A method for treating a relatively permeable formation containing heavy hydrocarbons in situ may include providing heat from one or more heat sources to a portion of the formation. The heat may be allowed to transfer from the heat sources to a selected section of the formation. The transferred heat may pyrolyze at least some hydrocarbons within the selected section. A mixture of hydrocarbons may be produced from the selected section. A production rate of the mixture may be controlled to adjust the time that at least some hydrocarbons are exposed to pyrolysis temperatures in the formation in order to produce hydrocarbons of a selected quality in the mixture.