Predicting Reservoir Quality in Organic-rich Bakken Shales, North Dakota, Using 3D Seismic and Petrophysical Analysis

A petrophysical analysis, using well logs along with empirical and theoretical models, is undertaken to understand and characterize organic-rich prospective intervals within the Bakken Formation in North Dakota. A more heuristic and data-driven approach for unconventional petrophysics modeling proposes a model based on a priori partitioning of the pore system and fluids enabling reasonable prediction of the volumetrics of the shale rock using a minimum suite of logs. The total porosity and water saturation curves estimated using the revised model agree favorably with effective porosity and water saturation core measurements in both the shale members of the Bakken Formation. This suggests that kerogen volume controls the total porosity of shale reservoirs, and the total porosity models should honor the influence of both the inorganic and organic phases of the rock with a more accurate fluid distribution. We include calculations and mapping of interval properties from log data and integrate the seismic with seismic AVA analysis and anisotropic fluid substitutions to explain the observed low velocity ratios within the thin Bakken shales as being due to hydrocarbon saturation and kerogen presence. The modeling of the Bakken shale rock frame needs the inclusion of both kerogen and pyrite content for it to be in better agreement with expectations from Greenberg-Castagna (GC-92) empirical shale trend. Integrating well-log analysis, rock physics, seismic modeling, multi-linear regressions, and supervised machine-learning predictions of organic-richness, total porosity, and water saturation contributes to identifying the distribution of promising reservoir quality areas within the Bakken shale members.