The term "diagenesis" refers to the physical and chemical changes imparted upon sediments subjected to pressures and temperatures lower than those of the metamorphic realm. Physical aspects of diagenesis such as compaction have been well studied, and the manner in which they affect various transport properties such as permeability are largely understood. Conversely, the relationship between subsurface chemical reaction, induration and permeability change remains unclear. Chemical reactions in the subsurface are manifestations of a system's disequilibrium. In natural systems, the numerous physical and chemical factors involved, render difficult the formulation of a model describing permeability as a function of chemical reaction. However, in general, processes such as dissolution cause permeability and porosity to increase while precipitation of clays or other low temperature minerals usually have the opposite effect. The focus of this paper is on the latter effect, where precipitation in a granular aggregate causes clogging of the pore network and results in an associated permeability drop. In some cases, precipitation of these secondary minerals may be so intense that permeability reduces to the point that hydraulic seals form [see Hunt, 1990].

This chemically-induced permeability reduction or "precipitation sealing" may drastically alter fluid flow in the subsurface. The formation of low permeability lithologic units can hinder the migration of hydrocarbons and other aqueous fluids, potentially preventing or enhancing the formation of economically valuable deposits or slowing the transport of various environmental pollutants. From a rheological point of view, large permeability reductions may indirectly result in significant weakening of a rock by allowing the generation and maintenance of fluid pressures in excess of hydrostatic. Such fluid "overpressures" form when intergranular pore fluids can not be expelled during compaction or other volume altering processes [Bredehoeft & Hanshaw, 1968; Barker, 1972; Magara, 1975; Bethke, 1986; Hunt, 1990], due to the low permeability of the rock. As pore pressure increases, the effective stress on the rock decreases, thereby resulting in a weakening of the material.

Precipitation sealing is generally induced by changing temperature conditions or by primary mineral dissolution which results in the supersaturation of various low temperature mineral phases. A number of experimental and field studies have been conducted, characterizing the expected equilibrium mineral assemblages and changes to fluid chemistry under diagenetic conditions [e.g. Boles and Franks, 1979; Moody et al., 1985; Thornton and Seyfried, 1985; Vavra, 1989; Hajash and Bloom, 1991; Huang and Longo, 1994]. In some sediments, hydraulic sealing may also be partly due to mass transfer resulting from stress induced solubility gradients. There has been an abundance of work pertaining to this process which is generally referred to as pressure solution [e.g. Rutter, 1976; De Boer et al., 1977; Sprunt and Nur, 1977; Robin,1978; Gratier and Guiguet, 1986; Tada et al., 1987; Hickman and Evans, 1991]. Generally, pressure solution is viewed as the dissolution of material at highly stressed grain contacts followed by precipitation of that same material immediately adjacent to the contact. This relocation of mass combined with the ensuing deformation is believed to reduce permeability in granular media [Sprunt and Nur, 1977; Angevine and Turcotte, 1983; Walder and Nur, 1984; Gavrilenko and Guéguen, 1993; Lemée and Guéguen, 1996].

In recent years, many theoretical models have attempted to quantify the relationship between chemical reaction and permeability in porous media [Lichtner, 1985; Dewers and Ortoleva, 1994; Steefel and Lasaga, 1994; Bolton et al., 1996; Aharonov et al., 1997a]. However, from the experimental standpoint, very little work has been done. Most of the experimental studies pertaining to the chemical aspects of diagenesis have not been equipped to measure permeability changes during reaction. The few existing experimental studies [Small et al., 1992; Main et al., 1994; Scholz et al., 1995] have shown that nucleation and growth of authigenic phases can cause drastic permeability reduction. In fact, Scholz et al. [1995] report permeability reduction with little or no concurrent decrease in porosity. These experimental results are confirmed by field studies which have shown that thin oxide/clay coatings on fracture surfaces can cause permeability to decrease by an order of magnitude [Fuller and Sharp,1992; Fu et al.,1994].

In this paper and its companion [Aharonov et al., 1997b], a joint experimental/theoretical approach is taken to better understand the relationship between chemical reaction and permeability change. Here, we present results from a series of experiments in which a quartz/feldspar aggregate equilibrates with de-ionized water at elevated temperatures and stresses. Feldspar dissolution and accompanying secondary mineral growth result in significant permeability reduction. During these experiments, there was little or no mechanical compaction, hence the permeability changes did not result from net porosity loss - only the nature and shape of porosity was altered due to precipitation. Based on the chemical and permeability data from the experiments, a conceptual model of fluid-rock equilibrium and kinetics was developed and tested. In the companion paper, Aharonov et al. [1997b] describe a theoretical model of permeability evolution as a function of chemical equilibrium state and attempt to fit those results to these experimental data.

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