Fluid-Rock Constraints on Porosity Models for Diagenesis of Siliciclastic Rocks
HUTCHEON, IAN, and MAURICE SHEVALIER, University of Calgary, Calgary, Alberta, Canada, and HUGH J. ABERCROMBIE, Geological Survey of Canada, Calgary, Alberta, Canada
Fluid phase buffering was described by Greenwood (1975) who showed that undetectable amounts of reacting minerals can buffer the composition of a coexisting fluid phase. During diagenesis, buffering of pH must be understood to predict porosity by mineral dissolution and precipitation reactions. Potential buffers of pH in pore waters are carbonates, silicates, and aqueous organic species. Water-rock interactions in sediments suggest that silicate mineral assemblages may buffer fluid compositions. Particular mineral reactions can be identified by comparing theoretical phase stabilities to measured aqueous activities. This process indicates that although the entire rock-water system may not come to equilibrium, metastable silicate reactions equilibrate in geologically short times.
The buffer index (Beta = -dC(a)/dpH), where C(a) is the concentration of strong acid, is a measure of the effectiveness of pH buffers. Changes in pH affect the distribution of aqueous species and, to accurately estimate beta, the aqueous distribution must be recalculated at each step. The reaction path/solubility speciation code EQ3/6 (Wolery, 1983) was modified to evaluate the buffer potential of carbonates, silicates, and organic species. Hypothetical solutions containing varying amounts of aqueous carbonate and organic species were allowed to equilibrate with the identified silicate mineral assemblages as HCl was added. Silicates, such as albite-smectite and kaolinite-illite, have the highest beta's over the normal pH range. Local, rock-dominated processes thus are interpreted to b more important to dissolution than the import of acids from external sources. A forward model for the dissolution and precipitation of minerals in a progressive burial sequence can be used to estimate changes in volume, and therefore porosity, due to chemical reactions. Physical compaction was not considered in the calculation.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)