TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Process: While controlling heat, the temperature within selected section is maintained within a pyrolysis temperature range and the pressure is controlled to at least 2 bar.The heating is controlled such that thermal conductivity of at least a portion of selected section is greater than 0.5 W/(m degreesC).The heating energy per day (Pwr), is calculated by the relation Pwr = h x V x Cv x rhoB, where V is the selected volume of hydrocarbon containing formation, Cv is the average heat capacity of the formation, h is the average heating rate of the formation (10 degreesC per day), and rhoB is the formation bulk density.    TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Apparatus: The heat sources comprises an electric heaters (803), surface burners, flameless distributed combustor or natural distributed combustor.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Composition: The mixture produced after treatment of hydrocarbon containing formation, contains condensable hydrocarbon (CH) having an American Petroleum Institute (API) gravity of at least 25 degrees, and non-CH each comprising 0.1-15 weight% (wt.%) of olefins. The non-CH contains ethene and ethane in molar ratio of 0.001-0.15. CH contains 5-30 wt.% of oxygen containing compounds such as phenol, less than 1 wt.% of sulfur compound, greater than 20 wt.% of aromatic compounds, less than 5 wt.% of multi-ring aromatics with more than 2 rings, less than 0.3 wt.% of asphaltenes and 5-30 wt.% of cycloalkanes. The non-condensable component contains 10-80 volume% of hydrogen. The mixture further contains 0.05 wt.% of ammonia, hydrogen, hydrogen sulfide and carbon dioxide. A portion of CH is hydrogenated by the produced hydrogen. EXAMPLE - (In weight%) A hydrocarbon containing formation such as tar comprising paraffin (7.08), cycloalkanes (29.15), monoaromatics (6.73), diaromatics (8.12), triaromatics (1.7) and tetraaromatics (0.02) was subjected to in situ thermal processing at a pressure of 28.6 bar. The resulting produced fluid mixture contained paraffin (26.45), cycloalkane (36.56), phenol (0.47), monoaromatics (28) and diaromatics (8.52), indicating increase in weight% of paraffin, cycloalkane, monoaromatics in produced mixture by in situ thermal processing.