Porosity and permeability data for the Mississippian Madison Group in southwestern Wyoming were compiled and evaluated to relate these properties to stratigraphic facies in the Madison Group. The study was performed to provide baseline data for a geologic model required to sequester carbon in the study area. Public domain geological and petrophysical data provided the basis for the evaluation. Using the available database of wire-line logs and core from wells that penetrate the Madison Group, we place the wells within the regional structural and sequence-stratigraphic framework and detail porosity-permeability relationships. The highest porosity and permeability in the study area is present in the lower portion of the formation in dolomitic packstone-to grainstone-dominated facies near the top of the transgressive systems tract. Wire-line logs were used to calculate porosity values that correlate well with the more limited core-based data. The porosity in the Madison Group has a tri-modal distribution with porosity related to depositional facies. The first group is characterized by low porosity (<4 percent) with highly variable permeability for low porosity units. The second has intermediate porosity (4–12 percent) with variable porosity-to-log permeability; the last is high porosity (>12 percent) with a strong log permeability to porosity covariance. While lateral variations in porosity related to depositional facies can be traced over tens of kilometers, natural fractures appear to be a significant control on permeability in the lower porosity portions of the Madison.