Similarly shaped capillary-pressure curves, classified according to pore-throat size sorting, the maximum threshold-entry radius (MTER), and percent recovery efficiency (RE), delineate petrophysical facies. Capillary-pressure curves of carbonates that have a well-sorted (WS) pore-throat size distribution are characterized by MTER occurring at less than 20% mercury imbibition and horizontal to subhorizontal plateaued injection curves resulting from a unimodal pore-throat size distribution. Capillary-pressure curves of carbonates that have a moderately sorted (MS) pore-throat size distribution are generally sinusoidal in shape and have MTER between 10 and 40% cumulative pore volume imbibition. Capillary-pressure curves of carbonates that have a poorly sorted (PS) pore-throat size distribution are generally oblique or diagonal and have no plateau and poorly defined MTER.
The concept of petrophysical facies is applied to the Red River Formation (Ordovician) of the Williston basin. Capillary-pressure curves are used to determine the spatial distribution of petrophysical characteristics within the Red River carbonates. Curve types are spatially clustered and subdivide the formation into petrophysical facies, which are laterally and vertically continuous. Capillary-pressure curves of dolostones that have high porosity and good reservoir potential are characterized by WS pore-throat size distributions, MTER within the range from 0.448 to 8.55 μm, and greater than or equal to 30% RE.
Optimum reservoir potential occurs in dolostones that have greater than 70% dolomite, WS pore-throat size distribution, greater than or equal to 13.5% apparent porosity (AP), more than 49% total pore system saturation by 500 psia and more than 87% total pore system saturation by 1000 psia during mercury porosimetry, 0.1 to 0.65 μm average pore-throat radius (APR), 0.18 to 1 μm median pore-throat radius (MPR), 0.6 to 3.6 μm MTER, and 46 to 63% RE. A positive correlation occurs between AP and RE where, as porosity increases to 21%, the RE increases to 64%.
Pore diameters that have optimum reservoir potential are not necessarily the largest found in dolostones. Moderate size pore diameters occurring in intercrystalline pore systems produce the highest porosity and RE. The AP is also a function of the pore-throat radius. Highest porosity occurs in dolostones that have relatively small pore-throat radii. As the APR increases from 1 to 3.2 μm, the AP slightly decreases. Dolostones that have APR larger than 3.2 μm have no AP.