Source rocks of petroleum are characterized by generation of hydrocarbons and the migration of these hydrocarbons out of the rock. Usually the process of generation is incomplete at depths of interest to the petroleum industry so that, when shale samples are heated in the laboratory, they first release the hydrocarbons which already have been generated in the subsurface and then at higher temperatures produce more hydrocarbons by the thermal breakdown of kerogen. These processes can be monitored by heating the samples in a stream of helium which sweeps the evolved hydrocarbons into a flame-ionization detector. For a heating rate of 9°C/minute the geologically generated hydrocarbons give a peak at approximately 130°C, whereas the hydrocarbons produced by pyrolysi give a second peak at approximately 130°C, whereas the hydrocarbons produced by pyrolysis give a second peak at approximately 480°C. In general, as sample depth increases, the relative size of the low-temperature peak increases and the temperature of the peak maximum for the second peak moves to higher temperatures.

If some of the original organic matter in a shale were isolated by the surrounding clay particles (in "microreservoirs"), the hydrocarbons generated from it would not have been able to migrate. Any migration out of the rock must have been from the connecting pores. Thus, migration would lead to a difference in composition between the hydrocarbons in the microreservoirs and in the pore spaces. A programmed-temperature pyrolysis method has been developed for determining the composition of the hydrocarbons in the microreservoirs and pore spaces. This was used to study a stratigraphic section which included a presumed source bed. In the nonsource rocks the composition of the hydrocarbons from the microreservoirs and pore spaces were very similar, but appreciable compositional differences ere found for the pores and microreservoirs of the source rock.