Oil shale is a fine-grained sedimentary rock containing a solid organic material called kerogen. The conventional measure of the quality of oil shale is oil yield as determined by the Fischer assay procedure. With this technique a sample of oil shale is heated in a standardized way to 500degreesC and the hydrocarbons released as vapor are cooled and condensed. The recovered oil is measured and the yield of oil is expressed as gallons of oil per ton of oil shale feed or as weight percent oil recovered from the rock. In the assay procedure, the oil is released from the shale by a thermal cracking reaction. This reaction also produces coke and uncondensed light gases consisting principally of H2, CO, Cl through C4 hydrocarbons H2S and CO2. The coke is produced as a carbonaceous deposit on the retorted shale.