An oil shale formation may be treated using an in situ thermal process. A mixture of hydrocarbons, H₂, and/or other formation fluids may be produced from the formation. Heat may be applied to the formation to raise a temperature of a portion of the formation to a pyrolysis temperature. Heat may be supplied to the formation by reacting an oxidant with hydrocarbons adjacent to wellbores to generate heat. Generated heat may be transferred to the portion substantially by conduction to pyrolyze at least a portion of hydrocarbon material within the portion.