Role of Mechano-Chemistry in Formation of Diagenetic Seals

Tom Dewers, Peter Ortoleva

In a rock with imposed confining and fluid pressure, the free energy of a given mineral grain is in part dependent on the texture (grain and pore size and shape, mineral content, fractures, etc) of the rock that surrounds it. This texture-dependent free energy arises due to textural modification of applied stress and interfacial energy effects and may be separated conceptually into mean and deviatory contributions. The former, averaged over the texture in a small parcel of rock, varies over a length scale larger than a single grain, and thus mediates supra-grain scale diffusional transport, whereas the spatial distribution of the latter varies within a single grain and controls changes in grain shape due to intergranular pressure solution and thus compaction. Chemical pot ntial gradients resulting from macroscopic inhomogeneities in rock texture may be of sufficient magnitude to induce appreciable diffusional mass transport on a macroscopic length scale. The resulting coupling of texture-mediated grain growth, dissolution, and solute transport can lead to the local enhancement of textural nonuniformities (mechano-chemical self-organization), giving rise to such features as stylolites and banded diagenetic cements. As these features may serve as pressure or permeability barriers, they are of considerable interest in basin evolution. Quantitative relations setting forth the texture dependence of grain solubility in reaction-transport models allow the prediction of where, when, and how fast stylolites and other features form, and the position and timing of n cleation of diagenetic phases. The spatial variation of free energy of texture-dependent grains thus strongly mediates subsequent textural evolution, and so is an important consideration in the formation of diagenetic hydrocarbon traps.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.