Reflectance spectroscopic mapping of diagenetic heterogeneities and fluid-flow pathways in the Jurassic Navajo Sandstone

摘要

Multiple episodes of fluids migrating through the Jurassic Navajo Sandstone have resulted in abundant and spatially variable diagenetic mineral changes. Depending on fluid chemistry, flow events have produced or removed varying amounts of iron oxides, clays, and carbonates with distinctive spectral reflectance signatures that can be used to map spatial heterogeneities in diagenetic mineralogy and paleofluid-migration pathways (including hydrocarbons and groundwaters). Field and laboratory reflectance spectroscopy shows that the common diagenetic minerals in the Navajo Sandstone have diagnostic visible, near-infrared, and short-wave infrared spectral characteristics in the 0.35–2.5-µm range. Comparisons of (1) geochemical data, (2) in-situ reflectance spectroscopy, and (3) airborne imaging spectroscopy for zones of variably altered Navajo Sandstone in southern Utah show that the minerals within alteration facies have distinctive spectral signatures. Reflectance spectroscopic mapping provides a method for evaluating the effects of diagenesis and fluids in this well-exposed reservoir sandstone. Reservoir heterogeneity in many eolian sandstones is largely controlled by diagenetic processes that can be difficult to evaluate on outcrop to reservoir scales (approximately tens to hundreds of meters to several kilometers). Imaging spectroscopy allows for the evaluation of mineralogy variations on these scales. The patterns of authigenic iron oxide, clay, and carbonate removal and precipitation trace the paths of different episodes of fluid flow and sandstone alteration. Mineral variations occur as kilometer-scale reaction fronts related to structural fluid conduits and as 100-m (330-ft)-scale changes that follow stratigraphy. These spectroscopic techniques provide important tools for reservoir evaluation, and the patterns observed serve as an analog to understanding regional diagenetic patterns in other subsurface eolian reservoirs.