Abstract

A detailed stratigraphic investigation of a 12-fold, common-depth-point seismic profile across the east flank of the Rock Springs Uplift, Wyoming and the Table Rock Anticline indicates that the stratigraphic trap in the Patrick Draw Field can be detected within the range of frequency values commonly recorded in field seismic operations. The principal sandstone reservoir in the Patrick Draw Field is located in the upper part of the Upper Cretaceous Almond Formation and attains a maximum thickness of 35 ft (11 m). The pinchout of the reservoir sandstone is manifested on the seismic profile by the westward termination of a continuous horizon of high-amplitude peak reflections believed to represent the approximate base of the reservoir sandstone. The horizon of high-amplitude reflections terminates as much as 0.75 mi (1.2 km) east of the actual sandstone pinchout, but when the 10- to 48-Hz frequency content of the seismic data and the 10- to 20-ft (3- to 6-m) thickness of the reservoir sandstone near its pinchout edge is considered, this discrepancy is minor. Selected sandstone units in the Lewis Shale and Fox Hills Sandstone also can be detected on the seismic profile. Critical to the seismic detection of the Patrick Draw stratigraphic trap and sandstone units in the Lewis and Fox Hills was the seismic-to-stratigraphy tie provided by a synthetic seismogram from the 17,337-ft-deep (5.3 km) Texaco Table Rock No. 15 gas well. Two-dimensional seismic model studies also aided in the stratigraphic interpretation of the seismic profile.

The recognition of Laramide angular unconformities in the area of the Rock Springs Uplift, by using the seismic profile and a detailed geologic cross section, is another significant result of the investigation. Two Laramide unconformities identified in the subsurface, an upper intra-Fort Union Formation unconformity and a lower unconformity that separates the Upper Cretaceous Lance Formation and the Paleocene Fort Union Formation, merge westward to form a single unconformity between the Lance and Fort Union in the adjacent outcrop belt. By interpreting the Lance-Fort Union unconformity in outcrop to be the product of two episodes of Laramide Uplift implies that, in addition to the latest Cretaceous episode of growth recognized by previous investigators, the Rock Springs Uplift also had a Paleocene episode of growth.