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The Curious Case of Hydrocarbon-Expulsion Fractures: Genesis and Impact on the Bakken Shales


The genesis and impact of hydrocarbon-expulsion fractures on the Bakken shales were investigated by integrating epifluorescence petrographic and pressure transient analyses. In early exploration vertical wells, drilling breaks from ten minutes to one minute per foot with gas increases from ten units to several hundred units were considered poor shows in the source rocks of the Bakken shales. Drill Stem Tests (DST) over the Bakken shales were reported with average production rates that reach several tens of barrels per day. These shales were overpressured with no matrix porosity in evidence and permeability in the micro Darcy scale. Production was assumed to come from fractures. Speculative conclusions were drawn about these fractures to be related to source-rock maturity, hydrocarbon expulsion and overpressuring. These conclusions were a significant promoter for exploration in the Bakken Formation. The curious case of hydrocarbon-expulsion fractures has encouraged to review a total of 64 Drill Stem Tests (DST) over different intervals that include the Bakken Formation and/or the underlying three Forks Formation. Resistivity logs, cores and thin sections were studied to conduct an integrated geological interpretation for pressure transient behaviors of the Bakken shales. The study highlights that the Bakken shales are naturally fractured and can be interpreted on resistivity curves separation. The Three Forks and Middle Bakken pressure transient behaviors show spherical flow which indicate that there is always contribution from the Bakken Shales. The Bakken pressure transient behavior shows dual porosity flow (naturally fractured) with low fracture system's storetivity (?) implying that fluid is mostly stored in the matrix. The study also revealed that the matrix gives up its fluid rapidly to the fracture system indicating high interporosity flow (?) and implying that hydrocarbon-expulsion fractures contribution is present. The significance of hydrocarbon-expulsion fractures resides in its ability to provide higher permeability pathways through the Bakken shales, and explains their high deliverability. The volume expansion due to hydrocarbon generation is invoked as a mechanism to increase pressures to levels of inducing expulsion fractures responsible for primary migration of hydrocarbons.