In situ retorting of oil shale, if technologically feasible and economically practicable, has several advantages over the more conventional mining and aboveground retorting system, particularly for deep, thick deposits under appreciable overburden. The problem of disposing of spent shale can be eliminated, air and water pollution problems can be minimized, aesthetic values can be enhanced. It may be possible to produce a higher grade oil, and it may be possible to effect a higher percentage recovery. Although hydraulic fracturing and chemical high explosives may be used to fracture shallow and thin beds, nuclear explosives may be required for adequate fracturing of the Green River oil shale in the center of the Piceance Basin of Colorado, where there are deposits as thick as 2000 ft. (610m) under as much as 1000 ft (306m) of overburden. Experiments have been conducted at the Bureau of Mines Laramie Energy Research Center in batch type retorts accommodating particle size ranges expressed to be present in a nuclear explosive created chimney. Capacities are 10 and 130 metric tons, accommodating pieces of shale as large in two dimensions as 20 in. (51 cm) and 4 ft. (1.2 m), respectively. In seven tests with the large retort, recoveries were 39% to 62% of Fischer assay, with shale charges assaying 8.0 to 10.1 weight of oil. In tests of lean shale (0.75 wt.%) in the small retort, the highest yield was 51% of Fischer assay. Field tests have been conducted and still are in progress near Rock Springs, Wyoming, in sections of oil shale ranging from 20 ft.(6.1 m) to 40 ft. (12.2 m) in thickness and from 68 ft. (20.7 m) to 145 ft. (44.2 m) in depth. Results of these tests have been somewhat encouraging and indicate that the creation of adequate permeability through carefully designed and controlled fracturing techniques is a critical factor.