A method for in situ heating of an organic-rich rock formation is provided. Preferably the organic-rich rock formation comprises kerogen. The method may include the steps of providing a first wellbore extending at least to a depth of the organic-rich rock formation, and providing a second wellbore also extending to a depth of the organic-rich rock formation and intersecting the first wellbore. The method may also include injecting air and a combustible fuel into the first wellbore, and providing a downhole burner in the wellbore so as to cause the air and the combustible fuel to mix and to combust at substantially the depth of the organic-rich rock formation. The method may further include, circulating combustion products into and up the second wellbore such that a pyrolysis zone is created from the first wellbore and second wellbores that provides substantially complete pyrolysis of the organic-rich rock formation between the first wellbore and the second wellbore. Operating conditions may be set or controlled so that the pyrolysis zones surrounding the first and second wellbore mate so to minimize underheated and overheated regions.