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Fracture system of non-marine sedimentary rock and effects on hydraulic properties: Brule Formation as an example
Abstract
The Brule Formation consists dominantly of non-marine siltstone with subordinate ash beds and sandstone lenses. As an aquifer, groundwater transport is mainly attributed to highly conductive secondary permeability in the form of fractures and partings. In the Brule Formation, horizontal and sub-horizontal fractures developed with a simple pattern. Most are parallel to lithologic boundaries and have a zonal appearance. Vertical and sub-vertical fractures developed into more complex patterns and are either non-systematic or systematic. The systematic vertical and sub-vertical fractures include: (1) fractures of NW-SE and NE-SW trend occurring throughout the southern panhandle of Nebraska; (2) fractures of NNW-SSE and ENE-WSW trend occurring in the area north of the Lodgepole Creek valley, particularly along the North Platte River valley; (3) fractures of WNW-ESE and NNE-SSW trend occurring primarily along the Lodgepole Creek and the Sidney Draw valleys. Stress field analysis indicates three stress fields, each with different characteristics, were developed after the deposition and lithification of the Brule Formation. These fields are believed to be associated with episodic Cenozoic uplift movements in western Nebraska. A common characteristic of all three is that they have a nearly vertical main stress axis with two nearly horizontal main stress axes of different orientation. The horizontal main stress axes of the stress fields appear to have rotated counter-clockwise from NNW-SSE to WNW-ESE. These fields produced three groups of high-angle shearing and extension fractures respectively, with dominant orientations of NNW-SSE, NW-SE and WNW-ESE. Primary permeability in the Brule siltstones accounts only for less than thirty-five percent of the bulk permeability. Secondary permeability is mainly, if not entirely, attributed to fractures and/or fracture networks. Specific storage $\rm (S\sb{s})$ values of this study are consistent between $1.60\times 10\sp{-3}$ to $\rm 2.70\times 10\sp{-3}\ m\sp{-1}.$ Values of specific yield $\rm (S\sb{y})$ are between 0.011 and 0.044. The dissolution of soluble materials, such as carbonate, is a very significant process involved in groundwater flow in Brule rocks. It was observed in outcrop that carbonate material was being dissolved from the rock matrix and moved into and along the fractures. Subsequent dissolution of soluble materials from these fracture planes produced large inter-connected channels for groundwater flow.
Subject Area
Geology|Hydrology|Environmental science
Recommended Citation
Wang, Song-Tao, "Fracture system of non-marine sedimentary rock and effects on hydraulic properties: Brule Formation as an example" (1998). ETD collection for University of Nebraska-Lincoln. AAI9826106.
https://digitalcommons.unl.edu/dissertations/AAI9826106