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The western portion of the Great Salt Lake contains two large Neogene basins, informally called the “North” and “South” basins. These basins are separated by an arch that trends northeast between Carrington Island and Fremont Island. Both basins are filled with Miocene, Pliocene, and Quaternary sediments and volcanic rocks. Each basin has an estimated maximum thickness of over 4300 m (14, 000 ft) of Tertiary rocks. Palynology indicates the oldest Tertiary sedimentary rocks present in both basins are Miocene, but a radiometric date indicates the presence of Oligocene rocks.

Structurally, the basins are slightly asymmetric, deeper on the east with an obvious boundary fault zone on the east flank of each basin. Faulting is present on the western flanks but of a lesser magnitude. The most common structural traps found in these basins are anticlinal closures, faulted noses, and fault closures. These structures are probably the result of continued differential subsidence of pre-Miocene blocks throughout Neogene time.

A total of 13 exploratory wells was drilled by Amoco in the Great Salt Lake, from June 1978 to December 1980, resulting in an oil discovery at West Rozel and oil and/or gas shows in eight other wildcat wells.

The West Rozel oil field produces from fractured Pliocene basalts at a depth of 640-730 m (2100-2400 ft). The trap is a faulted, closed anticline covering approximately 2300 acres. The discovery well, Amoco No. 1 West Rozel Unit (NW NW Sec. 23, T8N, R8W, Box Elder County), has an oil column of 88 m (290 ft) but produced at rates of only 2-5 BOPH with a gas-lift system. The oil is 4° API gravity, 12. 5% sulfur, and has a pour point of 75°F. Two development wells that have smaller oil columns (No. 2, 26 m [85 ft]; No. 3, 60 m [194 ft]) were equipped with a downhole hydraulic pump and produced oil at rates up to 90 BOPH. Additional development of the field was not initiated because of the high water cut and the high cost of operating an “offshore” field.

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