Abstract: A row of horizontally spaced-apart in situ oil shale retorts is formed in a subterranean formation containing oil shale. Each row of retorts is formed by excavating development drifts at different elevations through opposite side boundaries of a plurality of retorts in the row of retorts. Each retort is formed by explosively expanding formation toward one or more voids within the boundaries of the retort site to form a fragmented permeable mass of formation particles containing oil shale in each retort. Following formation of each retort, the retort development drifts on the advancing side of the retort are closed off by covering formation particles within the development drift with a layer of crushed oil shale particles having a particle size smaller than the average particle size of oil shale particles in the adjacent retort.

Abstract: A two-pass in situ oil shale retorting system uses two separate gas levels for withdrawing off gas from producing in situ oil shale retorts. During a first pass of retorting through an oil shale tract, spaced apart groups of in situ oil shale retorts are formed, leaving intervening barrier pillars of unfragmented formation between adjacent groups of retorts. During retorting, off gas from the retorts formed in the first pass is withdrawn to a first gas level drift system. Thereafter, during a second pass of retorting, intervening groups of in situ oil shale retorts are formed in the intervening barrier pillars between the groups of in situ retorts formed in the first pass. During retorting in the retorts formed in the second pass, off gas is withdrawn to a second gas level drift system that is formed at a different level and isolated from the first gas level drift system so that gas flow between the two gas levels is avoided.