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Observations of UV Fluorescence in Middle Bakken, Sanish and Three Forks Reservoirs, Williston Basin, North Dakota, Part 1. Leveraging Core Fluorescence to Visualize and Estimate Oil-In-Place

Abstract

In tight reservoirs such as the Middle Bakken-Sanish-Three Forks, where accurate quantification of porosity, permeability, and oil saturation can be difficult to ascertain; core fluorescence provides a critical visual representation of oil storage within these reservoirs and a means to calibrate reservoir characterization efforts. The basic observations are oil accumulations in the Middle Bakken-Sanish-Three Forks fluoresce from blue to locally yellow and the distribution, intensity, and color of this fluorescence in core presents a direct, detailed, yet qualitative estimation of oil storage (Oil-In-Place). However, when utilizing fluorescence, it is critical to distinguish between blue-yellow oil fluorescence and dull gold mineral fluorescence whose presence represents a lack of oil. Additionally, it is important to appreciate that mineral fluorescence can also be muted by the presence of clay, calcite-rich cements, and iron-rich dolomite. Core fluorescence, allied with the analysis of extract samples, reveals a conventional, although tight, reservoir character where oil storage is a function of pore space (porosity and permeability) and saturation. Variability in this oil storage is reflected in the distribution, intensity, and color of the oil fluorescence. Specifically, pore space is controlled by grain size and cementation. For the Middle Bakken and Sanish, this means sandy facies have the greatest propensity for pore space with considerably less in silty and argillaceous facies. Similarly, for the Three Forks, the pore space resides in dolosiltite rather than the finer grained dolomudstone. Importantly, it must be noted that diagenesis can have a dire effect on preservation of pore space especially due to the presence of an early generation of patchy calcite cement that sporadically occludes porosity. In prospective areas, oil storage is primarily a function of pore space presence with sandy or dolosiltite facies without calcite patches being oil fluorescent and bearing accumulated oil. Convolved with pore space, the saturation component of oil storage is also expressed in the oil fluorescence. Whereas in prospective areas where oil storage is primarily a function of pore space, in marginal or non-prospective areas oil storage is impacted by an absence of oil saturation reflecting a challenge to charge efficiency and/or trapping capability. In these areas, despite the presence of reservoir-quality facies, accumulated oil is lacking.