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Comparison of the Compositional & Mineralogic Control on Rock Strength Between the Middle Bakken and Three Forks Formation, Williston Basin, USA


The Middle Bakken Member and Upper Three Forks horizons, which are part of the prolific Bakken Petroleum system, are considered the dominant targets for unconventional hydrocarbon production in the Williston Basin. Although development of the Bakken play has evolved to leverage modern horizontal drilling and completion strategies, the geomechanical models driving these strategies are often derived from well logs that do not adequately capture the variability of brittle and ductile zones in these formations. Understanding the fundamental controls on rock strength at the sub-log scale is important when planning efficient completion strategies in the Bakken. We hypothesize that estimated UCS decreases as a function of the relative abundance of total clay minerals in the Middle Bakken and Three Forks. In this study, X-ray fluorescence (XRF), X-ray diffraction (XRD), and handheld micro-rebound hammer measurements were collected from the Middle Bakken and Upper Three Forks at inch-scale resolution from 5 cores across the Williston Basin to examine the relationship between XRF- and XRD-derived mineralogy and estimated unconfined compressive strength (UCS) estimated from rebound hammer measurements. Preliminary results indicate that there is a negative correlation between abundance of total clay minerals and UCS in the Middle Bakken and Three Forks, while the abundance of calcite and quartz have no systematic relationship with UCS. These relationships and the results of this study can be utilized for optimizing geomechanical models and inform how the inch-scale heterogeneity of these formations may impact completion strategies.