Kinetic constraints on illitization reactions and the effects of organic diagenesis in sandstone/shale sequences

Based on water-rock interaction modelling and kinetic considerations, the present study is aimed at testing the impact of organic matter maturation on two kinds of diagenetic reactions leading to the alteration of the petrophysical properties of sandstones: (1) Clay mineral conversion to illite with K-feldspar being a local potential source of K; and (2) Dissolution-precipitation processes produced in sandstones by the action of water-soluble organic species derived from adjacent shales.Assuming firstly that the chemical reactions take place in closed systems, the nature and timing of diagenetic reactions in marine sandstone/shale formations were modelled for a 50–120°C temperature range in order to improve understanding of the factors that control the illitization reaction with K-feldspar coexisting with aluminous clay. Illite is modelled here as a muscovite type mineral. We tested the effects of an energy barrier on illite growth by allowing or preventing the muscovite/illite precipitation reaction to occur, while using several illitization reaction rates. We also compared the stable mineral parageneses predicted for organic material-free systems with those predicted in the case of organic diagenesis (release of CO₂, CH₄, acetic and oxalic acids). Similarities and discrepancies between numerical results and natural mineral assemblages suggest that the illitization reaction depends on the nature of the reacting clays. Kaolinite conversion to end-member illite involves high-energy conditions (> 2 kcal mol⁻¹ ), which are not met when the pore water equilibrates with the mineral matrix from undersaturated conditions in a closed system. To overcome this barrier, the fluid should be oversaturated with respect to K-feldspar. An external source of K or a pH increase in an open system is necessary for this reaction. No particular effect of organic diagenesis on this reaction was found in the present study. On the other hand, smectite-to-illite conversion involves a lower energy barrier and can operate in closed systems where K-feldspars are the source of K. The maturation of organic matter may speed up the smectite-to-illite conversion rate by increasing the Gibbs Free Energy of illite growth.Interactions between sandstone and shale were examined by modelling the transfer of aqueous species of organic origin from shale to sandstone. Fluid expulsion from shale had little or no effect on diagenesis in adjacent sandstones during each flushing cycle, mainly because organic protons and ligands were neutralized in the source rock. However, the diffusion of shale-derived cations through sandstone as organometallic complexes (Mg > Ca > > Fe > > Al) appears to be an efficient process during carbonate cementation in sandstones, where oxidizing conditions enhance the decomposition of such complexes.