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. A formation to be treated may produce a relatively large amount of condensable hydrocarbons and/or a relatively large amount of non-condensable hydrocarbons. Hydrocarbons within the formation may have a relatively low initial elemental oxygen weight percentage.