Abstract: Evolution of the Wamsutter Arch, Wyoming Foreland, USA: Influence on Formation of Stratigraphic Traps in Upper Cretaceous Almond Shoreline Sandstones

William B. Hanson, Victor Carrillo, Scott Ritger

More than 525 million barrels of oil-equivalent hydrocarbons have been discovered in sandstones of the Almond Formation, which was deposited within a transgressive systems tract formed on the western margin of the seaway that occupied a retroarc foreland basin and divided the North American continent during much of Cretaceous time. Most of the discovered reserves are stratigraphically trapped in elongate north-south shoreline sandstones on the east-west-trending Wamsutter Arch of southcentral Wyoming. Structural relief within this part of the greater Green River basin is about 4,600 m, and the larger stratigraphic traps have tall hydrocarbon columns (460 to 500 m).

The Wamsutter Arch has a three-stage development history: the Almond transgressive systems tract (72-71 Ma) may thin slightly across the east and west ends of the present-day Arch; the north limb of the Arch formed as the Red Desert basin subsided during Lance time (68-66 Ma); the south limb of the Arch formed as the Washakie basin subsided during the Eocene (55-45 Ma). Decompaction thermal modeling, constrained by high-quality data, was used to estimate the maturation history of the dominantly type III kerogens associated with the Almond Formation. Almond humic kerogens are not fully mature at the locations of major stratigraphic traps on the crest of the Arch. The analysis suggests that source beds on the limbs of the Arch reached peak gas generation at the same time that critical s uth dip formed during early to middle Eocene time. This concurrence of events fed gas and condensate onto the crest of the Arch by migration paths within the narrow, laterally continuous north-south belts of shoreline sandstone.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994