ABSTRACT

The Bakken Formation consists of three members: The Upper Bakken and Lower Bakken are dark marine shales with high organic content, whereas the Middle Bakken consists of mixed carbonates and clastics and is the main reservoir unit, despite having low porosity and permeability. Dipole S-wave data acquired in a lateral well in the Middle Bakken Formation revealed this formation to be anisotropic. Backus upscaling of logs acquired in a nearby vertical pilot well in the same layers sampled by the lateral well gave estimates of the anisotropy that were too small to explain the S-wave anisotropy measured in the lateral well. The observed anisotropy was interpreted in terms of bedding-parallel compliant discontinuities such as microcracks and low-aspect-ratio pores. The presence of bedding-parallel microcracks and low-aspect-ratio pores may contribute to the permeability of the tight Middle Bakken reservoir, and the sensitivity of P- and S-wave velocities to the presence of microcracks and low aspect ratio pores suggested the use of sonic and seismic measurements for identifying the productive zones in the low-permeability Middle Bakken reservoir.

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