Gilsonite Veins of the Uinta Basin, Utah
Utah Geological Survey
Gilsonite is a solid hydrocarbon that forms a swarm of subparallel, northwest-trending, near-vertical, laterally and vertically extensive veins in the Uinta Basin of Utah and Colorado. The Uinta Basin hosts the world's largest deposits of gilsonite, and is the only place where gilsonite is economically produced in large quantities. Gilsonite was sourced from the Mahogany oil shale zone of the Eocene Green River Formation and is hosted in the Tertiary Wasatch, Green River, Uinta, and Duchesne River Formations. The veins formed in two stages associated with thermal maturation of the Mahogany oil shale. Overpressuring deep in the Uinta Basin expelled large quantities of thermal water from the reservoir rocks and hydrofractured the overlying and underlying strata. Subsequently, thick, liquid gilsonite was expelled from the reservoir rocks, forcing open the existing fractures in the overlying and underlying strata. The gilsonite later solidified in these fractures, probably primarily through cooling and polymerization. This study included examination and mapping of 59 veins, vein systems, and isolated vein outcrops totaling more than 120 miles in length. In addition, we collected 1474 Global Positioning System data points with associated attribute data, obtained field data from previous geologic mapping by the U.S. Geological Survey and U.S. Bureau of Land Management, and examined recent National Agriculture Imagery Program high-resolution color aerial photography. A total of 71 significant veins, vein systems, and vein extensions were documented in our study, having a total combined vein length of more than 170 miles. Gilsonite has a wide variety of uses including asphalt paving mixes and coatings; chemical components in metallurgical, adhesive, coating, binder, ink, and paint products; and in metal foundry and oil well drilling and well completions. Even though significant amounts of the approximately 45-million-short-ton original gilsonite resource have been mined, millions of tons of the valuable resource still remain. This resource tends to be in the deeper parts of the veins and in thinner, more remote veins that will likely be more expensive to mine than veins mined in the past. At the recent industry production rate of 60,000 to 80,000 tons per year, gilsonite could continue to be mined in the Uinta Basin for decades.
AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013