A pyrolysis technique that enables the hydrocarbon potential of a source rock, to be determined. It involves two heating steps, the first to volatilize hydrocarbons in the source rock (S1), and the second to pyrolyse the kerogen and convert the material to free hydrocarbons (S2). A separate third pyrolysis step generates trapped CO2 (S3). Maturation and source quality parameters are derived from the temperature at the top of the S2 peak, and the yields associated with each step, respectively.