Hydrocarbon Sealing Capacity of Paleosols, Wasatch Formation, Rifle, Colorado

Jarrad B. Berg¹, Frank G. Ethridge¹, Sally J. Sutton², William R. Almon³, and William C. Dawson^3
1 Dept. of Geosciences Colorado State University, Fort Collins, CO
² Colorado State Univesity, Fort Collins, CO
³ ChevronTexaco, Bellaire, TX

Two floodplain paleosol packages in the Eocene Wasatch Formation were sampled to determine how paleosol development, clay matrix and quartz grain content, and stratigraphic position affect sealing capacity. Outcrops sampled are located in the Piceance basin approximately 2 miles northwest of Rifle, CO in Hubbard Gulch. The Wasatch is divided into three members, in ascending order the Atwell Gulch, the Shire, and the Molina, in ascending stratigraphic order. Vertisol packages in the low net-to-gross Shire Member deposits are the main focus of this project.

Mercury injection capillary pressure analysis was used to evaluate the sealing capacity of 80 samples from two paleosol packages. Capillary pressure curves constructed from mercury injection data were used to compare sealing capacity at 10% mercury saturation. Sealing capacity values range from 467 to 7667 psia. Sealing capacity is largely controlled by texture and increases as clay content and paleosol development increase. Paleosol development was evaluated by the type and nature of pedogenic structures and the presence or absence of paleo-roots. Well developed paleosols exhibit multicolored mottling, angular to subangular peds, slickensides and paleo-root structures.

With one exception no systematic trends in lateral or vertical variation of sealing capacity in the paleosols was observed. Because of the nature of outcrop exposure lateral variations with respect to distance from contemporaneous paleo-channels was not determined. Samples collected directly above or below paleo-channels consistently exhibit poor sealing capacity, while samples further away from the paleo-channels vertically tend to have higher sealing capacity.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005