ABSTRACT: Comparison of Fort Union Formation Reservoir Types in Outcrop and in Nearby Abandoned Oil Fields, Southern Wind River Basin, Wyoming

KEIGHIN, C. W., ROMEO M. FLORES, and D. J. NICHOLS, U.S. Geological Survey, Denver, CO

Fort Union Formation sandstones range from fluvial, as in outcrops of the Castle Garden area and the nearby abandoned Thompson Ranch oil field, to lacustrine-deltaic, as in the Squaw Butte oil field.

Detailed examination of the more than 2000-ft-thick Paleocene (palynostratigraphic zones P1-P6) Fort Union sediments in the Castle Garden area, Fremont County, Wyoming, shows development of three potential sandstone-reservoir types: fluvial channel, crevasse channel, and crevasse splay. Fluvial channel sandstones are well developed in the lower (P1-P2), middle (P3-P4), and upper parts (P5-P6) of the Fort Union Formation. Fining-upward multilateral fluvial sandstones of the lower and middle parts of the formation may be traced on logs to the Thompson oil field; these sandstones merge to the north with multistory coarsening-upward deltaic sandstones in the Squaw Butte oil field. Laterally extensive (> 1.5 mi) thin to thick (5 to 20 ft) crevasse channel-splay sandstones commonly are i terspersed with the less extensive (< 0.75 mi), thick (> 20 ft) fluvial-channel sandstone reservoirs in the Castle Garden-Thompson Ranch oil field area. The deltaic sandstones in the Squaw Butte area are less than 15 ft thick and may be traced for more than 1.5 mi.

Success or failure of productive reservoir types in the oil fields may be predicted best from parameters measured in outcrops where sandstone heterogeneity is more readily observed and completely characterized than can be done in widely spaced drill holes. Knowledge of heterogeneous outcrop-reservoir types may be used to better define targets for renewed exploration in now-abandoned oil or gas fields, such as the Thompson Ranch and Squaw Butte fields.

AAPG Search and Discovery Article #91017©1992 AAPG Rocky Mountain Section Meeting, Casper, Wyoming, September 13-16, 1992 (2009)