"Oil Shale" is a kerogenous marlstone found in the Eocene Green River Formation over wide areas of Colorado, Utah, and Wyoming. Oil content averages about 20 gal per ton. Over the past 20 years experimentation in the in situ processing has been largely in the direction of downhole fracturing techniques. Nuclear fragmentation also has been considered. U.S. Bureau of Mines experiments on mine-run rock in surface retorts at the Laramie Research Station demonstrated the feasibility of in situ retorting once the necessary void volume had been created.

In the Occidental process a volume sufficient to create the required void space is mined beneath the ore which is then fragmented using conventional explosives to form a retort. Heat introduced at the top of the retort initiates retorting which then proceeds downward ahead of the advancing flame front. The oil produced is pumped from a sump at the bottom of the fragmented rock pile.

The Occidental process is not limited by room and pillar mining thicknesses and thus can exploit thicker

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oil-shale sections than other methods. Much less rock per barrel of oil is mined by the Occidental process than in other processes. The process is environmentally attractive in that surface installations are kept to a minimum because a surface retort is not required. Rock mined is readily revegetated raw oil shale.

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