One of the most important problems in petroleum exploration is the capability to predict the type, amount, and location of porosity in potential reservoir rocks ahead of the drill bit. This paper discusses the concepts needed to acquire such information from computerized chemical models, in order to calculate the type and the amount of diagenetic minerals, and hence the porosity modification. The location of porosity may also be predicted when physical mechanisms of mass transfer, such as fluid flow and diffusion, are appropriately coupled with the chemical reactions.

This paper attempts to classify available chemical models into three broad categories--speciation-solubility, reaction-path, and reaction-transport models--to provide a general background for the models referred to in this volume. A brief discussion of theory, inputs and outputs, names of a few (but not all) available codes, and their limitations and applications is presented.

The papers in this volume affirm the increasing reliability of predictions made by using geochemical models. However, the reliability of predictions can be improved if we continue correcting the inaccuracies in thermodynamic and kinetic data and increasing our understanding of mass-transfer mechanisms. In spite of the limitations discussed in this paper, the concepts of chemical modeling based on reversible and irreversible thermodynamics, equilibrium and disequilibrium, and open vs. closed systems, remain of great value in understanding natural rock-water systems. Other advantages of diagenetic simulations using geochemical modeling are: (1) they elucidate the chemical processes responsible for diagenesis, (2) they are much faster than empirical predictive models, which are based on umerous labor-intensive petrographic observations, and (3) they provide for many possible scenarios.

Chemical models are not only appropriate for predicting subsurface porosity but also can be applied toward the solution of problems involving environmental geochemistry and nuclear waste disposal.