Heating tar sands formations to visbreaking temperatures

A method for treating a tar sands formation, comprising:

• heating at least a section of a hydrocarbon layer in the formation from a plurality of heaters located in the formation;
• controlling the heating so that at least a majority of the section reaches an average temperature of between about 200° C. and about 240° C. resulting in visbreaking of at least some hydrocarbons in the section;
• maintaining a pressure in the section below a fracture pressure of the section and within about 1 MPa of the fracture pressure of the section;
• and
• producing at least some visbroken hydrocarbon fluids from the formation.