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Reactive Transport Modeling Approach to Studying Silicification of Carbonates

Abstract

Reactive transport models provide a systematic approach to evaluate diagenetic processes in terms of fluid flow, heat transport and reactions along flow paths, in complex geologic systems. We present a case study where reactive transport modeling was used to investigate the silicification of a carbonate buildup. Our modeling workflow consists of three steps of progressively increasing complexity: single cell “batch” experiments were used to evaluate the diagenetic potential of several combinations of fluids and lithologies; then, the most favorable fluids were applied in column experiments to test the fundamental controls on the process of calcite replacement by silica; finally, the spatial and temporal distribution of silicification of a carbonate build-up was simulated under four different flow regimes, in cross-sectional models. Our findings show that conditions that favor replacement of carbonate by silica include: 1) the interaction of fluids and volcanic rocks to produce silica-rich fluids; 2) high concentrations of CO2 to drive calcite dissolution; 3) an elevated geothermal gradient to transport fluids and heat; 4) rapid flow, for example through faults; 5) cooling, for example by entrained surface water; and 6) the occurrence of silica in the original sediments. We found that massive and pervasive silicification of a carbonate formation is difficult because the common parameter space for calcite dissolution and silica precipitation to occur is very narrow, which explains the relative paucity of large-scale carbonate silicification in natural settings.