Summary and implications
Recognition that water and minerals are reactive sources of hydrogen and oxygen has profound implications for models used to predict the generation of organic alteration products in sedimentary basins. The long-held assumption that hydrogen and oxygen in petroleum are derived solely from kerogen, in conjunction with compositional variations in kerogen as a function of increasing thermal maturity, has been used to constrain the composition, amount and timing of petroleum generation1,2,68,69,81. Examination of Fig. 1b reveals that traditional models predict generation of carbon dioxide and organic acids before mainstage petroleum generation, because the early stages of kerogen maturation result in a substantial decrease in kerogen–oxygen content (Fig. 1a). Similarly, the area under the methanegeneration curve in Figure 1b is limited by the hydrogen content of the initial kerogen. Derivation of hydrogen and oxygen from water and minerals eliminates these constraints and suggests that quantities of oxygen- and hydrogen-bearing alteration products may be substantially greater (Fig. 1c). Moreover, generation of carbon dioxide and organic acids may occur before, during and after mainstage petroleum generation, consistent with the occurrence of these species at more than 160 °C (Fig. 3). Inclusion of water-derived hydrogen in models for the prediction of hydrocarbon generation will affect the estimated quantities of natural gas to a greater extent than similar estimates for oil. Although water-derived hydrogen may play a central part in the reactions responsible for the primary generation of oil, the net hydrogen demand during cleavage of hydrogen-saturated kerogen fragments that constitute oil is relatively minor. By contrast, conversion of longchain saturated hydrocarbons, with hydrogen:carbon ratios approaching two, to methane with a hydrogen:carbon ratio of four requires the addition of substantial amounts of hydrogen. Because natural-gas