Abstract: A subterranean formation containing oil shale is prepared for in situ retorting by forming a fragmented permeable mass of formation particles containing oil shale in an in situ retort site. The retort is formed by excavating a lower level drift adjacent to a lower portion of the retort site and excavating an undercut within the retort site below a zone of unfragmented formation remaining within the retort site above the undercut. The bottom of the undercut slopes downwardly toward the lower level drift which opens into one side of the undercut, the slope being generally at the natural angle of slide of oil shale particles. The remaining zone of unfragmented formation is blasted downwardly toward the undercut in a series of lifts in sequence progressing upwardly in the retort site. The mass of formation particles formed during such blasting in lifts tends to slope downwardly toward the side of the retort adjacent the lower level drift.

Abstract: A method for forming a fragmented permeable mass of formation particles in an in situ oil shale retort is provided. A horizontally extending void is excavated in unfragmented formation containing oil shale and a zone of unfragmented formation is left adjacent the void. An array of explosive charges is formed in the zone of unfragmented formation. The array of explosive charges comprises rows of central explosive charges surrounded by a band of outer explosive charges which are adjacent side boundaries of the retort being formed. The powder factor of each outer explosive charge is made about equal to the powder factor of each central explosive charge. The explosive charges are detonated for explosively expanding the zone of unfragmented formation toward the void for forming the fragmented permeable mass of formation particles having a reasonably uniformly distributed void fraction in the in situ oil shale retort.

Abstract: An in situ oil shale retort is formed in a subterranean formation containing oil shale. Such an in situ oil shale retort contains a fragmented permeable mass of formation particles containing oil shale including a lower moiety and an upper moiety of said fragmented mass. The fragmented permeable mass of formation particles has top, bottom and side boundaries. A first portion of the formation is excavated from within the boundaries of the fragmented mass being formed to form a void. A second portion of the formation is excavated from within the boundaries of the fragmented mass being formed to form at least one additional void in communication with the first void and leaving a third portion to be expanded toward such an additional void. Fragmented particles from the excavation of such an additional void are deposited in the first void to provide a lower moiety of the fragmented permeable mass of formation particles.