Abstract

Objectives/Scope:

The goal of the work is to present a depositional and reservoir model for the Three Forks Formation of the Williston Basin. Analyzing the stratigraphy, core oil and water saturations, and petrophysical analysis aid in model creation. The Three Forks is a silty dolostone throughout much of its stratigraphic interval. The problem of reserve calculation is solved by an integrated approach.

Methods/Procedures/Processes:

Methods include analyzing core data and performing core descriptions. In addition, petrophysics (unconventional log analysis) including nuclear magnetic logs and pulsed neutron logs help delineate productive zones in the Three Forks Formation. Mineralogy is assessed with logs, XRD, and XRF data. Source rock maturity of the lower Bakken shale also helps define the Three Forks producing fairway.

Results/Observations/Conclusions:

The Bakken-Three Forks petroleum system is characterized by low-porosity and permeability reservoirs in the middle Bakken, Pronghorn, and Three Forks, organic-rich source rocks (lower and upper Bakken Shales), and regional hydrocarbon charge. The Three Forks has an intimate relationship with source rock maturity in the lower Bakken shale.

Reservoir facies, source rock maturity, and overpressure are key geologic factors for success in the Three Forks. Important technological factors include lateral length, lateral direction, fracture stimulation stages, amount of proppant, type of fracturing fluid, etc.

Migration of hydrocarbons from thermally mature lower Bakken shales is downward into the underlying Three Forks Formation.

Applications/Significance/Novelty:

The Three Forks producing area is a continuous accumulation. The limits of the play are expanding with step out drilling and existing fields have merged.

Downward migration of hydrocarbons can be significant in unconventional plays.

Interdisciplinary Components:

Geology and petrophysical approach are needed to characterize Three Forks reservoirs. Core analyses and descriptions combined with NMR and ECS logs help delineate the reservoir for accurate oil in place calculations.

Introduction

The Mississippian-Devonian Bakken Petroleum System of the Williston Basin is characterized by low-porosity and permeability reservoirs, organic-rich source rocks, and regional hydrocarbon charge. The unconventional play is the current focus of exploration and development activity by many operators. The Bakken petroleum system is a giant continuous accumulation in the Williston Basin (Sonnenberg et al., 2017; Sonnenberg, 2020).

The structure of the Williston Basin at the top of the Three Forks is illustrated by Figure 1. The basin is semi-circular in shape and prominent structural features are the Nesson, Billings, Little Knife, Poplar and Cedar Creek anticlines. Many of the structural features have a documented ancestral origin and influenced Paleozoic sedimentary patterns (Gerhard et al., 1990). Recurrent movement on Precambrian faults or shear zones is seen elsewhere in the Rocky Mountain region (Weimer, 1980). The Nesson anticline is the location of the first oil discoveries in the 1950s. The first oil production on the Nesson anticline was from the Silurian Interlake Formation in 1951 and subsequent oil production was established from the Mississippian Madison Group.

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