Effects of Sand Cementation on Pore Pressure in Basin Models

Basin models have traditionally described sediment porosity loss with compaction models that are functions of a vertical effective stress (VES). These models are unrealistic in depths where porosity loss by cementation becomes important. Consequently, the predicted pore fluid pressure resulting from these models may also be unrealistic. The pore pressure estimate further affects calculated sediment porosities and thermal calculations.

To begin addressing these concerns, a series of 1D synthetic models and a 3D basin model were investigated. Sand porosities in these models were described with 1) a simplistic VES-dependent model, as well as 2) a temperature-dependent porosity model (VES-T; Schneider et al., 1996) available in the Shell basin modeling software. The VES-T porosity model was calibrated to petrographic data and published sand porosity models. The derived range of the parameters allowed a crude and fast approximation of the porosity in sands in a manner consistent with the physically more meaningful and accurate Touchstone™ software. Porosities in other lithologies were described with common VES-dependent models.

The VES-T porosity model was used to study porosity and overpressure evolution in shallow and deep sand layers in a 3D model of a major sedimentary basin. The porosity model for the shallow sand was calibrated to petrographic data. A range of sand lithologies was considered for the deep sand. The application of the VES-T model to the deep sand led to a (counterintuitive) pore pressure (and porosity) underprediction in the shallow sand. In order to preserve the pore pressure match with data, mudstone permeability had to be reduced throughout the model by a factor of 0.1 compared with the permeability in the original basin model, which is well within the range of uncertainty. The consequences of these changes include 1) significantly higher calculated porosities and pore pressures in the non-sand sections of the basin model and, 2) correctly predicted shallow sand porosities using parameters appropriate for given sand lithology. The modeled sand cementation in the 3D model contributed between ~±3 MPa or ~±8 % of the total overpressure.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California