An integrated study delineating karst and fracture features affecting reservoir performance in a Mississippian reservoir, Cheyenne County, Colorado

As part of a project to improve geologic and engineering models of mid-continent fractured and karst-modified reservoirs using new 3D seismic attributes, we are conducting an integrated study of a Mississippian reservoir in Cheyenne County, Colorado. As distribution of production has been scattered and variable throughout the area, the focus of this study has been to integrate new 3D seismic attributes with geological data to increase probability of identifying fractures and karst features that are affecting reservoir performance. Core and wire-line log data have been used to determine lithofacies, diagenetic signatures, and petrophysical properties for the reservoir. Core analysis reveals a complex history for the reservoir. Lithofacies range from mudstone to grainstone, and depositional environment is interpreted as a normal marine shelf. The entire section has been heavily dolomitized. The porosity is mainly intercrystalline and moldic. In the productive zones, porosity is mainly solution-enhanced moldic and intercrystalline. Chalcedony, megaquartz, and baroque dolomite have been identified as hydrothermally precipitated minerals. Geometric attributes from a 5.5 square mile (14.2 km2) 3D seismic survey over the reservoir show subtle lineaments that parallel regional structural trends. These lineaments potentially reflect fracture orientations that may have controlled karst development on the Mississippian surface. Seismic attributes, correlated with wire-line log and core data are used to help delineate these surface karst features and reservoir compartments, as well as to guide the distribution of petrophysical properties in a reservoir model. Steady reservoir pressure and quick decline curves lead us to the hypothesis that each well has a small drainage area. This could be due to compartmentalization of the reservoir by the tectonically related lineaments, as well as, a lithofacies mosaic within the Spergen. The results of this study provide key input to the synthesis of a best-practices workflow for characterizing karst-modified reservoirs in other areas.