The permeability of stylolite-bearing limestone

Abstract

Stylolites are planar features that form due to intergranular pressure solution. Due to their planar geometry and relative abundance in limestone reservoirs, their impact on regional fluid flow has attracted considerable interest. We present laboratory permeability data that show that stylolites can be considered as conduits for flow in the stylolite-bearing limestones measured. A combination of analysis techniques shows that this is due to a zone that surrounds these stylolites that is more porous and contains larger pores than the host rock. Our data also show that the water permeability of a sample containing a stylolite parallel to fluid flow is typically lower than its permeability to gas, explained here as a result of the expansion of minor amounts of clay found in the stylolite, and that, due to their microstructural similarities, tectonic and sedimentary stylolites affect sample permeability similarly. Finally, we show that the permeability anisotropy that develops in the rock mass due to the presence of sedimentary stylolites makes it appear as though the stylolites are acting as barriers to fluid flow, and may explain the discrepancy between laboratory measurements and field-scale observations. This approach can provide estimates for the equivalent permeability, and permeability anisotropy, for stylolite-bearing limestone reservoirs worldwide.