THE FRACTURES OF OKLAHOMA'S BASEMENT: FROM CORE TO REGIONAL SCALE (Invited Presentation)

The dramatic increase of widespread seismicity in Oklahoma from 2009-2016 has been attributed to the reactivation of pre-existing, critically-stressed, and seismically unstable faults due to decades of wastewater injection. 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. Two conditions required for pore-fluid pressure driven fault reactivation are the nature, distribution, & geometry of basement-hosted faults, and pathways for downward fluid pressure migration. In this study, we focus on pathways for fluid pressure migration by characterizing fractures and faults in crystalline basement material from the core to the regional scale. We document fracture spacing, orientation, and alteration in core samples and outcrops from northern and southern Oklahoma. At the regional scale, we document the orientation and upward continuation of seismically visible fractures (faults) in north-central Oklahoma via analyses of 3D seismic data. Our multidisciplinary analyses show: 1) the presence of pervasive vertical fractures in basement core from near the sediment-basement interface; 2) that many fractures, observed in both core and outcrop, are mineralized or otherwise altered, 3) that an outcrop scale strike-slip fault is characterized by densely-spaced parallel slip zones within a broader 260m-wide damage zone, and 4) that on the regional scale, Oklahoma’s buried igneous basement is riddled with pre-existing faults, many of which penetrate into the overlying sedimentary sequences, including the Arbuckle and Simpson groups. Our results document both fracture- and fault-related pathways for fluid pressure migration into the basement from the Arbuckle Group.