An economic method for in situ maturing and production of oil shale or other deep-lying, impermeable resources containing immobile hydrocarbons. Vertical fractures are created using horizontal or vertical wells. The same or other wells are used to inject pressurized fluids heated to less than approximately 370.degree. C., and to return the cooled fluid for reheating and recycling. The heat transferred to the oil shale gradually matures the kerogen to oil and gas as the temperature in the shale is brought up, and also promotes permeability within the shale in the form of small fractures sufficient to allow the shale to flow into the well fractures where the product is collected commingled with the heating fluid and separated out before the heating fluid is recycled.