Hydrocarbon sealing capacity of paleosols, Wasatch Formation, Rifle, Colorado
Date
2005
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Abstract
Hydrocarbon reservoirs require a seal to keep the hydrocarbons from migrating out of the reservoir. Therefore, recognition of seal rocks and determination of seal properties are extremely important for exploration. Seal rocks are commonly fine-grained shale and mudstone. Previous seal rock studies have mostly been limited to marine shales. Other fine-grained rocks that may provide seals for hydrocarbon deposits include paleosols. Two floodplain paleosol packages in the Eocene Wasatch Formation were sampled to determine how paleosol characteristics, 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, 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, five lithofacies, and four microfacies. 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. No significant differences in sealing capacity were found between the upper and lower paleosol packages, or among lithofacies. Microfacies, defined primarily on the basis of quartz grain size, show an inverse relationship with sealing capacity. For all samples analyzed as seals correlation analysis suggests that high sealing capacity correlates with high matrix and low grain content, smaller pore-throat diameters, relatively low TOC and relatively high grain density. In general good seals have one class of pore throats and minimal variation in pore-throat size distribution. Visibly good seals exhibit multicolor mottling, angular to subangular peds, slickensides, and paleo-root structures; all features common in well-developed paleosols. Because of the nature of outcrop exposure, lateral variations with respect to distance from contemporaneous paleo-channels were 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. Using available data it was not possible to determine the cause of the relationship between sealing capacity and paleosol development. The textures visible in the present day paleosols may be primary (depositional), or secondary (from soil forming and or diagenetic processes).
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Paleopedology -- Colorado -- Rifle Region
Hydrocarbon reservoirs -- Colorado -- Rifle Region