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 may be produced from the selected section. A
quality of the produced mixture may be controlled by varying a location
for producing the mixture.