Hydrocarbon recovery from impermeable oil shales

An in situ method for maturing and producing oil and gas from a deep-lying, impermeable formation containing immobile hydrocarbons, comprising the steps of:

• (a) pressure fracturing a region of the hydrocarbon formation, creating a plurality of substantially vertical, propped fractures;
• (b) injecting under pressure a heated fluid into a first part of each vertical fracture, and recovering the injected fluid from a second part of each fracture for reheating and recirculation, said pressure being less than the fracture opening pressure, said injected fluid being heated sufficiently that the fluid temperature upon entering each fracture is at least 260° C. but not more than 370° C., the injected fluid having a volumetric thermal density of at least 30,000 kJ/m3 as calculated by the difference between the mass enthalpy at the fracture entry temperature and at 270° C. and multiplying by the mass density at the fracture entry temperature, and the separation between said first and second parts of each fracture being less than or approximately equal to 200 meters;
• (c) recovering, commingled with the injected fluid, oil and gas matured in the region of the hydrocarbon formation due to heating of the region by the injected fluid, the permeability of the formation being increased by such heating thereby allowing flow of the oil and gas into the fractures;
• and
• (d) separating the produced oil and gas from the recovered injection fluid.