Total organic carbon
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Total organic carbon (TOC) is a measure of the dry weight per cent of organic carbon within hydrocarbon source rocks. Hydrocarbons, including natural gas (principally methane, ethane and propane), may be generated by the heating of organic carbon through burial over geological time.
A shale with low concentrations of organic carbon (typically below about two per cent) probably won’t have the capacity to produce oil or gas in useful quantities. It should also be noted that a high TOC value is not necessarily an indicator of shale gas potential. The gas prospectivity of a rock is heavily influenced by the kerogen types within the organic matter, including that derived from plankton, algae, spores, pollen or plant fragments. These can affect gas production, rock porosity and the rates at which gas may be released.