Novel Approach for Elastic Anisotropy Assessment in Organic-Rich Shales: A Case Study from Early-Mature Region of the Bakken Formation

ABSTRACT The middle Bakken (MB) is a major unconventional oil producing formation in the U.S. with an enormous contribution of upper (UB) and lower (LB) Bakken members, source rocks. The three Bakken members form a crucial petroleum system in the Williston, where UB and LB, the target of this study, have similar lithologies, composed of siliceous, pyritic black shale, and middle Bakken (MB) which is a calcareous siltstone and fine-grained sandstone. Shales are characterized by their anisotropic nature due to layering. Therefore, isotropic modeling can lead to errors in stiffness estimations. The main objective of this study was to develop anisotropic rock physics modeling of the Bakken formation to assess the elastic behavior and determine the key parameters controlling the velocity dispersion with respect to layering. Maturity effect was also investigated by including kerogen content into the system via solid substitution modeling. Laboratory data was scattered against the lab-scale rock physics templates to capture the dominant pores morphology with respect to beddings in the Bakken shale. Furthermore, this study captured the implications of different pore structures on controlling the degree of anisotropy in the medium. The results showed that UB and LB shales exhibit large anisotropy due to high presence of clay minerals, and fractures. INTRODUCTION The Bakken formation is a major oil producer in the U.S. The members constituting the Bakken formation are upper (UB), middle (MB), and lower (LB) Bakken members. Both UB and LB units, which are the subject of this study, are source rocks with similar lithologies, composed of siliceous, pyritic black shale (Laalam et al., 2022a; Malki et al., 2022a; Sennaoui et al., 2022a; Sennaoui et al., 2022c). Total organic carbon (TOC) in the Bakken shales may reach 40 wt.%, with potential oil in place in a range of 10-500 billion barrels (Aoun et al., 2023a; Ozotta, et al., 2022a; Sennaoui et al., 2023a; Sorensen et al., 2010), but ultra-low porosity and permeability characteristics (Afari et al., 2022; Laalam et al., 2022b; Ozotta et al., 2021c; Sennaoui et al., 2023b).