The Role of Mudstone Baffles In Controlling Fluid Pathways In A Fluvial Sandstone: A Study In the Pennsylvanian-Permian Fountain Formation, Northern Colorado, U.S.A

Abstract

Abstract Along the western edge of the Denver Basin, the arkosic Pennsylvanian–Permian Fountain Formation exposes hematite-bearing redbeds that have been partially whitened by the passage of reducing paleo-fluids. The fluids migrated mainly parallel to bedding, producing whitening displayed today by about 15% of the exposed surface area of the formation. The relationship between whitening and outcrop-scale stratigraphy suggests that the position of relatively impermeable beds controlled which of the more permeable beds carried the migrating fluids. In the study area the Fountain Formation displays repetitious vertically stacked, fining-upward sequences typically capped by paleosol mudstones. Coarse-grained channel-fill sandstones directly overlying the paleosol mudstones are proportionally the most whitened beds, and hence may have provided the most permeable pathways for the whitening fluid. Fine-grained sandstones immediately beneath the capping mudstones, however, display more whitening than seen in mid-sequence sandstones not in contact with the mudstones. Within a given bed, rock within 30 cm stratigraphically of the paleosol mudstones is considerably more likely to be whitened than is the rest of the bed. This stratigraphic proximity of whitening to the paleosol mudstones suggests that the mudstones served as baffles to fluid flow, causing fluids to focus and migrate laterally along the mudstone boundaries. The importance of the paleosol mudstones in controlling the flow pathways of the whitening fluids suggests that such flow barriers may strongly influence subsurface fluid movements within fluvial sandstone sequences. Fluorescent dolomite and kaolinite cements in the whitened rocks, coupled with inclusions of oil and bitumen, indicate that hydrocarbons likely contributed to the reducing nature of the fluids that removed the hematite. In the study area, whitening decreases from south to north, as does the proportion of coarse channel sandstones. Whitened rock in the north shows more intense alteration of feldspar than is observed farther south or in adjacent red rock. This greater alteration, which includes replacement by fluorescent kaolinite, suggests that, compared to farther south, the smaller volume of coarse sandstones experienced a higher fluid/rock ratio. Stronger association of fluorescent cements with the increased feldspar alteration in the northern part of the study area also indicates greater fluid flux along the more tightly focused flow pathways.