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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 68 (1984)

Issue: 11. (November)

First Page: 1837

Last Page: 1837

Title: Anschutz Ranch East--Finding and Defining a Giant Oil Field in Thrust Belt: ABSTRACT

Author(s): Jeffrey J. Lelek


Anschutz Ranch East is the giant oil field of the western United States thrust belt, with over 3/4 billion bbl of oil equivalent in place. The original prospect was not so impressive. Extensive Tertiary cover precluded precise prospecting by surface mapping. An early well penetration of a shallow recumbent fold tied to conventional 2-D seismic reflection lines hinted at a Triassic-Jurassic structure 1,500 acres (600 ha.) in extent. Soon after Amoco discovered Anschutz Ranch East in 1979, production pressure data revealed that the field was very large; but it was development drilling and 3-D seismic that prompted the geologic interpretation of 14,600 acres (5,900 ha.) under closure. Still being delineated nearly 5 yr after its discovery, the current size estimate is near 4 000 acres (1,600 ha.). The field, therefore, serves as a good case example of the difficulties involved in geologic mapping using remote data.

Anschutz Ranch East is part of a prolific oil trend extending more than 40 mi (64 km) along the leading edge of a major regional thrust plate. Situated in the extreme southwest corner of Wyoming and adjacent Utah, the field produces from separate reservoirs in two en echelon structures. The more significant west lobe is a highly overturned, narrow anticline with more than 2,000 ft (610 m) of hydrocarbon column. The producing Triassic-Jurassic Nugget Sandstone, largely eolian in origin, is just over 1,000 ft (305 m) thick, with more than 75% of the rock capable of contributing to production. Geologically, the field has passed through several stages of geometric interpretation: the faulted stage, the concentric fold stage, and the angular stage. Development of these three interpretation was based largely on outcrop examples of fold geometries in thrust belts of both the eastern and western United States.

Production rates in the first half of 1984 averaged approximately 36,000 bbl of condensate, 6,500 bbl of natural gas liquid, and 215 mmcf of gas per day. The reservoir fluid is a rich gas condensate originally about 150 psi (1,034 kPa) above the dew-point pressure. Complexities involving reservoir fluid properties, stratigraphic influences on fluid flow, effects of structural deformation, and fracture systems have required close cooperation between geologists and reservoir engineers in planning for field development. With this cooperation to maximize recovery, the recovery factor for hydrocarbons in place is expected to be about 70%.

More than 90% of the field's recovery will come from the west structure, which is just over 1 mi (1.6 km) wide and probably less than 6 mi (9.7 km) long. Fields of this size might be easily missed in thrust belt exploration.

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