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The hydrogen index is defined as the amount of the S2 pyrolysis peak (in mg hydrocarbons) normalized to grams of total organic carbon (TOC) obtained from the usual Rock-Eval procedure. This value has been used to determine the type and, in part, the maturity of the organic matter in source rocks. When the same S2 peak (in mg hydrocarbons) is normalized to the grams of reactive organic carbon (ROC) actually consumed in the pyrolysis (measured by difference), one should expect a reasonably constant value between 1,110 and 1,330 mg S2/g ROC if all of the products of pyrolysis are hydrocarbons. However, this is not the case. The values of S2/ROC range from 700 to 1,400 mg S2/g ROC for samples having hydrogen index values above 200 mg HC/g TOC. Below a hydrogen index of approx mately 200 mg HC/g TOC, the value of S2/ROC decreases sharply. The low values of S2/ROC mean that FID-nondetectable carbon is being released probably as CO and CO2. The sharp decrease of this value in samples with lower hydrogen indices indicates that a large proportion of the reactive carbon is transformed to the oxidized species. This is true not only for type III kerogens, but also for the more mature type II kerogens with lower hydrogen indices. The amounts of CO and CO2 can be estimated from a plot of S2/ROC versus S2/TOC. The calculated amounts of CO and CO2 produced exceed that of CH4 in source rocks having a hydrogen index of 100 mg HC/g TOC. The effects of this phenomenon are significant both with respect to natural gas compositions an with respect to the role of CO2 as a possible medium for hydrocarbon migration.
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