An open system pyrolysis of a first hydrocarbon source rock sample obtained from a natural system is performed within a pyrolysis chamber by maintaining the pyrolysis chamber at a substantially constant temperature. Hydrocarbons are recovered from the pyrolysis chamber released by the first hydrocarbon source rock sample. A thermo-vaporization is performed within the pyrolysis chamber on the pyrolyzed sample at a substantially constant temperature. A first hydrocarbon expulsion efficiency of hydrocarbon source rock is determined. A second hydrocarbon rock sample is ground to a grain size less than or equal to or less than 250 micrometers. A second pyrolysis is performed on the ground hydrocarbon source rock sample by maintaining the chamber at a substantially constant temperature. A second hydrocarbon expulsion efficiency of the hydrocarbon source rock in the natural system is determined. The first hydrocarbon expulsion efficiency is verified using the second hydrocarbon expulsion efficiency.