It is worth noting that ‘oil shale’ is neither ‘oil’ nor ‘shale’ and should in act be called ‘kerogen saturated marl’.
Most oil shale is a fine-grain sedimentary rock containing significant amounts of an organic bituminous material known as kerogen and can be a precursor to conventional oil. Oil shale composition varies from location to location due to the wide range of environments that oil shale has formed. Essentially oil shale is immature petroleum, usually located at shallower depths than conventional oil.
The U. S. Geological Survey (USGS) defines oil shale as ‘organic-rich shale that yields substantial quantities of oil by conventional methods of destructive distillation of the contained organic matter, which employ low confining pressures in a closed retort system.’ (Retorting involves heating in the absence of oxygen at temperatures of up to 500oC.) Furthermore the USGS defines oil shale as ‘any part of an organic-rich shale deposit that yields at least 10 gallons (3.8%) of oil per ton of shale’.
The groups of oil shales have been identified:
Three types of components of oil shale known as ‘macerals’ have been identified: