A hydrocarbon containing 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 and pressure applied to the formation may be controlled so that a majority of the hydrocarbons produced from the formation have carbon numbers less than 25. Conditions may be controlled to produce low quantities of olefins in non-condensable hydrocarbons produced from the formation.