Bakken Petroleum System Characterization Using Dielectric-Dispersion Logs

The Bakken Petroleum System (BPS) is composed of both conventional and unconventional units exhibiting significant variations in lithology, rock texture, clay content, total organic carbon (TOC), accompanied by high connate-water salinity, presence of disseminated pyrite grains, and low porosity. These petrophysical attributes lead to inconsistency in water-saturation estimates obtained from various subsurface logs, such as NMR log, resistivity log, dielectric dispersion log, and induction log. An inversion-based interpretation method is applied to process dispersive electrical conductivity and dielectric-permittivity logs acquired at four dielectric-dispersion log-acquisition frequencies in the range of 10 MHz to 1 GHz. The Lichtenecker-Rother (LR), Stroud-Milton-De (SMD) models are coupled with the PS model, a mechanistic pyrite-clay dispersion model, to jointly process the dielectric-dispersion logs for simultaneous estimation of water saturation, water salinity, cementation index, and homogeneity index. Using the proposed interpretation method, water-saturation estimate for a specific depth is obtained as a range of possible values within a desired accuracy. These estimates were compared against those obtained from the induction resistivity log, NMR log, Quanti-ELAN solver, the service company's dielectric inversion, and Dean-Stark core measurements. Our estimates of water saturation and those obtained using the service company's dielectric inversion exhibit a best match with Dean-Stark core water saturation in the Middle Bakken and Three Forks formations. The water-salinity estimates range from 150,000 to 350,000 ppm. The homogeneity index obtained using our method indicates the presence of layering and heterogeneity in the Lower Three Forks and Middle Bakken formations. The cementation index indicates high tortuosity and cementation in Upper and Lower Bakken formations.