SANDSTONE COLORATION AND IRON OXIDES: AN INDEX TO FLUID FLOW IN JURASSIC RESERVOIRS, SOUTHEASTERN UTAH

Iron oxides (e.g., hematite, goethite) account for the vivid coloration in permeable Jurassic Navajo, Page, and Entrada sandstones of southern Utah. Sandstone coloration is an index to both the character of the fluids which caused both precipitation (oxidizing) and bleaching (reduction), and the overall hydrologic setting. Most of these sandstones were likely originally red from small amounts of early, disseminated iron oxides. Bleaching by reducing solutions later removes iron on nearly microscopic scales such as those along individual eolian laminae, deformation bands, and lithologic contacts, to regional scales extending through a formation over tens of kilometers. Bleaching is dependent on the mobility of iron, fluid composition and flow paths. Bleaching is a function of permeability, lithology, sedimentology, stratigraphy, and structure.

Field, laboratory, and numerical modeling studies on iron mineralization in southern Utah suggest that iron is mobilized and removed by reducing water that moved along conduits (e.g., faults or fractures) and then outward into adjacent permeable rocks. The reducing fluids need only contain small quantities of reductant such as hydrocarbon, methane, organic acids, or hydrogen sulfide. When reduced waters carrying the iron meet and mix with shallow, oxygenated ground water, iron oxides are precipitated to form a variety of iron-oxide concretionary cements in the porous sandstones. Multiple iron-oxide mineralization events and concretionary geometries are evident and can be explained as the result of permeability heterogeneities in the host rock, presence of favorable nucleii for precipitation, a self-organization process, or the influence of microbes. This study emphasizes the nature of the reducing fluids that mobilize iron, fluid pathways, the mixing fluid compositions, and the relation to oxidizing fluids that precipitate iron oxides.