Abstract

A fragmented permeable mass of formation particles containing oil shale and having a substantially uniformly distributed void fraction is formed in an in situ oil shale retort in a subterranean formation containing oil shale. The fragmented mass is formed by explosive expansion towards a horizontal free face. At least one generally horizontally extending void is formed in the subterranean formation and zones of unfragmented formation are left above and below such a void. An array of horizontally spaced apart substantially vertical columnar explosive charges is formed in at least one of the zones of unfragmented formation. The array of explosive charges includes a plurality of central explosive charges which are spaced apart from side boundaries of the retort and a plurality of outer explosive charges surrounding the central charges. Explosive in each outer explosive charge extends nearer the free face toward which unfragmented formation is to be explosively expanded than any portion of the explosive in a central explosive charge. Additionally, each outer explosive charge has a scaled depth of burial which is no greater than the scaled depth of burial of each central explosive charge. The explosive charges are thereafter detonated for explosively expanding the zone of unfragmented formation toward the void, forming the fragmented permeable mass of formation particles in the in situ oil shale retort.