Effects of inorganic minerals in source rocks on hydrocarbon generation from organic matter

The effects of inorganic minerals in source rocks on gas generation and isotopic fractionation of organic matter were studied by hydrous pyrolysis using two sets of whole rocks and their kerogens, which were separated from the low maturity shale of the Yanchang Formation in the Ordos Basin and the Xiamaling Formation in Huayuan area of North China. The liquid and gaseous hydrocarbon yields indicated that hydrous pyrolysis with whole source rocks were lower than those in pyrolysis of kerogen, but the amounts of CO₂ and H₂ in the former experiments were much higher, and the i-C₄/n-C₄ ratios in the whole rock hydrous pyrolysis were also lower. The results indicated that the presence of inorganic minerals in source rocks inhibited the interaction between water and organic matter, and the reaction mechanism between them also changed. Stable isotopic analyses revealed that little difference exists between the carbon isotopes of methane generated from two different types of hydrous pyrolysis, whereas the isotopic values of CO₂ were much more enriched due to the inorganic minerals present in source rocks, and the hydrogen isotopic values of gaseous hydrocarbons decrease significantly. These results demonstrated that inorganic minerals in source rocks have participated in and affected the isotopic compositions of hydrocarbon gases, and there exists indirect hydrogenation controlled by the presence of hydrogen.