do not always correspond to the best estimates of all three companies, nor did the three companies apply the methods described next in all cases. Results of independent reservoir performance studies, based on the companies' reservoir descriptions, were used to design production facilities, to select initial completion production facilities, to select initial completion intervals, and to develop a long-range operating plan for the field.

Geologic Setting
The first step in the development of a reservoir description should always be an intense geological study. Such a study is fundamental for explaining the anomalies present in the reservoir, such as a tilted water-oil contact, fracture system, lithologic units, and shale continuity.The principal oil and gas accumulations at Prudhoe Bay Field are in rocks of Permo-Triassic age. Fig. 1 shows the geologic setting for the Prudhoe Bay Field and gives some clues as to the geologic history of the area. The Prudhoe Bay Field is a combination structural and stratigraphic hydrocarbon trap situated on the south flank of the Barrow Arch. The northern limit of the field is established by a series of normal faults, downthrown to the north, which have up to 1,000 ft (305 m) of displacement.The Permo-Triassic sediments were derived from a source area situated north of the present coastline. Various unconformities in the Prudhoe Bay area indicate that the Barrow Arch was at or near sea level several times during its history.
JPT
P. 933