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 i nvolve both physical and chemical modifications that together create d ramatic permeability variations at the outcrop scale. On the fracture surface, infiltered detritus combined with mineral and organic coating s have reduced pore sizes and, hence, permeabilities (0.03-0.44 md) by an order of magnitude over values characteristic of the adjacent sand stone (0.32-1.53 md). Prominent zones of orange-brown discoloration co ntain evidence of oxidation reactions and form an envelope of variable thickness around the fractures. Authigenic iron oxides are not unifor mly distributed within these zones, but rather are concentrated as loc al bands of pervasive mineralization commonly known as liesegang bands . Petrographic evidence suggests that most of the iron that now reside s in oxidized authigenic phases was derived from solutes mobilized thr ough dissolution of older iron-bearing authigenic minerals. Permeabili ty and pore sizes within the oxidation zone are bimodal and vary from high values similar to those in adjacent unoxidized sandstone to low v alues, associated with the zones of pervasive mineralization, that app roach 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 fl uid and contaminant transport cannot be realistically modeled using av erage rock properties.