In situ thermal processing of a coal formation to produce hydrocarbons having a selected carbon number range

A coal 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. An average temperature and/or pressure within the formation may be controlled to inhibit production of hydrocarbons that have carbon numbers greater than 25. A small amount of hydrocarbons having carbon numbers greater than 25 may be entrained in vapor produced from the formation.