Reservoir-scale CO 2-fluid rock interactions: Preliminary results from field investigations in the Paradox Basin, Southeast Utah

Despite a long history of detailed study, the extensive Jurassic sandstone outcrops of the Colorado Plateau in Southeast Utah, USA continue to provide opportunities to examine reservoir-scale processes. There are a number of large-scale CO 2 accumulations in these reservoirs and locally also natural and man-induced CO 2-rich springs and geysers, often producing or associated with travertine deposits. These rocks have therefore been exposed to CO 2 and/or CO 2-rich waters over a substantial period of time, and as such may provide information on resulting geochemical and geomechanical processes. Salt Wash Graben is a WNW-ESE trending structure that cuts across a northerly plunging anticline to the south of Green River, Utah. The Salt Wash Graben lies to the south of the well-studied ‘Crystal Geyser’, and itself contains several CO 2-rich springs and abundant travertine deposits. Rocks of the Slick Rock and Earthy members of the Jurassic Entrada Sandstone outcrop in the core of the anticline immediately to the north of the graben. These are locally extensively bleached, the bleaching being most pronounced towards the base of the stratigraphically lowest exposed strata, forming a light coloured area in aerial photos. Field investigations of the nature of this bleaching and its relationship to faults and fractures, travertines and CO 2 springs, have been supported by detailed analysis of aerial photographs and other satellite-based remote sensing data. The remote sensing data provides a geological context that serves to highlight the different structural and stratigraphic controls which were observed between the regionally extensive bleaching and the bleaching in the Salt Wash Graben. Diagenetic analyses have identified the geochemical processes responsible for bleaching (loss of iron-staining) and other fluid-rock interactions, notably increased porosity in bleached sandstones and differences in carbonate cementation. Simple laboratory experiments have also attempted to replicate these processes by reacting samples of unbleached Entrada Sandstone with CO 2-rich formation waters. In this paper, we present a preliminary hypothesis that attempts to link the features described in the Salt Wash Graben, which suggest that CO 2-rich fluids may have produced the observed features. If this hypothesis is proved correct, the Salt Wash Graben offers considerable potential to study in detail the flow-constrained geochemical processes at reservoir scale.