Abstract: 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 synthesis gas production temperature. A synthesis gas producing fluid may be introduced into the formation to generate synthesis gas. Synthesis gas may be produced from the formation in a batch manner or in a substantially continuous manner.