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Abstract

A numerical model is developed to calculate the rates at which minerals precipitate or dissolve in basin strata as groundwaters migrate along temperature and pressure gradients. The calculation is based on the assumption that minerals maintain local equilibrium with migrating groundwater and their solubilities depend only on temperature or temperature and pressure. The model integrates predicted groundwater flow patterns with geochemical reaction path modeling; this approach allows us to predict the rate at which minerals dissolve and precipitate in complex geochemical systems open to groundwater flow and mass transfer. The model is formulated and solved in geologic time and basin distance scales and can therefore be applied to study basin-wide diagenesis related to long-distance fluid migration. The calculation can adjust sediment porosity from the net volume of precipitation and dissolution, therefore accounting for feedback effects of chemical diagenesis on porosity, which in turn affects permeability and fluid flow. The model is used to study the rates and nature of diagenetic alteration in several hydrologic systems, including (1) diagenesis of quartz by flow through a wavy sandstone, a sloping aquifer and a faulted aquifer, (2) cementation of amorphous silica and its feedback effect on thermal convection, (3) cementation of anhydrite in the Lyons Sandstone, Denver basin, and (4) diagenesis by migrating brines in the deep aquifers of the Illinois basin. The sample calculations shed light on the rates and patterns of chemical diagenesis that Likely accompany fluid migration in sedimentary basins. When the predicted results can be compared to diagenetic patterns observed in basin strata, the model provides an interpretation for the origin of diagenetic alteration.

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