Enhanced carbon dioxide release and other possible effects of clay minerals during thermal evaluation of sediments

Thermal evaluation of potential source rocks (PSR) include measurements of CO₂ evolved (>400°C) designated as IO (Oxygen Index). For whole rock analysis this index frequently exceeds the value derived from organically bonded oxygen. The surplus of CO₂ stems mainly from the reaction of carbonates with active acidic volatiles released from the associated clays. The magnitude of this effect is specific for every clay mineral, but for any particular mineral assemblage it correlates directly with the clay/carbonate ratio. The largest discrepancy between IO values and organically bonded oxygen occurs when O.M. is low (ca. 0.5-1%), clays dominate and sufficient carbonates are present to react. Clay minerals and kerogen probably do not react, as such. However, the active volatiles released upon heating attack even before depolymerization. The liquid monomers produced then can react with both clay minerals and their volatiles. These interactions could result in changes of the IH factor (Hydrogen Index), as well as the distribution of products.