NOVELTY - A hydrocarbon containing formation is in situ treated by providing heat from heat source(s) (801) to portion(s) of the formation; allowing the heat to transfer to a selected section of the formation; controlling the heat from heat source such that an average temperature within the selected section of the formation is less than 375degreesC; and producing a mixture from the formation.    USE - Used in situ treatment of hydrocarbon containing formation (e.g. a formation containing coal, oil shale, and/or heavy hydrocarbons) to produce synthesis gas (claimed). The synthesis gas can be converted to paraffins to produce other products, e.g. diesel, jet fuel, or naphtha products. It can also be used in a catalytic methanation process to produce methane. It is used for production of methanol, gasoline and diesel fuel, ammonia, or middle distillates. It may be used to heat the formation as a combustion fuel. It is used for synthesizing organic and/or inorganic compounds such as hydrocarbons or ammonia. It is used to generate electricity, by combusting it as a fuel, by reducing the pressure of the synthesis gas in turbines, and/or using the temperature of the synthesis gas to make steam (and then run turbines). It is used in an energy generation unit, e.g. molten carbonate fuel cell, solid oxide fuel cell, or other type of fuel cell. Hydrogen in synthesis gas is used to upgrade oil.    ADVANTAGE - The invention can yield a mixture of relatively high quality hydrocarbon products, hydrogen, and other products. It can also reduce energy input costs.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a system for heating hydrocarbon containing formation comprising a heater (803) in an opening in the formation, an oxidizing fluid source, and a conduit. The conduit provides an oxidizing fluid from the oxidizing fluid source to a reaction zone in the formation during use. The oxidizing fluid is selected to oxidize at least some hydrocarbons at the reaction zone during use, such that heat is generated at the reaction zone. The system can allow heat to transfer by conduction from the reaction zone to a pyrolysis zone of the formation during use.    DESCRIPTION OF DRAWING(S) - The figure shows an embodiment of using pyrolysis water to generate synthesis gas in a formation.    Heat source (801)    Heater (803)