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Abstract

A method for controlling the average particle size of a fragmented permeable mass of formation particles formed in an in situ retort in a subterranean formation containing oil shale is provided. At least one void is excavated in the subterranean formation, leaving zones of unfragmented formation above and below the void. Such a zone of unfragmented formation has naturally occurring cleavage planes and a substantially horizontal free face adjoining the void. An array of a plurality of substantially vertical blastholes is formed in at least one of the zones of unfragmented formation and each such blasthole is loaded with explosive for forming a substantially horizontal array of explosive charges. The spacing distance between adjacent blastholes or explosive charges in the array is from about 10 to about 15 times the average distance between cleavage planes when the oil shale formation has an average grade of more than about 20 gallons per ton. The spacing distance between adjacent blastholes or explosive charges in the array is from about 15 to about 22 times the average distance between cleavage planes when the oil shale has an average grade of less than about 20 gallons per ton. The explosive charges are detonated for explosively expanding the zone of unfragmented formation toward the void to form a fragmented permeable mass of formation particles in the in situ retort.

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First Claim