Origin of brecciated intervals and petrophysical analyses: the Three Forks Formation, Williston Basin, North Dakota, U.S.A.

Abstract

The Three Forks Formation of the Williston Basin in North Dakota is currently of interest to the petroleum industry and can be subdivided into three units: upper, middle, and lower. The Three Forks is Devonian in age and lies stratigraphically below the Bakken Formation. The upper 50ft of the Three Forks is included in the Bakken Petroleum System. The upper and middle units of the Three Forks are the focus of this study. These units can be subdivided into four lithofacies: (1) a green to gray or red mudstone, (2) a disrupted to mottled, interbedded silty dolostone and mudstone, (3) a silty, tan dolostone, (4) and an interbedded silty dolostone and mudstone. The middle Three Forks consists of a deepening upwards sequence capped by a basin wide mudstone marker. The upper Three Forks represents a shallowing upwards sequence capped by an intertidal lithofacies. The Three Forks contains environments ranging from supratidal to a restricted, basinal mudstone. The Three Forks, primarily the middle, contains multiple brecciated units that are predominantly the result of reworking of sediments through storm influence. These brecciated units are most commonly found in lithofacies TF2 and TF3, and grade from intraclasts in disrupted dolostone beds to floating clasts in mudstones; however the clasts themselves rarely exhibit any grading. In addition to storm reworking, there are brecciated units in the upper Three Forks that appear to be the result of dissolution collapse. The Three Forks is a petrophysically complex unit to evaluate, as it contains thin bedded sequences, clays, and conductive minerals that all contribute to inherently low resistivity values. To evaluate water saturation in the Three Forks, all the input variables are analyzed and compared to core data. The density porosity curve demonstrates a favorable comparison to core data when a matrix density of 2.78 g/cm3 is applied. Resistivity is corrected for the presence of pyrite, and other variables are determined using the triple porosity model. All of these inputs lead to the development of a pseudo-Archie equation for evaluation of fluids within the upper and middle Three Forks that compares favorably to core saturations. In addition to water saturation, a Vsh log is created which demonstrates that the upper Three Forks contains a lower mud content than the middle Three Forks. Data from core indicates that the middle Three Forks has slightly higher porosities and permeabilities, 6.9% and 0.198md, than the upper Three Forks, 6.4% and 0.070md. However, fluid saturations from core and log analyses indicate that the upper Three Forks has a higher hydrocarbon potential. Regional mapping of the petrophysical attributes and isopachs reveal that the hydrocarbon potential of the Three Forks is stratigraphically constrained. Thickness of the source rock, the Lower Bakken Shale, and stratigraphical vertical distance from the source are the most important factors for hydrocarbon potential within the upper units of the Three Forks. While stratigraphical constraints are the most dominant factor, hydrocarbon potential can be enhanced by structure (i.e. Nesson Anticline and Billings Nose).