Fractured, altered, and faulted basement in northeastern Oklahoma: Implications for induced seismicity
Introduction
Seismic activity in northern Oklahoma began to rapidly increase starting in 2008 and has been linked to wastewater disposal wells (Ellsworth, 2013; Keranen et al., 2013, 2014). These induced/triggered earthquakes occur along pre-existing faults in the crystalline basement (Keranen et al., 2013; McNamara et al., 2015; Kolawole et al., 2019), many of which remain unrecognized until rupture. Most of the earthquakes occur 2–3 km below the top of the basement even though wastewater is commonly injected into the overlying Cambrian-Ordovician carbonates of the Arbuckle Group (Keranen et al., 2013; Schoenball and Ellsworth, 2017). The dramatic increase in seismicity has led to renewed interest in the basement rocks of the area.
As exposures are non-existent aside from a small discontinuous granite outcrop in the town of Spavinaw (Ireland, 1930), direct observations of basement rocks and faults from northern Oklahoma are few and far between. Faults have therefore been primarily identified and studied via well logs or geophysical expressions such as offset of seismic reflectors and/or magnetic anomalies. Direct description of basement fault rocks has been limited to a few observations of sheared and cataclastic material from drill cuttings (Denison, 1966, 1981). Consequently, literature values for intact crystalline granite are commonly assumed for hydrological and mechanical modeling of the basement (e.g. Schoenball et al., 2018), sometimes with modification using empirical crustal permeability relationships (e.g. Barbour et al., 2017; Pollyea et al., 2019).
This study documents numerous features in northeastern Oklahoma basement rocks which are incongruent with the common assumption of intact crystalline granite. As matrix permeabilities of basement rocks are typically extremely low, hydrological properties are usually governed by fractures. Mineralized fractures are present in all recovered cores of basement rock from the area. Hydrothermal alteration changes the mineralogy of rocks and thus their mechanical properties (e.g., Callahan et al., 2019; Laubach et al., 2019), and this study indicates that all basement rocks of the area have been hydrothermally altered. Fault rocks are found in multiple cores as well, providing the closest available analog to the seismically-active faults. As basement rocks from the main seismically active areas are not currently available as core, sample locations in the current study are mostly east of the main area of seismic activity (Fig. 1), with the closest being approximately 45 km from the 2016 Mw 5.8 Pawnee earthquake.
Section snippets
Basement lithology
The basement rocks of the area consist of Mesoproterozoic granites and rhyolites, with minor andesites (Fig. 1, Fig. 2) and have been described and categorized into four groups by Denison (1966, 1981). On the basis of occurrence and mineralogy, three of these units are considered to represent a comagmatic volcanic-intrusive complex which forms most of the basement subcrop in the study area (Denison, 1981); recent geochemical data are consistent with this interpretation (Hamilton et al., 2020a).
Materials and methods
Materials for this study consist of eight azimuth-unoriented cores housed by the Oklahoma Geological Survey's Oklahoma Petroleum Information Center (OPIC) (Fig. 1, Table 1). These cores represent three of the four basement groups described by Denison (1966, 1981); core from the Central Oklahoma Granite Group was not available. Cored basement sections are typically short (<1 m to a few tens of meters) with the exception of the Amoco SHADS 4, which recovered over 140 m of basement section (Derby
Fractures
Every core of northeastern Oklahoma basement rock inspected contains fractures, nearly all of which (aside from those which are clearly artificial) contain secondary minerals. These fractures usually occur throughout the entire recovered sections – for instance, mineralized fractures are still common over 140 m below the top of basement in the Amoco SHADS 4. The vast majority of fractures appear to have formed as opening-mode fractures, though many show indications of subsequent shear (Section
Analysis and discussion
Mineralized fractures appear to be ubiquitous in the basement rocks of northeastern Oklahoma, with abundant evidence for multiple generations of fracturing and mineralization. A variety of origins are possible, given the age of the rock and what is known of the tectonic history. Some, particularly the epidote-bearing fractures, are likely nearly as old as the rock – for instance, the presence of saussuritized plagioclase (present in several intervals of the Amoco SHADS 4, and at least partially
Summary
Hydrothermal alteration and mineralized fractures are ubiquitous in the northeastern Oklahoma basement. Many of these fractures host multiple generations of minerals, with early silicates followed by carbonate and occasionally pyrite. Near-vertical fractures are present in virtually every core from the area, suggesting a high fracture density. In the only core with a large recovered interval, fracture density remains high well over 100 m below the basement unconformity – in fact, fracture
Author statement
M. Hamilton: Conceptualization, Investigation, Writing - Original Draft, Visualization. B.M. Carpenter: Investigation, Writing - Review & Editing, Supervision. C. Johnston: Investigation, Visualization. F. Kolawole: Investigation. S.C. Evans: Conceptualization, Funding acquisition. R.D. Elmore: Conceptualization, Investigation, Writing - Review & Editing, Supervision, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Funding was provided by a grant to R.D. Elmore and S.C. Evans from the Oklahoma Governor Emergency Seismicity Funds (Task 4.3) through the Oklahoma Geological Survey. Valuable assistance with core viewing and sampling was provided by the staff at OPIC. Colin Pennington, Jack Williams, Robert Holdsworth and two anonymous reviewers provided helpful comments on earlier drafts of this manuscript.
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