Organic–inorganic rock–fluid interactions in stylolitic micro‐environments of carbonate rocks: a FIB‐TEM study combined with a hydrogeochemical modelling approach

Abstract

AbstractStylolites and the interfaces to the host limestone have been investigated by means of a multidisciplinary analytical approach (thin section microscopy, FIB‐TEM, organic geochemistry and petrography). Carbonate dissolution assuming different boundary conditions was simulated by applying a generic hydrogeochemical modelling approach. It is the conceptual approach to characterize and quantify traceable organic–inorganic interactions in stylolites dependent on organic matter type and its thermal maturity, and to follow stylolite formation in carbonates as result of organic matter reactivity rather than pressure solution as a main control. The investigated stylolite samples are of Upper Permian (Lopingian, Zechstein), Middle Triassic (Muschelkalk) and Late Cretaceous (Maastrichtian) age and always contain marine organic matter. The thermal maturity of the organic matter ranges from the pre‐oil generation zone (0.4–0.5% Rr) to the stage of dry gas generation (>1.3% Rr). The results of the generic hydrogeochemical modelling indicate a sharp increase of calcite dissolution and the beginning of stylolite formation at approximately 40°C, which is equivalent to a depth of less than 800 m under hydrostatic conditions considering a geothermal gradient of 30°C and a surface mean temperature of 20°C. This temperature corresponds to the pre‐oil window when kerogens release an aqueous fluid enriched in carbon dioxide and organic acids. This aqueous fluid may change the existing pore water pH or alkalinity and causes dissolution of carbonate, feldspar and quartz, and clay mineral precipitation along the stylolite. Dissolution of limestone and dolostone leads to reprecipitation of calcite or dolomite opposite of the dissolution side, which indicates only localized mass redistribution. All these integrated hydrogeochemical processes are coupled to the generation of water during organic matter maturation. In all of the calculated hydrogeochemical scenarios, H2O is a reaction product and its formation supports the suggested hypothesis.