THE FRACTURES OF OKLAHOMA’S BASEMENT: GEOMETRY, ALTERATION, AND IMPLICATIONS FOR INDUCED SEISMICITY

The widespread seismicity experienced in Oklahoma, from 2009-2019, was attributed to the reactivation of pre-existing, critically-stressed, and seismically unstable faults due to decades of wastewater injection and temporally increased disposal rates. The vast majority of the noted earthquakes occurred in the Precambrian igneous terranes that underlie much of Oklahoma, whereas the majority of injection, during that time period, occurred in the Arbuckle Group. Currently, while seismicity is significantly reduced compared to its peak levels, it remains elevated. This ongoing seismicity, combined with interests for CCS and low-enthalpy geothermal projects in the state of Oklahoma, requires continuing characterization of Oklahoma’s crystalline basement. This study focuses on the structural and geochemical characterization of a number of basement cores from NE Oklahoma. The main results of these combined studies show: 1) the presence of pervasive vertical fractures in basement core from near the basement-sediment interface; 2) abundant fracturing and alteration of basement material throughout the cored intervals; 3) the documentation of multiple hydrothermal alteration events; 4) evidence for fluid movement in the basement from shallower and deeper depths; and 5) the ubiquitous presence of faults. Overall, the observations of extensive fracturing and hydrothermal alteration, combined with the widespread occurrence of faults raises concerns about the stability of fault zones at shallower depths. Documenting the structural features and mineralogical changes in the seismogenic basement rocks provides valuable insights into the thermo-tectonic history of the basement and can contribute to improved seismic hazard assessments in the region as it relates to wastewater injection, CCS, and geothermal production.