Anatomy of a Gas Reservoir Complex - Outcrop to Subsurface, Wind River Basin, Wyoming

C. WM. Keighin, Romeo M Flores, Clarke Mueller, Ronald C. Johnson

Natural gas is produced from Tertiary strata from the Muddy Ridge and Pavillion fields, northwestern flank of the Wind River basin, Fremont County, Wyoming. Pavillion and Muddy Ridge Fields are approximately 10 miles from the outcrop, and situated on local structural closures developed on a southeast-plunging regional anticline. Pavillion Field was discovered in 1960, and has produced 60 BCFG from the Wind River Formation and 80 BCFG from the Shotgun Member of the Fort Union Formation. Muddy Ridge Field was discovered in 1961, and has produced 28 BCFG from the Shotgun and Lower Members the Fort Union Formation. Development drilling is continuing at the present time.

Detailed examination of Paleocene (lower and Shotgun Members of the Fort Union Formation) and Eocene (Indian Meadows and Wind River Formations strata, exposed at Little Dome and Merriam Anticlines and Shotgun Bench, and in the nearby fields, clearly reveals the extent and complex nature of reservoirs and seals. Sandstones in the outcrops of the Fort Union Formation may reach an aggregate thickness of approximately 200 ft; sandstone and conglomerate complexes in the Indian Meadows Formation may be greater than 100 ft in thickness. Individual sandstones are typically laterally discontinuous, but may be traced for more than 1 mile. Heterogeneity of the sandstone/conglomerates is controlled by variations in internal scour surfaces, internal sedimentary structures, and grain size. Sandston s formed in low sinuosity fluvial channels, represent most of the heterogeneous reservoirs in the subsurface. Sandstones are fine to medium grained, and often grade into interbedded pebble- to cobble-size conglomerates; enclosing siltstones and mudstones act as seals. Outcrops of the Wind River Formation exhibits abundant conglomerates (bedded and chaotic), minor sandstones, and mudstones with diamictites. Tracing the basal scour surface of the Wind River Formation over 1-1/2 miles reveals up to 700 ft of downcutting, and a northwest-southeast trending paleovalley infilled by braided streams and debris flows of alluvial fans. This extensive valley fill serves as heterogeneous reservoirs in the subsurface.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995