A pyrolysis-gas chromatography system has been developed for the rapid evaluation of potential source rocks. For the determination of organic richness and maturation, this system uses the pyrolysis methods previously described. However, the determination of hydrocarbon production type and the identification of contamination (by migrated hydrocarbons or drilling additives) are accomplished by gas chromatographic (GC) analysis of the thermal extracts and pyrolysis products of rock samples or isolated kerogens. The production type is recognized either qualitatively by GC fingerprint traces or quantitatively by hydrocarbon composition (C₁ to C₆, C₆ to C₁₁, C₁₁⁺) from the kerogen (Peak II) pyrolysate. Oil-prone erogens are recognized by GC traces with a full spectrum of C₁ to C₂₈ hydrocarbons, or by high concentrations of C₁₁⁺ compounds. In contrast, gas-prone kerogens are characterized by the predominance of light hydrocarbons from C₁ to C₄ and higher contributions of aromatic compounds. Mixed-type production is intermediate in character between the two. Contaminants are identified from the GC analysis of the thermally extractable material in Peak I. Possible mineral-organic matter reactions during sample heating make interpreting data from whole-rock samples more difficult.