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Abstract

In situ oil shale retorts are formed in spaced-apart rows, with adjacent rows of such retorts being separated by load-bearing inter-retort pillars of unfragmented formation sufficiently strong for preventing substantial subsidence. Each retort contains a fragmented permeable mass of formation particles containing oil shale. An air level drift is excavated in formation directly above the inter-retort pillar so that the roof and/or floor of the air level drift is spaced above the upper boundaries of the retorts in such adjacent rows. This causes the roof of the air level drift to be in compression, rather than in tension, which stabilizes the roof and avoids dangerous rock falls. During retorting operations, air is introduced at the upper edge of each retort through lateral air inlet passages sloping downwardly from the air level drift. Off gas and liquid products are withdrawn from each retort through a production level passage at the bottom of each retort at the edge opposite the air inlet. The production level passages connect to a main production level drift extending between adjacent rows of retorts. The roof of the main production level drift is excavated in formation directly below the inter-retort pillar so that the roof of the production level drift is spaced below the lower boundaries of the retorts in adjacent rows. This places the roof of the production level drift in compression, avoiding the likelihood of rock falls.

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