remains of large lakes such as the deposits of the Green River Formation of Wyoming and Utah, USA. Large lake oil shale basins are typically found in areas of block faulting or crustal warping due to mountain building. Deposits such as the Green River may be as much as and yield up to 40 gallons of oil for each ton (166 L/t) of shale. Oil shale deposits formed in the shallow seas of continental shelves generally are much thinner than large lake basin deposits. These are typically a few meters thick and are spread over very large areas, extending up to thousands of square kilometers. Of the three lithologic types of oil shales, siliceous oil shales are most commonly found in such environments. These oil shales are not as organically rich as lake-deposited oil shales, and generally do not contain more than 30 gallons of extractable oil per ton of oil shale. Oil shales deposited in lagoonal or small lake environments are rarely extensive and are often associated with coal-bearing rocks. These oil shales can have high yields– as much as 40 gallons per ton (166 L/t) of oil shale. However, due to their small areal extent, they are considered unlikely candidates for commercial exploitation. Formations in the United States The United States has two significant oil shale deposits which are suited for commercial development due to their size, grade and location. The Eocene Green River Formation covers parts of Colorado, Wyoming and Utah; the second significant deposit is Devonian oil shales in the eastern United States. In both places, there are sub-basins varying in volume and quality of the reserves. Oil shale in the Green River Formation is found in five sedimentary basins namely, Green River, Uinta, Piceance Creek, Sand Wash and Washakie. The first three have undergone some significant exploration and attempts to commercialize the oil shale reserves since the 1960s. The Green River Formation includes deposits from two large lakes which covered an estimated area of over during the Early to Middle Eocene. These lakes were separated by the Uinta uplift and the Axial Basin anticline. For significant periods during their 10 Ma life, the lakes became closed systems allowing many changes in size, salinity and sedimentation. The deposition of oil shales resulted from abundant blue-green algae that thrived in the lakes. The oil shale that underlies almost in the eastern United States was formed in a