Computer simulation evidence for enthalpy driven dehydration of smectite clays at elevated pressures and temperatures

Monte Carlo molecular modelling studies of smectite–water interactions at elevated pressures and temperatures are reported. These simulations are the first to mimic conditions encountered by clays in real sedimentary basins, down to a burial depth of 9 km. To achieve this, geostatic and geothermal gradients of 1·5 x 107Pa km-1 and 30°C km-1 respectively are applied. The density of interlayer pore water exceeds that of the bulk, except at very low water content. Hydrated smectite is therefore enthalpically stable at burial depths of up to 1·5 km. Below this depth the remaining fluid prefers the bulk phase. This prediction is consistent with the observation that diagenetic release of interlayer water, and consequent conversion of hydrated smectite to anhydrous illite, occurs at burial depths greater than 1·5 km.