Petrophysical facies of the Ordovician Red River Formation, Williston Basin, USA

A petrophysical facies is a mappable rock unit of similar petrophysical characteristics, imparted by the depositional and diagenetic environment. Capillary-pressure curves are the basis for delineating a petrophysical facies. Important parameters that affect the shape of a capillary-pressure curve are sorting of the pore-throat size distribution, the maximum threshold-entry radius, and percent recovery efficiency. Curve types based on these parameters are spatially clustered both laterally and vertically within the Red River Formation. The ordering of petrophysical facies, vertically up-hole, is related to the maximum threshold-entry radius which range from very small radii 'A' to very large radii 'F', with 'C' and 'D' of moderate size as defined in the construction of curve type designation. In the Red River depositional sequences, petrophysical facies are ordered by 'B-C-D-C-B', where either 'A', 'E', 'F', or PS is found at the bottom or top of the sequence, defining a sandwiched sequence of pore throats with the optimum reservoir facies in the center. The maximum-threshold entry radius, apparent porosity, and recovery efficiency are logged for each lithologic unit. Large maximum threshold-entry radii are associated with poorly sorted pore-throat size distributions and/or little to no porosity. Recovery efficiency is only meaningful for dolostones with moderate to high porosity.