A hydrocarbon containing formation may be treated using an in situ thermal process. A mixture of hydrocarbons, H
, 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. The mixture may be produced from the formation through production wells. A spacing between production wells, and operating conditions of production wells and heat injection wells, may allow the produced mixture to have a desired ratio of condensable hydrocarbons to non-condensable hydrocarbons.