MR028-02
Basement Structure, Fluid Migration Pathways, and Mechanisms of Induced Fault Reactivation in U.S. Mid-Continent

Thursday, 17 December 2020: 04:04
Virtual
Folarin Kolawole, University of Oklahoma, Norman Campus, School of Geosciences, Norman, OK, United States, Brett M Carpenter, University of Oklahoma Norman Campus, School of Geosciences, Norman, OK, United States, Ze'ev Reches, University of Oklahoma, School of Geosciences, Norman, OK, United States, Estella A Atekwana, University of Delaware, Department of Earth Sciences, Newark, DE, United States, Micah Mayle, Colorado State University, Geosciences, Fort Collins, CO, United States, Rob L Evans, Woods Hole Oceanographic Institution, Woods Hole, MA, United States and Kerry Key, Lamont-Doherty Earth Observatory, Palisades, NY, United States
Recorded Presentation
Abstract:
The recent widespread seismicity of the Central and Eastern U.S. (CEUS) is attributed to the reactivation of pre-existing basement faults by wastewater injection. However, only limited details are known for basement structure and mechanical stability of basement faults.

We examined the patterns of basement fractures and faults across the Mesoproterozoic Granite-Rhyolite Province of U.S. Mid-Continent as a controlling factor of induced seismicity. We found a tectonic fabric with dominating inherited trends of NW-SE and NE-SW that are well-oriented for strike-slip reactivation in the current regional stress field. We tested the hypothesis that the triggering depth (>3 km) of most induced seismicity events is controlled by pore pressure from downward migration of the injected saline fluids. For this testing, we deployed a rapid magnetotelluric (MT) imaging of the 2016 Mw5.8 Pawnee earthquake. The imaging close to the mainshock hypocenter (4.5 – 5.6 km) revealed that the sedimentary cover and shallow basement (<2 km) are characterized by high/moderate conductivity, whereas the deeper basement displays low conductivity. These observations indicate that this fault reactivation was induced by remote stress/pressure changes, and not by direct fluid migration.

Further, we analyzed the basement fault zones and fracture networks the faults at/near the basement-sedimentary interface. The multiscale (outcrop, drone, & satellite) characterization of 2 km2 of exposed basement in southern Oklahoma reveals a ~260 m-wide fault damage zone with a distinct system of sub-vertical clusters of fractures trending ~245°. This fabric could serve as the fluid migration pathways into the basement.

Finally, we perform rock-mechanics testing of the frictional stability of the Oklahoma basement rock and associated mineral alteration samples (granite, rhyolite, altered calcite, and epidote) under temperature/pressure conditions relevant for depths up to 9 km. The tested samples became unstable (a-b < 0) for depth equivalent conditions of 3-5 km, which is the depth interval of most seismic events in Oklahoma. Our analyses quantify the inherited fabric and mechanical stability in the Mid-Continent basement that facilitates the seismic reactivation by regional fluid injection.

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