Abstract

Gilsonite is a solid hydrocarbon that forms a swarm of subparallel, northwest trending, near vertical, laterally and vertically extensive veins in the Uinta Basin. Gilsonite is sourced from the Mahogany oil shale zone of the Green River Formation and is hosted in the Tertiary Wasatch, Green River, Uinta, and Duchesne River Formations. The veins formed in two stages due to thermal maturation of the Mahogany oil shale. Overpressuring deep in the Uinta Basin hydrofractured overlying strata and expelled large quantities of thermal water. Subsequently, thick, liquid gilsonite was expelled forcing open the existing fractures. The gilsonite later solidified, probably primarily through cooling and polymerization.

The gilsonite veins in Utah are the largest in the world and have a long, colorful history of profitable mining. Consequently, gilsonite has been studied and described in a large body of work dating back to the 1880s. However, there has always been some ambiguity in the descriptions of the veins, especially in their locations and extent, so there were no accurate summaries. The goal of our study was to use Global Positioning System (GPS) and Geographic Information System technology to compile accurate maps of the gilsonite veins, collect vein attribute data, and combine these data with existing data into an accurate, up-to-date gilsonite resource study with an extensive bibliography and historical production data.

We examined 56 veins, vein systems, and isolated vein outcrops totaling more than 116 miles in length. We collected 1418 GPS data points with associated attribute data, obtained field mapping data from previous geologic mapping by the USGS and the U.S. Bureau of Land Management, and examined recent National Agriculture Imagery Program high-resolution color aerial photography.

Even though significant amounts of the original gilsonite resource of about 45 million tons have been mined, there are millions of tons of resource remaining. 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. At the recent industry production rate of 60,000 to 80,000 tons per year, gilsonite will continue to be mined in the Uinta Basin for decades.