The generation of oil is a process that begins to occur at some point during the burial history of a source rock. This "onset of maturation" is dictated largely by temperature and residence time. However, the nature of the source rock itself also influences the hydrocarbon product being expelled from the source rock. The vast majority of the world's oil can be ascribed to source rocks of the following types. (1) Marine mudrocks deposited in anoxic conditions and dominated by phytoplankton organisms; this type of source rock can have a carbonate or clay inorganic matrix and total organic carbon values from 1 to 30% (commonly 4 to 10% when immature). Examples of this classical oil source rock would be the source rocks of western Canada, the Middle East, and the North Sea. ( ) Specific coal facies such as torbanites and cannel coals, which contain a mixture of hydrogen-rich plant detritus (e.g., spores, pollen, cuticle, resin, and algae); deposition was probably in open-water areas of an overall coal-swamp environment. Examples of hydrocarbons from this type of source include the Gippsland basin, Canadian Beaufort Sea, and Southeast Asia. (3) Lacustrine organic-rich deposits, rich in freshwater algae, which ultimately result in high-wax crude oils. Examples are relatively rare, but include major source rocks in the Uinta basin and China.

The effect of increasing maturity on marine mudrocks of the Devonian Duvernay Formation of Alberta illustrates oil generation from this type of source rock. The data base in this unit consists of 40 conventional cores, ranging from immature to completely overmature, and 80 oils from separate accumulations sourced from the Duvernay. An illustration of oil generation in a coaly source rock is provided by a single core from the Lower Cretaceous of the Beaufort-Mackenzie basin plus many of the oils and condensates reservoired in that area.

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