In situ retorting and extraction methods for oil shale and heavy oil formations require large amounts of heat. Oil shale formations require heat to convert kerogen into oil, and heavy oil formations require heat to lower oil viscosity. We present a method by which the amount of heat required to raise the temperature of a hydrocarbon- and water-saturated" rock formation is calculated from downhole geochemical data typically provided by a nuclear spectroscopy log. The heat capacity of various inorganic minerals, organic materials, and water, along with enthalpies from water evaporation, mineral decompositions, and pyrolysis, are used in the calculation. The heat requirements, expressed in units of kilojoule per kilogram, estimated from downhole geochemical data are found to agree with those calculated from laboratory-determined mineralogy within 1% at 300 degrees C for most formations. Mineral composition sensitivity and error propagation were investigated. These results suggest that heat requirements for oil shale and heavy oil retorting and extraction can be determined from downhole measurements, obviating the need to acquire and analyze core samples in the laboratory in most cases.