POROSITY AND PERMEABILITY VARIATIONS IN FRACTURED AND LIESEGANG-BANDED BREATHITT SANDSTONES (MIDDLE PENNSYLVANIAN), EASTERN KENTUCKY - DIAGENETIC CONTROLS AND IMPLICATIONS FOR MODELING DUAL-POROSITY SYSTEMS

Middle Pennsylvanian fluvial sandstones in eastern Kentucky (Breathitt Group) manifest visible evidence of alteration related to fluid flow localized through near-vertical joints. Fracture-related alterations involve both physical and chemical modifications that together create dramatic permeability variations at the outcrop scale. On the fracture surface, infiltered detritus combined with mineral and organic coatings have reduced pore sizes and, hence, permeabilities (0.03-0.44 md) by an order of magnitude over values characteristic of the adjacent sandstone (0.32-1.53 md). Prominent zones of orange-brown discoloration contain evidence of oxidation reactions and form an envelope of variable thickness around the fractures. Authigenic iron oxides are not uniformly distributed within these zones, but rather are concentrated as local bands of pervasive mineralization commonly known as liesegang bands. Petrographic evidence suggests that most of the iron that now resides in oxidized authigenic phases was derived from solutes mobilized through dissolution of older iron-bearing authigenic minerals. Permeability and pore sizes within the oxidation zone are bimodal and vary from high values similar to those in adjacent unoxidized sandstone to low values, associated with the zones of pervasive mineralization, that approach those values observed for the fracture skin. The large magnitude of permeability variation around fracture systems in these sandstones documents the presence of a dual porosity system and suggests that fluid and contaminant transport cannot be realistically modeled using average rock properties.