"Total Resource Energy Extraction (TREE sm) uses underground coal conversion to fuel in situ recovery of crude petroleum from oil shale, tar sand and heavy oil deposits. Conventional rubbling or fracturing processes prepare the fossil energy resource for permeation by hot flue gases from underground coal conversion. The hot flue gases are generated initially by combusting low btu product gas from underground coal gasification. As the gas heating value declines below combustible limits, the production of process energy is sustained by burning the remaining coal resource. The temperature of the flue gas is controlled by recycling the cooled gases which have transferred sensible heat to the fossil energy resource. During this permeation process, the flue gases sweep the liquid petroleum to production wells for conventional product collection. Shale oil production with TREE technology is 40% higher than conventional in situ combustion retorting."