What is oil shale? Oil shale is a fine-grained sedimentary rock that contains various amounts of organic matter. Oil in the shale is contained within a waxy, bituminous substance called kerogen.
How is the oil released from the rock? In order to release the oil from the kerogen, the rock must be heated to about 650 to 700 degrees Fahrenheit in an oxygen-free environment. This process is called “retorting.” Two different methods have been used in the past to recover oil from the oil shale rock: surface retorting and in situ retorting. On average, approximately three quarters, or less, of the oil contained within the kerogen may be released using these techniques, depending on the physical properties of the rock itself (porosity, permeability, cementation, etc.).
Surface retorting usually involves recovering the oil shale by using standard surface mining or underground mining techniques and then feeding the rock into a kiln for heating. After the oil has been removed from the rock, the spent shale is returned to the mined-out portions of the surface pit or under-ground mine workings, or maintained in a permanent surface spoil dump.
In situ retorting is normally done at depth, leaving the oil shale beds in the ground. This is accomplished by drilling a number of bore holes from the surface into the deposit and then heating the rock underground by using one of several methods to release the oil. The extracted oil can then be pumped to the surface through production wells for gathering and further processing. This process may also involve fracturing the oil shale beds using standard techniques to increase the porosity and permeability of the deposit to facilitate the flow of the released oil.
Where does oil shale occur? The United States holds more than 50 percent of the world’s known oil shale resource, most of which is contained in deposits found in Colorado, Utah and Wyoming. These oil shale deposits are found within a total area of approximately 16,000 square miles between the three states, with about 72 percent of located on federal lands. The total reserve contained within these deposits is estimated to be 2.6 trillion barrels of in-place oil, and 1.5 trillion barrels of recoverable oil.
The oil shale deposits of Wyoming are located predominantly within Sweetwater County, and to a lesser extent within Uinta, Lincoln, Sublette and Carbon counties. The oil shale beds are found almost exclusively in the Upper Eocene Age rocks of the Green River Formation in the Green River Basin, the Washakie Basin and the Fossil Basin of southwestern Wyoming. The beds vary greatly in thickness, ranging from several feet to several hundred feet thick. Quality is also highly variable from area to area, with an overall average grade of approximately 15 gallons of oil per ton of shale. The depths to the beds range from zero feet at the outcrop to well over 3,000 feet below the surface at the depositional center of the basins.
Map of Oil Shale Withdrawals
The Green River Basin of southwestern Wyoming is underlain by approximately 1,895,125 acres (2,960 square miles) of “classifiable” oil shale deposits. Classifiable deposits are defined as those containing oil shale beds that are at least 15 feet in thickness that will yield a minimum of 15 gallons of oil per ton of shale. Total reserve for the basin is estimated to be 417 billion barrels of in-place oil.
The classifiable oil shale deposits within the Washakie Basin of Wyoming are restricted to the western flank of the basin in Sweetwater County, covering about 302,470 acres (473 square miles). The estimated total reserve for this basin is 57 billion barrels of in-place oil.
Estimates for the oil shale resources contained within the Fossil Basin are not available.