CONTROLS ON PALEO-FLUIDS AND FLUID FLOW IN THE PENNSYLVANIAN-PERMIAN FOUNTAIN FORMATION

The Pennsylvanian-Permian Fountain Formation is an arkosic conglomeratic sandstone that was deposited in a braided river environment along the ancestral Rocky Mountains. Within the otherwise pinkish red to orange sandstone are distinct whitened zones that range from entirely whitened beds to single whitened laminations. Whitened zones such as these usually indicate that a reducing fluid migrated through the formation. This paleo-fluid could have been aqueous basinal fluids, hydrocarbons, or a combination of both. The Fountain Formation is composed of fining upward packages and the whitened zones appear in predictable sequences within these packages and are dominant in certain facies. This indicates that the paleo-fluid followed preferential pathways through the formation. Seven stratigraphic sections were measured along a fifty mile transect near Fort Collins, CO. The whitened zones were described in detail and analyzed to determine what features controlled the pathway of the paleo-fluids. Completely whitened zones make up about 7% of the measured sections and commonly occur directly above and below paleosols and fine-grained overbank deposits. Coarse channel sandstones are often located above paleosols and the lower portions of these sandstone units seem to make up the main conduit for fluid flow. Whitening within the laminations of coarse-grained channel fill sandstones account for another 8% of whitened material and are typically in the lower to middle portion of these units. Seventy six thin sections were made from samples taken from five of the measured sections to determine if any mineralogical differences, other than Fe-oxide staining, exist between the red and white zones, as well as to more precisely describe the lithology of the measured sections. Understanding and being able to predict the pathways of migrating fluids in complex and highly variable systems such as the Fountain Formation is important for modeling hydrocarbon migration within basins as well as for reservoir characterization.