ABSTRACT
This paper describes a numerical model for the calculation of the rates of minerals that precipitate or dissolve in basin strata as groundwaters migrate along temperature and pressure gradients. The calculation incorporates chemical reaction and mass balance equations into a transient flow model that predicts flow velocities, flow directions, and temperature and pressure distribution along flow path. The model integrates predicted groundwater flow patterns with geochemical reaction path modeling; this approach allows us to predict the rates of precipitation and dissolution in geochemical systems open to mass and heat transfer. In steady-state runs, the model calculates the instantaneous rates of precipitation and dissolution; in transient runs, the model can trace the volume of cements that precipitate in basin strata from migrating groundwater as well as the volume of minerals that dissolve. The program then uses the net volume of precipitation and dissolution to modify porosity. Such an approach accounts for the effect of precipitation and dissolution on sediment porosity, which in rum affects permeability and fluid flow. The program is integrated with the stochastic model to evaluate how the spatial variability in permeability affects the pattern of diagenetic reactions. The model is used to predict chemical reactions during several hydrologic processes, including (1) diagenesis of quartz and calcite by flow through a wavy sandstone, (2) cementation of amorphous silica and its feedback effect on thermal convection, and (3) regional diagenesis by migrating brines in 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 nature, the model places important constraints on the origin of diagenesis.
