Reservoir Rock Characteristics Of The Madison Limestone In The Williston Basin

Porosities and water saturations from logs were compared with these properties from core analyses for three oil reservoirs in carbonate formations in the Williston Basin. The physical properties and fluid flow characteristics of cores from these reservoirs were also analyzed. Average Sonic or resistivity log porosities are generally higher than the corresponding core porosities, but average neutron log porosities are nearly equal to the corresponding core porosities. Average water saturations from Sonic or resistivity logs are lower than the corresponding core saturations. Core analysis data show only a very general trend of increasing permeability with increasing porosity, and of increasing residual water saturation with decreasing permeability, confirming the high degree of heterogeneity of the rocks. The effective permeabilities to two-phase gas and oil flow are very small. Mercury injection measurements show that porosity is preponderantly associated with pore entry radii smaller than 0.5 micron, and that less than half the pore volume contributes significantly to the absolute permeability of the cores. X-ray analyses confirm that dolomite is a significant mineral constituent, but anhydrite infilling and other secondary features obscure the effect of dolomitization on the porosity and permeability development of the rock.