Sensitivity of seismic reflections to variations in anisotropy in the Bakken Formation, Williston Basin, North Dakota
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The Upper Devonian–Lower Mississippian Bakken Formation in the Williston Basin is estimated to have significant amount of technically recoverable oil and gas. The objective of this study is to identify differences in the character of the seismic response to Bakken interval between locations with high and poor production rates. The predicted seismic responses of the Bakken Formation will hopefully help achieve such discrimination from surface seismic recordings. In this study, borehole data of Bakken wells from both the Cottonwood and the Sanish Field were analyzed, including density information and seismic P and S wave velocities from Sonic Scanner logs. The Bakken Formation is deeper and thicker (and somewhat more productive) in the Sanish Field and is shallower and thinner in the Cottonwood Field. The Upper and Lower Bakken shale units are similar and can be characterized by low density, low P and S wave velocities and low Vp/Vs ratios. The Sonic Scanner data suggest that the Upper and Lower Bakken shales can be treated as VTI media while the Middle Bakken may be considered as seismically isotropic. Models of seismic response for both fields were constructed, including isotropic models and models with variations in VTI, HTI, and the combination of VTI and HTI in the Bakken intervals. Full offset elastic synthetic seismograms with a vertical point source were generated to simulate the seismic responses of the various models of Bakken Formation. This sensitivity study shows pronounced differences in the seismic reflection response between isotropic and anisotropic models. P-P, P-SV and SV-SV respond differently to anisotropy. VTI anisotropy and HTI anisotropy of the Bakken have different character. In particular, types of seismic data (P-P, P-SV, and SV-SV) and the range of source-receiver offsets that are most sensitive to variations in anisotropic parameters and fluid saturation were identified. Results suggest that bed thickness, anisotropy of the Upper and Lower Bakken shales, fractures/cracks and fluid fill in the fracture/cracks all influence the seismic responses of the Bakken Formation. The paucity of data available for “poorly” producing wells limited the evaluation of the direct seismic response to productivity, but sensitivity to potentially useful parameters was established.