| Description |
With shale resources becoming increasingly more important in the global energy fuel mixture, a thorough understanding of heterogeneities present within individual units is crucial to an accurate assessment of resource potential. A wide variety of core data collected from the Lower Blue Gate Member, Mancos B, Juana Lopez Member, and Tununk Member of the Mancos Shale shed light on the scale and variety of heterogeneities present in this siliciclastically influenced mudstone/shale system. 11 lithofacies were identified over five cored intervals throughout the Uinta Basin, and a depositional model was thereby developed. Three environments of deposition are identified: the prodelta, the mudbelt, and the sediment starved shelf. Within this depositional model, subtle proximal to distal trends are interpreted. Mechanisms of transport and deposition of sediment are different within each of the three environments, with river influence dominant in the prodelta environment, combined flow as well as hyperpycnal flow dominating the mudbelt environment, and hypopycnal settling of sediment dominating in the sediment starved shelf. Individual parasequences are identified in each cored interval based on identification of flooding surfaces. Each parasequence (roughly 15 feet thick) records the local landward progression of depositional environments. Proximal to distal trends relating overlying and underlying parasequences is used to determine stacking patterns throughout each cored interval. Highstand systems tracts record aggradational stacking patterns. Transgressive systems tracts demonstrate a retrogradational stacking pattern. Lowstand systems tracts record a significant landward shift in facies at their base, and are aggradational to progradational. Systems tracts are 20-50 foot-scale in thickness. Two potential shale gas reservoir targets emerge from this study, and each target fits a very limited sequence stratigraphic interval. High gas storage potential along with a relative brittleness responsive to hydraulic fracturing is expected in the most landward lithofacies with the coarsest grained siltstone (lithofacies 9), deposited in the proximal mudbelt environment. In the sequence stratigraphic model presented herein, this lithofacies occupies the late, proximal highstand systems tract, or the lowstand systems tract. High organic carbon content, as well as high porosity values and increased calcite content (which may impart brittleness) are found in a separate lithofacies (lithofacies 8) that is deposited exclusively in the distal sediment starved shelf during the transgressive systems tracts. The systematic classification of environments of deposition allows refined interpretations of the sequence stratigraphic framework of the Mancos Shale without the necessity to correlate mudstones to updip, more proximal, shallow marine expressions of the depositional system. |
