The role of diagenesis in the formation of fluid overpressures in clastic rocks

Abstract

We have developed a model of fluid flow and pressure development in sedimentary basins that incorporates pore volume loss due to mechanical compaction and to chemical diagenesis (quartz cementation, grain contact quartz dissolution and illitization). Mechanical compaction is modeled to be a function of effective stress. In this model, pore volume loss due to mechanical compaction will be retarded when overpressure develops. The diagenetic processes are modeled as being kinetically controlled and the reaction progress depends only on the temperature history. Hence pore volume loss due to chemical compaction is not retarded by overpressure. By including diagenetic effects on overpressure development, the pressure model should be more generally applicable than models that consider mechanical compaction to be the sole process that reduces porosity. To demonstrate the potential importance of chemical compaction in the formation of fluid overpressures in different settings, we calibrated our model with data obtained from the Halten Terrace offshore mid-Norway and from the Gulf of Mexico. In both cases, the diagenetic processes have the potential to control on the timing and magnitude of overpressuring. From 25% and up to 80% of the present-day overpressure may be caused by pore volume loss resulting from diagenetic reactions. Pressure build-up from diagenetic processes also potentially controls the timing of hydraulic fracturing. If diagenetic processes are actively contributing to overpressure generation, then unrealistically low shale permeabilities are not needed to retain overpressures for geologic time periods (>10 My).