Thesis
My M.S. thesis at WVU was a petrographic study of the Bakken Formation middle member from two cores (~30 thin sections) associated with the Nesson anticline in North Dakota's Williston basin.
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The goal was originally to explore Bakken petroleum system models and literature (open vs. closed-fluid systems) by petrographically assessing horizontal microfractures previously observed within the Bakken middle member. I was, and still am, especially interested in Leigh Price's unpublished manuscript (2001), and the proposed petroleum system model and associated mechanisms (like horizontal microfracture propagation) therein.
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Download the .pdf of my thesis.
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While I used SEM-EDS, XRD, and thermogravimetric TOC analysis, epifluorescence provided the most useful observations in my study.
My research was funded through the ORISE professional internship program, at the DOE/NETL in Morgantown, West Virginia.
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Here is the abstract for a paper that was submitted to the Rocky Mountain Association of Geologists' Bakken Guidebook:
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ABSTRACT
Open, uncemented horizontal microfractures observed in the Bakken Formation’s middle member are often interpreted as having been induced by coring or core storage. Another less common interpretation is that these HMFs were hydraulically propagated by overpressured hydrocarbons during Bakken upper and lower member hydrocarbon generation. The latter interpretation implies that HMF networks were present in the subsurface prior to coring and are therefore may be fracture-porosity hydrocarbon reservoirs.
Petrographic analysis, including epifluorescence, of Bakken middle member thin-sections from two cores at oil-window maturity was undertaken (Warner, 2011); the results of which are synthesized in this paper. Some horizontal microfractures exhibited extensive submicroporosity networks emanating approximately 150 µm above and below the microfracture aperture, interpreted to be secondary in nature. Such secondary porosity development is unlikely to occur during coring or core storage; it is much more feasible that such development required temperatures, fluid flux, and geologic time more likely in the subsurface. These submicroporosity-enhanced, open, uncemented horizontal microfractures (HMFs) were most abundant in dolomitic rock and were rare or absent in calcite-dominated lithology, suggesting subsurface propagation because dolomite is only significantly more brittle than limestone at subsurface temperature and pressure conditions. Additionally, HMFs were found to be most abundant within 10 feet of organic-rich source rocks, suggesting hydrocarbon fluid overpressure may have been the driving force for HMF propagation.
Extreme organic-richness of Bakken source rocks, the stratigraphic location of the Bakken between effectively impermeable limestones, the structural simplicity of the Williston basin, and Late Cretaceous Laramide orogeny far-field tectonic horizontal stresses contemporaneous with Bakken oil generation each also provide explanation for a subsurface HMF propagation mechanism.
HMFs require reevaluation of proposed horizontal microfracture propagation mechanisms, as well as Bakken petroleum system dynamics.
They say a picture is worth a thousand words - here are a couple photomicrographs which I took for my thesis:
**IMAGE**
Plane polarized transmitted light photomicrograph of apparently nonporous silty dolomicrite. Faint horizontal microfractures can be seen in blue. These are representative of the open, uncemented horizontal microfractures seen in Bakken middle member cores.
**IMAGE**
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This is the same view as above, but in epifluorescence. Wide-band UV excites the fluorochrome in the epoxy filling the porosity (fracture, intergranular, intragranular, etc.), causing it to fluoresce bright blue. This fluorescence reveals a well-connected horizontal microfracture network with submicroporous porosity emanating above and below them! This type of porosity suggests secondary development, which can probably rule out its generation syn- or post-coring.