Passive and Active Feedback Between Diagenesis and Rock Deformation

Peter Eichhubl
Department of Geological and Environmental Sciences, Stanford University, Stanford CA 94305-2115 [email protected]

Chemical mineral alteration under diagenetic conditions commonly takes place by dissolution/ precipitation reactions in an aqueous pore fluid. These reactions are accompanied by diffusive or advective mass transport from sites of dissolution to sites of precipitation. Structural discontinuities such as joints and faults frequently act as preferred pathways for fluid flow and thus for mass and heat transport. Local anomalies in temperature and fluid composition result in chemical disequilibrium and enhanced chemical alteration. Such structurally controlled or enhanced diagenetic reactions may be far from thermodynamic equilibrium, are therefore kinetically favored, and occur at shallower depth than similar reactions associated with regional burial diagenesis. Structurally controlled chemical alteration may enhance fluid flow through widening of pathways by dissolution or restrict flow and further mass transport by cement precipitation. In addition, diagenetic reactions may change the petrophysical properties, making the rock more or less conducive to fracture. This positive or negative feedback between chemical change and deformation lacks a direct cause-effect relationship and is therefore considered passive.

The active feedback between chemical mineral reactions and deformation involves a direct functional relationship between chemical potential and stress or pressure. This relationship is associated with pressure solution, sintering, and the uptake or release of pore fluid by hydration or dehydration reactions. In pressure solution, normal stress enhances dissolution, thus accommodating a contractile strain. Sintering results from the thermodynamic tendency of grain boundaries to minimize surface free energy, resulting in the reduction in pore volume and thus in contraction. If contraction is constrained a tensile sintering stress can build up and lead to fracture. Under diagenetic conditions, sintering will be most effective in fine-grained rock under chemically reactive conditions. Dehydration reactions can result in an increase in pore fluid pressure and thus in a decrease in effective stress that equally favors brittle failure.

Such active and passive feedback between chemical alteration and rock fracture implies that the prediction of reservoir flow and petrophysical properties requires an integrated structural and diagenetic approach.

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