Abstract: A method is provided for operating an in situ oil shale retort in a subterranean formation containing oil shale. The retort contains a fragmented permeable mass of formation particles containing oil shale, within top, bottom and side boundaries of unfragmented formation. A drift is in communication with a lower region of the fragmented mass for withdrawal of liquid products of retorting and an off-gas comprising gaseous products of retorting. A gas-sealing bulkhead is placed across the product withdrawal drift and a water spray is installed in the drift through the bulkhead. An upper region of the fragmented mass is ignited for establishing a combustion zone therein. A retort inlet mixture comprising an oxygen-supplying gas is introduced into the top of the fragmented mass for advancing the combustion zone downwardly through it to thereby establish a retorting zone on the advancing side of the combustion zone for producing the liquid and gaseous products.

Abstract: A method for recovering liquid and gaseous products from an in situ oil shale retort containing a fragmented permeable mass of formation particles containing oil shale is provided. A hot ignition gas comprising oxygen at a first selected concentration is introduced into the fragmented mass for heating the fragmented mass top surface. The percentage of the fragmented mass surface that is at a temperature no less than the ignition temperature of oil shale is determined. Thereafter, the concentration of oxygen in the ignition gas is increased by an amount proportional to the determined percentage. Such heating of the fragmented mass top surface establishes a combustion zone in the retort. After the combustion zone has spread horizontally across the retort, introduction of the hot ignition gas is discontinued. Thereafter, an oxygen-supplying gas is introduced into the retort for advancing the combustion zone downwardly through the fragmented mass.