Generation of hydrocarbons in source rocks is controlled by their temperature history with burial, as well as by quantity, type and structure of their kerogens. The degree to which hydrocarbon generation processes have advanced in a given source rock can be recognized on the basis of elemental analysis of kerogens (H/C-O/C diagrams), as well as by analysis of yield and composition of extractable hydrocarbons. The “liquid window” or main phase of petroleum generation corresponds to a subsurface temperature interval of 60–150 °C. The overall chemical evolution of kerogens and their reaction products in this interval are exemplified here for several source rock series on the basis of organic carbon mass balance calculations. In this way the changes of the ratio of “reactive carbon” to “dead carbon” with maturity increase become obvious.