Bakken Petroleum System (BPS) is composed of both conventional and unconventional units exhibiting significant variations in lithology, rock texture ranging from highly bioturbated to finely laminated, clay content, total organic carbon (TOC), accompanied by high connate water salinity, presence of disseminated pyrite grains, and low values of porosity. These petrophysical attributes of the BPS lead to inconsistency in the water saturation estimates obtained from resistivity induction log, NMR log, dielectric dispersion log, Techlog Quanti-ELAN, and Dean-Stark core measurements.
Inversion-based interpretation of dispersive electrical conductivity and dielectric permittivity logs acquired at four dielectric-dispersion tool frequencies across 400-feet depth interval in a science well intersecting the BPS. Three geoelectromagnetic mixing models, namely Lichtenecker-Rother (LR) model, Stroud-Milton-De (SMD) model, and PS model, a mechanistic pyrite-clay dispersion model, are coupled to jointly process the dielectric dispersion logs to simultaneously estimate water saturation, formation water salinity, cementation index, and homogeneity index.
Water saturation estimate for a specific depth obtained using the proposed interpretation method is not one single value but a range of possible values within a desired accuracy. These estimates were compared against those obtained from resistivity induction log, NMR log, Quanti-ELAN solver, Schlumberger's dielectric inversion, and Dean-Stark core measurements. Our estimates of water saturation and those obtained using the Schlumberger's dielectric inversion exhibit the best match with Dean-Stark's core water saturation in the Middle Bakken and Three Forks formations. However, in the part of Lodgepole, Scallion, and Upper Bakken formations, our estimates of water saturation are closer to those obtained from NMR logs, which disagrees with the extremely high estimates obtained using the Schlumberger's dielectric inversion. The estimated values of water saturation and formation water salinity in Middle Bakken are in the ranges of 0.5 to 1 and 205 kppm to 250 kppm, respectively. Homogeneity index obtained using our method indicates the presence of layering and heterogeneity in the Lower Three Forks and Middle Bakken formation. The cementation index indicates high tortuosity and cementation in the Upper and Lower Bakken formations.
