Predicting Depths of Gypsum Dehydration in Evaporitic Sedimentary Basins

Abstract

The conversion of gypsum to anhydrite upon burial and heating is accompanied by about a 39% volume decrease, a four-fold increase in thermal conductivity, and consumption of heat. Accurate back-stripping of an evaporite basin and tracking its thermal evolution requires a knowledge of the depth of the gypsum-anhydrite transition. This depth depends on temperature, fluid pressure, lithostatic pressure, and activity of water in the pore fluid. Using a finite-difference heat-conduction program, we have determined the depths of transition for gypsum in evolving basins beneath commonly associated sediments and in several tectonic environments represented in the program by different basal flows and sedimentation rates. The activity of water is kept at 0.93 (precipitation f gypsum from seawater). All physical properties are recalculated at each time step as temperature increases and porosity decreases with burial. (An algorithm for calculating thermal conductivities using parallel-series mixtures is presented.) The modeling results show that overlying lithologies and the tectonic environment are two important factors. Gypsum converts to anhydrite at shallow depths (^sim400 m) when overlain by poor conductors like shale or gypsum in a rift environment, and at great depths (hypothetically >4 km) when overlain by good conductors like salt in a stable cratonic region. Sedimentation rate and the transient heat sink effect of the endothermic reaction have little effect on transition depth.