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.

Abstract: A method for igniting an in situ oil shale retort containing a fragmented permeable mass of formation particles containing oil shale is provided. A void space is in the retort between the top surface of the fragmented mass and the top boundary of overlying unfragmented formation. A hot ignition gas comprising oxygen is introduced into the void space to form a combustion zone across the surface of the fragmented mass. An oxygen-supplying gas is then introduced into the void space for sustaining the combustion zone and for advancing the combustion zone downwardly through the retort. The combustion zone is then extinguished and a cool inert gas is introduced into the retort to cool carbonaceous materials comprising the surface of the fragmented mass to a temperature below the self-ignition temperature of such carbonaceous materials, while leaving carbonaceous materials below the fragmented mass surface at temperatures greater than the self-ignition temperature of such materials.