We characterized the dependence of ultrasonic velocities on confining pressure (Pc) and pore pressure (Pp) of six argon-saturated cores from three formations associated with the Bakken play in the Williston Basin (Lodgepole, Middle Bakken and Three Forks). We cycled Pc under constant Pp to simulate reservoir stress changes associated with depletion and injection. The ultrasonic velocities (Vp and Vs) in the axial direction were measured along the loading path, based on which the elastic moduli and effective stress coefficient were derived. Common to all specimens, both Vp and Vs under injection are consistently higher than under depletion at the same Pc and Pp. Derived elastic moduli assuming material isotropy qualitatively agree with logging data, but are consistently higher than those based on static measurements. We found the effective stress coefficient (α) with respect to Vp and Vs is close to unity when the simple effective stress is no more than 10 MPa, regardless of wave type, lithology and loading path. α for Vp and Vs generally increases for higher simple effective stress (Pc-Pp) and beyond unity, which is contrary to the trend obtained through static deformation and against theoretical expectations. It implies that Vp and Vs become more sensitive to Pp rather than Pc as (Pc-Pp) rises. This apparent increase of α with (Pc-Pp) is a priori unresolved, but can be plausibly attributed to the fact that the change of (Pc-Pp) altered the rock microstructure, which essentially rendered the pore pressure more effective. Submission Files Included in this PDF File Name [File Type] cover letter.docx [Cover Letter] Ma et al. IJRMMS.docx [Manuscript File] Ma et al. IJRMMS_figures.pdf [Figure] declaration-of-competing-interests_Ma.docx [Conflict of Interest] To view all the submission files, including those not included in the PDF, click on the manuscript title on your EVISE Homepage, then click ‘Download zip file’.