CHARACTERIZATION OF PRE-EXISTING STRUCTURES IN THE BASEMENT OF OKLAHOMA WITH IMPLICATIONS FOR INDUCED SEISMICITY

Recent widespread seismicity in Oklahoma, attributed to high-rate wastewater injection, has been associated with previously unknown faults in the Precambrian igneous basement. We analyzed the Oklahoma Precambrian basement structure via characterization of the dominant structural trends on multiple scales: satellite images, field outcrops and 3-D seismic reflection. The exposed Precambrian basement only cover a 305 km² area in southcentral Oklahoma. Our outcrop and satellite mapping in this area revealed widespread fracture and fault systems with predominant bimodal sets of NW-SE and NE-SW trends. These mapped trends display a striking similarity to the unexposed trends of present-day earthquake lineaments and fault-plane solutions in Oklahoma. We have identified two styles of discontinuity zones within the exposed basement: (1) Zones in coarse-grained granite display sparse fracturing with discrete zones of intense damage that include a predominant fracture set with cross-cutting minor sets, which together form rhombohedral blocks. We noted systematic increase of fracture density toward the zone core and a multi-scale, dense pattern of anastomosing fractures with local gouge development; (2) Zones in fine-grained granite display overall dense fracturing in which dominant fracture sets are conjugate in orientation, overprinted with other minor fracture sets. Our analysis of 3-D seismic data revealed subsurface basement faults in the area of seismicity revealed steeply-dipping faults that splay into the sedimentary cover forming ‘flower structures’. These faults may connect intra-sedimentary zones of fluid injection to the basement. In summary, our observations reveal dominant NE-SW and NW-SE fault trends that serve as a weak tectonic fabric within the Precambrian basement of Oklahoma. We envision that the observed faults from seismic data, and fracture patterns in the exposed basement could define pathways for fluid diffusion between the sedimentary cover and the basement.