RESERVOIR ARCHITECTURE AND CONNECTIVITY OF THE TRAIL MEMBER FLUVIAL SANDSTONES, ERICSON SANDSTONE, MESAVERDE GROUP

The Trail Member of the Cretaceous Ericson Sandstone, exposed along the length of the Rock Springs Uplift, represents a low-accommodation fluvial system, with a very high net-to-gross relative to both underlying and overlying strata. It is dominated by trough-cross stratified sandstone with frequent, but often discontinuous, mud and silt interlayers. Individual beds are laterally discontinuous and pinch out regularly, but both lateral and vertical amalgamation of sandy fluvial channel elements is common. Though this strata is overwhelmingly sand-rich in the study area, measured sections and photogrammetric modeling of this fluvial system show both spatial and temporal variations in the character and connectivity of these sands that have important implications in this and analogous systems in the subsurface. Near both the base and the top of the section in the field area, sands are more laterally accreting and less vertically stacked, suggesting that accommodation was lowest at this time, leading to frequent avulsion and lateral migration of channels. In the middle of the stratigraphic section, sands are generally less laterally extensive, and multi-story channel complexes are more common. Both of these configurations have very different reservoir implications, and ongoing work shows that these trends extend more regionally throughout the Rock Springs Uplift. In addition, within the exposed potential reservoir sands of the field area, key facies identified in outcrop show important variability in both porosity and permeability. Through both large-scale observation of architectural elements and much finer-scale investigation of rock properties and facies relationships, it is shown that effective reservoir connectivity and drainage patterns can be much more complex than anticipated, even in sandy high net-to-gross systems such as this.