Process for in situ recovery of carbonaceous values from underground petroleum impregnated media such as oil shale, under controlled radiation from a microwave distributing source adjacent the media, e.g. in a borehole, effected in the absence of air, such that the radiation is distributed at least initially at incrementally increasing power and/or at least initially in intermittent interval cycles of on and off duration of the microwaves, for selective pyrolysis of the organic content of the media to liquid, vapor and gas form as the case may be under autogenous pressure in the pores of the media for driving the organic constituents therefrom for appropriate recovery, e.g. via the borehole, including breakup of larger molecules, e.g. hydrocarbons, for selective increase in the noncondensible gas quantities in proportion to the liquid and/or condensible oil vapor quantities, plus pyrolysis scavenging of residual carbon coke by gasification thereof to noncondensible gas constituents, optionally using a portion of the recovered noncondensible gases to produce electrical energy for energizing the microwave source, and
conjoint probe apparatus having an adjustable extendable probe end for embedding in the porous media for in situ sensing of changes in the dielectric constant of the carbonaceous constituents undergoing microwave pyrolysis, and a mechanism for indicating the sensed changes, for adjusting the radiation in dependence upon such changes, the degree of extension of the probe end being adjustable in dependence upon the sensed frequency of the attendant radiation.