The concentration of organic material in source rocks as represented by the weight percent of organic carbon. A value of approximately 0. 5% total organic carbon by weight percent is considered the minimum for an effective source rock, although values of 2% are considered the minimum for shale gas reservoirs; values exceeding 10% exist, although some geoscientists assert that high total organic carbon values indicate the possibility of kerogen filling pore space rather than other forms of hydrocarbons. Total organic carbon is measured from 1-g samples of pulverised rock that are combusted and converted to CO or CO₂. If a sample appears to contain sufficient total organic carbon to generate hydrocarbons, it may be subjected to pyrolysis.