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SPWLA Distinguished Speaker Series. The Bakken petroleum system (BPS) can be considered a hybrid play because it is composed of both conventional and unconventional elements. The conventional aspects include the presence of separate reservoir intervals (Scallion, Middle Bakken, Sanish and Three Forks) and source rock intervals (Lower Bakken and Upper Bakken shales) along with more problematic intervals (Basal Bakken). This is in direct contrast to most unconventional shale plays, in which a single lithologic or stratigraphic interval comprises both the source rock and reservoir. The unconventional aspects of the BPS include very low permeability conventional reservoir sections, as well as combined shale-rich source and reservoir intervals. Additional complexity results from stacked depositional environments with significant variations in lithofacies, mineralogy, total organic carbon (TOC), and rock textures ranging from highly bioturbated to finely laminated.
Historically, development programs and petrophysical analyses in the Bakken were centered on a volume of shale calculated via deterministic models using triple combo log data that were focused primarily on the Middle Bakken reservoir. Production type-curves generated from such analyses showed reservoir recovery factors that were inconsistent with actual production data as well as knowledge of the reservoir. Additionally, rock mechanical properties used to model hydraulic fracturing performance and real-time measurements of microseismic events recorded during hydraulic fracturing indicated fracture height growth that extended into surrounding formations.
Based on these results, a series of science wells were drilled, cored over the entire BPS and logged extensively using advanced logging devices to better understand the overall system. Results from these wells provide a basis to refine production type curves and to re-calculate stock-tank oil originally in place (STOOIP). The formation evaluation program consisted of conventional triple combo logs supplemented with advanced downhole measurements including: (1) triaxial resistivity for thin-bed analysis; (2) nuclear magnetic resonance for porosity, free fluid and kerogen identification; (3) dielectric dispersion for water saturation; (4) geochemical spectroscopy for mineralogy and total organic carbon (TOC); and (5) dipole sonic for dynamic rock properties. Petrophysical models were developed using both deterministic and probabilistic methods to integrate the measurements acquired for analysis of porosity, saturation, and mineralogy, and describing the hydrocarbon production potential of the BPS more accurately. The advanced evaluation results will enable the development of computation models in areas of the basin where only minimal logging suites such as triple combo logs exist as data. Petrophysical models that encompass the entire BPS will be the basis for updated STOOIP calculations that can be used to revise production type curves and improve confidence in estimated recovery factors that have better agreement with measured production results.
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