 The heat is transferred to the shale through the pipe walls rather than being injected directly into the rubble, thereby avoiding dilution of the product hydrocarbons with the heating gas.The oil shale rubble is enclosed by a low-cost earthen impoundment structure to prevent environmental contamination and to provide easier and more rapid reclamation after the extraction process is finished.Heat from the spent shale is recovered for enhancing the process's energy efficiency by passing cool gas through pipes and then using it for preheating adjacent capsules.