MODELING POROSITY EVOLUTION AND CEMENTATION OF SANDSTONES

The porosity evolution of sandstones is modeled when the source for ce ment is dissolution of quartz at stylolites. The model is based on lab oratory measured kinetics of the Arrhenius type, and a 'Poisson'-model for the porous medium, which yields the specific surface and tortuosi ty as Functions of the porosity. Closed form expressions for the poros ity as a function of time and temperature is found in both the isother mal case and the case of constant burial. These simple expressions are valid when the precipitation step is slower than the diffusive supply of silica from the stylolites. The parameters in the expressions for the porosity evolution are few, and the most uncertain one involves th e product of the specific surface, the Arrhenius-prefactor and the sup ersaturation of silica. The porosity evolution is also studied when di ffusion of dissolved silica away from the stylolites is slower than th e precipitation step. This cementation regime, characteristic for high temperatures or a large stylolite separation, is shown to produce a t hin, almost completely cemented boundary layer, of nearly zero porosit y, close to the stylolites. (C) 1998 Elsevier Science Ltd. All rights reserved.