Seismic petrophysics applied to the Bakken Formation, North Dakota

Abstract

This study uses well log analysis along with empirical and theoretical equations to characterize properties of the unconventional resource in the Bakken Formation of North Dakota. Cross-plot analysis of petrophysical and elastic properties indicates that density (ρ), P- and S-impedance, Young’s modulus (E), shear modulus (μ), and μρ are good total organic carbon (TOC) content indicators. Shales with high TOC content (greater than 10 wt%) have lower velocities and densities (<2.31 g/cm3) than lower TOC cases. High and low TOC rocks exhibit Vp/Vs values between 1.65 and 1.75. Both the calibration of Greenberg and Castagna’s trends and Vernik’s predictive methods can be helpful for shear-wave velocity and impedance prediction in the Bakken Formation. A total porosity range between 6% and 10% corresponds to Bakken shales with TOC content ≥ 10 wt% with P-impedance (AI) and S-impedance (SI) cutoff values of 25,000 g/cm3·ft/s and 15,000 g/cm3·ft/s, respectively. Elastic moduli decrease as TOC content and hydrocarbon saturation increase. The high TOC shales correlate with E, μ, and μρ values between 11–24 GPa, 2.5–8.75 GPa, and 10–18 GPa·g/cm3, respectively. We present empirical equations, calibrated for the Bakken Formation members, to be used for quick-look calculations and comparisons. The methodology presented here can help inform analysis for selecting promising target areas in unconventional resource plays.