ABSAROKA THRUST, ANSCHUTZ RANCH AREA, UTAH AND WYOMING

Oil and gas fields in the Anschutz Ranch area typify the productive trend in southwestern Wyoming and northcentral Utah. Two nearlyparallel trends in the Absaroka plate appear to be the result of the formation of ramp anticlines. Each thick competent unit in the stratigraphic section, (Paleozoic carbonates, Mesozoic Thaynes Formation and Nugget Sandstone-Twin Creek Limestone) forms a hanging wall anticline within a productive trend. The western anticline (Anschutz Ranch) is associated with the truncation of the Paleozoic section against the Absaroka thrust, and the eastern anticlines (Anschutz Ranch East) are associated with the truncation of the Mesozoic section. In the Anschutz Ranch East field two separate ramps (and anticlines) are documented. Palinspastic reconstruction indicates ‘that the Paleozoic section was shortened due to folding by 24%, and the Mesozoic section (folded beyond the requirements of the classic Rich model) Was shortened by 27%. This shortening associated with the ramp anticlines applies to a stratigraphic package 12,000 ft thick and 12.3 mi(65,OOO ft) wide. The Paleozoic-cored anticlinal trend, which resulted from ramping through 4,500 ft of competent Paleozoic rocks, extends for nearly 50 mi along strike with oscillations of closure and minor strike offsets. The Mesozoic anticlinal trend, which is cored by competent rock units 1,500 ft to 2,500 ft thick is not as continuous as the Paleozoic trend; individual closures range from only 1 to 6 mi in length. Data presented suggest a clear relationship between the length of the ramp anticline and the thickness of the stratigraphic section traversed by the ramp.