Abstract: In situ oil shale retorts are formed in spaced apart rows, with adjacent rows of such retorts being separated by load-bearing barrier pillars of unfragmented formation sufficiently strong for preventing substantial subsidence at the ground surface. Each retort contains a fragmented permeable mass of formation particles containing oil shale. Separate air level drifts are excavated on an upper level of the retorts within alternating barrier pillars, and separate production level drifts are excavated at a lower production level of the retorts within intervening barrier pillars between the barrier pillars having the air level drifts. Each air level drift extends between a pair of adjacent rows of retorts adjacent upper edges of the retorts in the adjacent rows, and each production level drift extends between a pair of adjacent rows of retorts adjacent lower edges of the retorts on sides of the retorts opposite the air level drifts.

Abstract: A module of in situ oil shale retorts are formed in a row of retort sites in a subterranean formation. Each retort has top, bottom, and side boundaries of unfragmented formation and contains a fragmented permeable mass of formation particles. Two cross drifts are excavated through the retort sites along the row. One of the drifts is at a lower elevation near the floor of voids to be formed in the retort sites and along one side boundary of the retort sites. The other drift ramps upwardly at an end of the row for extending through the retort sites at a higher elevation near the roof of the voids excavated in the retort sites and along the opposite side boundaries of the retort sites. A horizontally extending slice is excavated at the elevation of the higher drift extending substantially to the side boundaries of a retort site for commencing a void within the retort site. The balance of the void is formed by benching from the slice to the elevation of the lower drift.

Abstract: Formation is excavated from within the boundaries of a retort site in formation containing oil shale for forming at least one retort level void extending horizontally across the retort site, leaving at least one remaining zone of unfragmented formation within the retort site. A production level drift is excavated below the retort level void, leaving a lower zone of unfragmented formation between the retort level void and the production level drift. A plurality of raises are formed between the production level drift and the retort level void for providing product withdrawal passages distributed generally uniformly across the horizontal cross section of the retort level void. The product withdrawal passages are backfilled with a permeable mass of particles.

Abstract: A non-subsidence method for developing an in situ oil shale retort tract in a subterranean formation containing oil shale includes forming a number of spaced apart rows of in situ oil shale retorts, leaving intervening zones of unfragmented formation between adjacent rows of retorts for supporting the overburden loads without substantial subsidence. Each retort contains a fragmented permeable mass of formation particles containing oil shale. The retorts in each row are separated by gas barriers that provide support for the overburden load above each row of retorts. After retorting, a stabilizing material is introduced into the void spaces in the spent in situ oil shale retorts for increasing the compressive strength of the fragmented masses of spent oil shale particles in the spent in situ retorts. Thereafter, separate rows of in situ oil shale retorts are formed in corresponding intervening zones of unfragmented formation.

Abstract: An in situ oil shale retort is formed in a subterranean formation containing oil shale. Formation is excavated from within a retort site for forming a plurality of vertically spaced apart voids extending horizontally across different levels of the retort site, leaving a separate zone of unfragmented formation between each pair of adjacent voids, and leaving one or more pillars within each void for providing temporary roof support for unfragmented formation above each void. A plurality of horizontally spaced apart vertical blast holes are drilled in each zone of unfragmented formation below the voids. A pillar within a first void is offset horizontally from at least a portion of a pillar in a second void excavated directly below the first void. This provides an access region in the first void above at least a portion of the pillar in the second void so that vertical blast holes can be drilled into a zone of unfragmented formation below the pillar in the second void from the access region in the first void.

Abstract: An array of in situ oil shale retorts is formed in a development region in a subterranean formation containing oil shale. At least one void is excavated in each retort site, and remaining formation within each retort site is explosively expanded toward the void for forming a fragmented permeable mass of formation particles containing oil shale in each in situ retort. Overburden loads over an area of the development region are carried largely by the fragmented masses and partly by unfragmented partitions between retorts. Subsidence of overburden following explosive expansion is controlled at the boundary of the development region to avoid an abrupt change in the overburden load supported largely by the fragmented masses inside the boundary and the overburden load supported by unfragmented formation outside the boundary.

Abstract: Oil shale formation is explosively expanded toward a limited void volume for forming an in situ oil shale retort in a subterranean formation containing oil shale. In one embodiment, a void in the form a narrow vertical slot is excavated within a retort site, leaving at least one portion of unfragmented formation within the retort site adjacent a vertical free face of the slot. Explosive is placed in a row of vertical blasting holes in the remaining portion of unfragmented formation adjacent the vertical free face. The blasting holes are mutually spaced apart along the length of the slot, and the row of blasting holes extends generally parallel to the vertical free face.

Abstract: An in situ oil shale retort is formed in a subterranean formation containing oil shale. Underground workings excavated within the formation provide a means for access to a retort site in the formation. At least one void is excavated in the retort site via access provided by the underground workings, leaving a remaining portion of the unfragmented formation within the retort site adjacent the void. Explosive placed in the remaining unfragmented formation adjacent such a void is detonated in a single round for explosively expanding the unfragmented formation toward such a void for forming a fragmented permeable mass of formation particles containing oil shale. Prior to such explosive expansion, a barrier of unfragmented formation is left between such a void and underground workings providing means for access to such a void. At least one gas flow passage extends through the barrier of unfragmented formation between the means for access and the retort site.

Abstract: Oil shale formation is explosively expanded toward a limited void volume for forming an in situ oil shale retort in a subterranean formation containing oil shale. In one embodiment, a void in the form of a vertical slot is excavated within a retort site, leaving at least one portion of unfragmented formation within the retort site adjacent a vertical free face of the slot. Explosive is placed in at least two rows of vertical blasting holes in the remaining portion of unfragmented formation adjacent the vertical free face. The blasting holes in each row are mutually spaced apart along the length of the slot and longitudinally offset from blasting holes in the next adjacent row, and the row of blasting holes extends generally parallel to the vertical free face.

Abstract: A group of spaced apart in situ oil shale retorts is formed in a subterranean formation containing oil shale. At least one void is excavated in each retort site, and remaining formation within each retort site is explosively expanded toward such a void for forming a fragmented permeable mass of formation particles containing oil shale in each retort. A vertically extending partition of substantially unfragmented formation forms a gas barrier between the fragmented masses in a pair of adjacent retorts. Such a gas barrier yields structurally but retains sufficient integrity to inhibit gas flow between the fragmented masses of adjacent retorts. Such a gas barrier is sufficiently thin that it independently supports substantially the same proportionate amount of load from overburden at elevations above the retorts as the fragmented masses on either side of the gas barrier.

Abstract: A bridging plug for a circular blast hole has a relatively longer major axis and a relatively shorter minor axis. The plug is hinged to pivot about the minor axis. A tension line is secured to the minor axis for lowering the plug into an explosive placement hole. The plug pivots into an angular open position when lowered into the hole. An upward force on the tension line jams the hinged portions of the plug in a closed position to plug the hole. An explosive charge can be placed in the hole at the location defined by the closed plug.

Abstract: An in situ oil shale retort is formed in a subterranean formation containing oil shale. The retort with top, bottom and side boundaries contains a fragmented permeable mass of particles containing oil shale. A base of operation is excavated at a working level in the formation. Means are excavated through the formation for access to a location underlying the base of operation. In one embodiment, a void in the form of a vertical slot is excavated in the site between the means for access and an elevation below the bottom of the base of operation, leaving a remaining portion which is to be expanded toward the void. This is designed to leave a horizontal sill pillar of intact formation between the top of the void and the bottom of the base of operation with a vertical thickness sufficient to maintain a safe base of operation after explosive expansion of formation to form the fragmented permeable mass of particles.