Stylolite Porosity in Carbonate Reservoirs

William C. Dawson

Despite the common presence of stylolites in carbonate reservoirs, the influence of stylolites on porosity has not been investigated systematically. Because stylolites result from pressure solution, stylolitization is inferred to have been an important source of calcite. Hence, the prevalent views are: (1) porosity decreases toward stylolites, (2) stylolites are permeability barriers, and (3) stylolites invariably degrade reservoir quality. Nevertheless, many stylolitized carbonate rocks are porous, and the distribution of some porosity is genetically related to stylolites. Most importantly, and contrary to prevalent views, porosity may increase toward stylolites. Examples of stylolite porosity exist in the Paradox, Delaware, and Williston basins. Several types of styloli e-associated porosity are discernible: open stylolite seams, fabric-selective megaporosity halos, vugs, microporosity halos, fractures, and crystal molds.

Stylolite porosity results from post-stylolitization marginal dissolution, where stylolite seams have served as conduits for CaCO₃-undersaturated basinal fluids. This interpretation implies that stylolites may be reopened (probably by partial uplift and unloading) during mesodiagenesis. Subsequently, open stylolites provide conduits for diagenetic fluids and hydrocarbons through otherwise impermeable strata. Stylolite porosity may significantly enhance reservoir quality. The flow of CaCO₃-undersaturated fluids along stylolites and marginal stylolite dissolution have been duplicated experimentally at confining pressures simulating up to 15,000 ft of burial.

Petrographic and experimental data indicate that diagenesis may continue after stylolitization ceases, and post-stylolitization dissolution may partly reverse porosity loss induced by stylolitization. Thus, deeply buried carbonates may have reservoir potential where stylolite porosity exists.

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