Reservoir Quality Prediction Through Simulation of Compaction and Quartz Cementation

R. H. Lander, O. Walderhaug, A. Lyon, A. Andersen

We have developed a forward numerical model that simulates the effects of compaction and quartz cementation through time on rock porosity and composition. Model predictions are consistent with measured porosity, intergranular volume (IGV), and quartz Cement fractions for samples from the Jurassic of the Norwegian Shelf, the Tertiary of the Gulf of Mexico, and the Ordovician of the Illinois basin.

Quartz cementation is modeled as a precipitation rate controlled process where quartz cement is derived from dissolution at nearby stylolites and grain contacts according to the method of Walderhaug (1994). Dissolved silica then diffuses short distances from sites of dissolution to sites of precipitation. Within the model frame of reference (based on a 1 cm³ of material upon deposition) no silica dissolution is modeled. Quartz surface area available for precipitation of quartz cement is a function of the proportion and size of detrital quartz grains, the fraction of clay coatings, and the extent of cementation.

Compaction is modeled by an exponential decrease in IGV as a function of effective stress. The formulation includes a term for defining a minimum possible IGV. This compaction model is consistent with compaction arising from grain rearrangement, ductile grain. deformation, and brittle failure of grains. In addition, the model considers the effect of fluid overpressures.

The model is better able to predict porosity for quartz cemented sandstones that occur within basins characterized by large fluid overpressures than models dependent on long-range transport of silica in solution. Models based on long range silica transport require widespread fluid fluxes greatly in excess of that indicated by transient flow simulations. Furthermore our model is consistent with the occurrence of high IGV at depth (eg., Paxton and others, 1990) unlike models that depend on intergranular quartz dissolutions a source of quartz cement. In addition the model accurately reproduce the experimental results of Pittman and Larese (1991) for compaction of lithic grain rich samples.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995