Abstract: An in situ oil shale retort containing a fragmented permeable mass of formation particles having a generally uniform permeability across horizontal cross-sections of the retort is provided. To form the retort, underground formation is explosively expanded by explosive charges placed in a selected pattern. The combination of spacing distance and powder factor of central explosive charges is selected to produce a center portion of the fragmented permeable mass having a first average particle size (d.sub.1) and a first average void fraction (e.sub.1). The powder factor for perimeter explosive charges and the spacing distance between central charges and adjacent perimeter charges are selected to produce a perimeter portion of the fragmented permeable mass having a second particle size (d.sub.2) and a second void fraction (e.sub.2). The second average particle size (d.sub.2) and the second average void fraction (e.sub.2) are related to the first average particle size (d.sub.