ABSTRACT

The Elk Basin field in the northern Bighorn basin is a giant structural trap with cumulative production surpassing 500 million bbl, principally from a Paleozoic common pool. Abundant well data and seismic information have been used in a stratigraphic and structural study focusing on the Greybull (Lower Cretaceous) gas pool and on deeper formations along this structural complex. These data support an interpretation of the Elk Basin field as a thrust-fold complex, underlain by a listric thrust fault zone which probably emanates from Precambrian basement at an angle of 45° or less. The fault steepens upward and dies out in steeply dipping Mesozoic clastics that are attenuated and cut by extensional faults at the surface.

The little known Greybull Sandstone pool at Elk Basin field, which is now used for gas storage, was discovered in 1920, and contained estimated primary recoverable reserves of 54 bcf of gas at an average depth of about 2,500 ft (760 m). The Greybull lies stratigraphically between the Dakota and Morrison Formations, and is composed of two distinct sandstone units, called “A” and “B” at the North Clark’s Fork field in southern Montana. The lower “B” unit at Elk Basin is a fluvial river-channel deposit which ranges up to 150 ft (45 m) in thickness and nearly 2 mi (3 km) in width. The upper “A” unit is a series of shoreline sandstone deposits oriented northwest-southeast. Individual, porous “A” sandstone bodies range from a few feet to more than 20 ft (6 m) in thickness at Elk Basin. These two Greybull Sandstone units are part of a common gas pool covering about 2,000 acres (800 ha.) of the crestal closure of the Elk Basin anticline. Seismic modeling indicates that Greybull Sandstone channels over 60 ft (18 m) thick may be detected by reflection character changes in CDP seismic data.

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