This invention relates to in situ recovery of shale oil, and more particularly, to techniques involving excavation of a void and explosive expansion of oil shale formation toward such a void in preparation for forming an in situ oil shale retort. 
The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits.  It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil.  It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce liquid and gaseous products.  It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil". 
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ.  The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes. 
The recovery of liquid and gaseous products from oil shale deposits have been described in several patents, such as U.S. Pat.  Nos.  3,661,423; 4,043,595; 4,034,596; 4,034,597; 4,034,598; and 4,118,071, which are incorporated herein by this reference.  These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded for forming a stationary, fragmented permeable mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort.  Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale.  One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat.  No.  3,661,423, includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass.  In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale.  By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort. 
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting".  Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products, and a residual solid carbonaceous material. 
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone.  The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn.  An off gas is also withdrawn from the bottom of the retort.  Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process.  The products of retorting are referred to herein as liquid and gaseous products. 
It is desirable to form a fragmented mass having a distribution of void fraction suitable for in situ oil shale retorting; that is, a fragmented mass through which oxygen-supplying gas can flow relatively uniformly during retorting operations.  Techniques used for explosively expanding formation toward the void space in a retort site can affect the permeability of the fragmented mass.  Bypassing portions of the fragmented mass by retorting gas can be avoided in a fragmented mass having reasonably uniform permeability in horizontal planes across the fragmented mass.  Gas channeling through the fragmented mass can occur when there is non-uniform permeability. 
A fragmented mass of reasonably uniform void fraction distribution can provide a reasonably uniform pressure drop through the entire fragmented mass.  When forming a fragmented mass, it is important that formation within the retort site be fragmented and displaced, rather than simply fractured, in order to create a fragmented mass of generally high permeability; otherwise, too much pressure differential is required to pass a retorting gas through the retort.  Preferably the retort contains a reasonably uniformly fragmented mass of particles so uniform conversion of kerogen to liquid and gaseous products occurs during retorting.  A wide distribution of particle size can adversely affect the efficiency of retorting because small particles can be completely retorted long before the core of large particles is completely retorted. 
The general art of blasting rock formation is discussed in The Blaster's Handbook, 15th Edition, published by E. I. DuPont de Nemours & Company, Wilmington, Del.
The prior art has disclosed techniques for forming a fragmented permeable mass of particles in an in situ oil shale retort, wherein formation from within a retort site is excavated to form a void in the form of a narrow slot having vertically extending free faces.  Blasting holes can be drilled parallel to the vertical free faces in rectangular zones of formation adjacent opposite sides of the slot.  Explosive is placed in the blasting holes and detonated in a desired time delay sequence for explosively expanding formation in such rectangular zones toward the free faces for forming the fragmented mass.  Explosive within the retort site can be detonated for expanding separate vertical segments of formation from within the retort site toward the free faces in a time delay sequence progressing into such formation away from the free faces.  In such a blasting pattern there is progressively less void space into which vertical segments of formation of the same size can be expanded.  Stated another way, the segments of formation farthest from the free face encounter increasing confinement when blasting progresses into the formation away from the free face.  In some instances such confinement can inhibit desired breakage and movement of formation being expanded. 
Moreover, explosive placed in a rectangular zone of formation and blasted toward rectangular void volumes can create an inefficient use of explosive energy along the side boundaries of the zone being blasted.  The natural cratering effect of explosive when detonated causes fragmentation of formation to occur in an outwardly diverging pattern from the explosive charge.  Fragmentation from such explosive expansion can be askew to the desired rectangular side boundaries of the retort being formed, resulting in an inefficient use of explosive along the boundaries. 
It would be beneficial to provide a blasting pattern in which the cross-sectional shape of formation being expanded can reasonably match the side boundaries of the retort being formed, so that use of explosive energy is reasonably efficient.  It would also be beneficial to provide a blasting pattern in which oil shale formation expanded toward a vertical free face has reasonably good lateral relief throughout the retort site as expansion progresses into the retort site away from the free face, to provide reasonably good breakage and movement of formation being expanded.