Spatial Variability of Free-Fluid Porosity in the Middle Bakken Reservoir: Applying Traditional Mapping Techniques to Nuclear Magnetic Resonance Log Data
Jonathan Payne and Peter Hook
Horizontal wells targeting the Middle Bakken oil play have been drilled throughout much of the U.S. portion of the Williston Basin. Many of these wells have been drilled in areas in which production is widely known to be commercially economic. However, reliable prediction of reservoir-grade rocks in the Middle Member of the Bakken Formation has been problematic since the emergence of the play. Historically, these reservoir predictions have been hindered, at least in part, by reliance upon conventional porosity logs. Analysis of 37 wells, from which both conventional porosity logs and nuclear magnetic resonance (NMR) porosity data are publicly available, suggests that bulk density-derived porosity datasets do not always allow for identification of the presence of reservoir-grade rocks in the Middle Member of the Bakken Formation. In contrast, NMR log data provide valuable information for characterizing those permeable porosity systems, and can be shown to accurately predict the spatial distribution of reservoir-grade rocks associated with commercial hydrocarbon production. By applying long-established techniques for calculating vertical average porosity and porosity-feet quantities from free-fluid porosity datasets, a suite of regional sub-surface maps has been constructed which reliably identify areas of high-grade Middle Bakken reservoir rocks. Further, free-fluid porosity maps can be employed to define both the spatial limits of commercial productivity and 'sweet spots' within the play, specifically if those maps can be integrated with pore pressure information and fluid saturation data.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012