Karst and Fracture Features Affecting Reservoir Performance in a Mississippian Reservoir, Cheyenne County, Colorado

Natalie B. Givens¹ and Susan Nissen^2
1 University of Kansas, Lawrence, KS
² Kansas Geological Survey, Lawrence, KS

As part of a project to improve geologic and engineering models of mid-continent fracture and karst-modified reservoirs using new 3-D seismic attributes, we are conducting an integrated study of a Mississippian reservoir in Cheyenne County, Colorado, which has scattered and variable production. The focus of this study has been to integrate new 3-D seismic attributes with geological data to increase probability of identifying fractures and karst features that are affecting reservoir performance. Core and wireline log data have been used to determine lithofacies, depositional facies, diagenetic signatures, and petrophysical properties for the reservoir, as well as well-bore-scale fracture distribution and orientation. Core analysis reveals a complex history for the reservoir. Depositional environment is interpreted as a restricted lagoon with a migrating shoal. Lithofacies range from mudstone to packstone-grainstone; however, the entire section has been heavily dolomitized, obscuring primary depositional structures. Porosity is mostly intercrystalline, except where it is moldic in the productive zones. Fractures identified in the cores are filled. Geometric attributes from a 6 square mile 3-D seismic survey over the reservoir show subtle lineaments that parallel regional structural trends. These lineaments may reflect fracture orientations that controlled karst development on the Mississippian surface. Seismic attributes, correlated with log and core data are used to help delineate fractures and reservoir compartments, as well as to guide distribution of petrophysical properties in a reservoir model. The results of this study provide key input to the synthesis of a best-practices workflow for characterizing fractured and karst-modified reservoirs in other areas.