Semester

Spring

Date of Graduation

2011

Document Type

Thesis

Degree Type

MS

College

Eberly College of Arts and Sciences

Department

Geology and Geography

Committee Chair

Richard Smosna.

Abstract

The Devonian-Mississippian Bakken Formation of the Williston basin does not outcrop. All rock samples are obtained by coring. Open, uncemented, horizontal mode I (joints, with no sense of shear) microfractures (with apertures typically 5 to 25 microns) in the Bakken Formation's middle member have debated origins: natural or induced by coring. Samples were taken from two cores associated with the Williston basin's Nesson anticline, and horizontal microfractures were characterized, counted, and compared with geologic variables to determine what controls, if any, existed. Results support the theory that some Bakken middle member horizontal microfractures propagated naturally in the subsurface.;The majority of horizontal microfractures were clearly induced, exhibiting shear morphology. A minority of horizontal microfractures, all mode I joints with no sense of shear, were observed to have submicroporosity (less than 1 micron wide; effectively not resolvable with standard transmitted light optics) emanating approximately 150 microns above and below the microfracture apertures. Such horizontal microfractures with submicroporosity zones are termed HMFs in this study. Epifluorescence microscopy was necessary to visualize microporosity-associated submicroporosity. If secondary, submicroporosity would likely have required elevated temperatures, fluid flux, and geologic time to have generated. Coring alone is unlikely to have generated such submicroporosity; subsurface propagation of HMFs explains the observation better.;Three observations further suggested HMFs were propagated in situ. First, HMFs were found to be most abundant in dolostone lithology, and were rare or absent in calcite-dominated lithology. The association with dolomite suggested subsurface propagation because dolostone is more brittle than limestone only at subsurface temperature and pressure conditions. If HMFs had propagated as a result of coring, calcite and dolomite lithologies would have experienced similar microfracturing, and no disparity would have been observed. Second, HMFs were most abundant within a ten-foot proximity of a source-rock. The working hypothesis that in situ HMF propagation was driven by hydrocarbon generation and expulsion explains the source-rock proximity trend. Third, HMFs were most abundant where macroporosity (pores resolvable with standard transmitted light optics) was least. This either suggests the existence of macropores prevented microfracture propagation, or the minerals leached during HMF secondary submicroporosity development precipitated in, and closed, nearby macroporosity. No evidence was found to validate either explanation; however, the latter suggests in situ processes.;The Bakken was buried to the oil window in the Late Cretaceous. Also during the Late Cretaceous, the Nesson anticline underwent growth and uplift, evident from sediment isopach maps in the Williston basin; far-field horizontal stresses from the Laramide orogeny are thought to be responsible. A maximum horizontal stress regime concurrent with Bakken oil generation and expulsion could account for HMF propagation as their observed characteristics suggest; superlithostatic fluid pressures are not necessary to explain the propagation.

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