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The seismic profile shown here in Figures 2 and 3 is part of a regional northeast to southwest common-depth-point (CDP) line shot by Amoco Production Company in 1969, which traverses the northeastern Big Horn basin of Wyoming (Figure 1). A small-scale reproduction of the original analog section was published earlier by W.R. Sacrison (1978, Figure 7, p. 43), and permission from Amoco and Sacrison to publish this reprocessed, larger scale part of the section is gratefully acknowledged.

The combination of high quality seismic data and critical well control make this particular example a very instructive one. A unique opportunity to study and test some of the concepts of thrustfold geometry and genetics in the Wyoming foreland province is afforded by this profile.

The thrust-fold features so clearly shown on this seismic profile are typical of many of the oil field structures in the Big Horn basin. The largest feature, on the left (west), is South Elk Basin field, an important Paleozoic structural accumulation. South Elk basin is an independent closure lying down plunge to the south of the giant Elk basin oil field. The Paleozoic accumulation at Elk basin is contained in a common pool with an oil column measuring more than 700 m (2,300 ft) (Stone, 1967, p. 2070). To the right (east) of the South Elk basin feature are the Little Polecat and Big Polecat thrustfold trends which both contain small Paleozoic oil fields on local closures updip to the southeast of the profile (Figure 5).

As shown on the True Scale Structural Cross Section (Figure 4), the Continental Oil Company Unit No. 41 well, drilled in early 1972, penetrated most of the Paleozoic section above Precambrian basement in the hanging wall block of a large thrust fault, cut the thrust zone, then cut overturned Paleozoic rocks, and right-side-up Paleozoic rocks, and finally bottomed in Precambrian basement rocks in the footwall block. This interpretation is supported by log and sample data as well as dipmeter data through the critical interval above and below the fault zone. In addition, abundant well data define the structure of the South Elk Basin field at all levels down to the Madison reservoir This combination of excellent seismic and well data across the South Elk basin structure, places definite constraints on the geometry of the faulting and folding along this classic structure.

Just east of the Elk basin trend is the Little Polecat thrust-fold trend, which plunges northwest and dies out under the north-trending Elk basin thrust zone (the two trends meeting at an angle of about 45°; Figure 5). The seismic profile crosses the Little Polecat trend about 3.2 km (2 mi) southeast of its junction with Elk basin, and about half a mile (0.8 km) down plunge from a control well drilled to test the Pennsylvanian Tensleep Formation. A synthetic seismogram made from the sonic log of this well has aided in reflection identifications. Clearly the folding and faulting are much less intense along the Little Polecat trend than at South Elk basin.

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