Structural characteristics of shallow faults in the Delaware Basin

Abstract

The Delaware Basin of Texas and New Mexico is experiencing elevated levels of seismicity. There have been more than 130 earthquakes with local magnitudes of at least 3.0 recorded between 2017 and 2021, with earthquakes occurring in spatiotemporally isolated and diffuse clusters. Many of these events have been linked to oilfield operations such as hydraulic fracturing and wastewater disposal at multiple subsurface levels; however, the identification and characterization of earthquake-hosting faults have remained elusive. There are two distinct levels of faulting in the central region of the basin where most earthquakes have been measured. These fault systems include a contractional basement-rooted fault system and a shallow extensional fault system. Shallow faults trend parallel to and rotate along with, the azimuth of S HMAX, are vertically decoupled from the basement-rooted faults, accommodate dominantly dip-slip motion, and are the product of more recent processes including regional exhumation and anthropogenic influences. The shallow fault system is composed of northwest–southeast-striking, high angle, and parallel trending faults which delineate a series of elongate, narrow, and extensional graben. Although most apparent in 3D seismic reflection data, these narrow elongate graben features also are observed from interferometric synthetic aperture radar (InSAR) surface deformation measurements and can be delineated using well-located earthquakes. In contrast to the basin-compartmentalizing basement-rooted fault system, shallow faults do not display any shear movement indicators, and they have small throw displacement given their mapped length, producing an anomalous mean throw-to-length ratio of 1:1000. These characteristics indicate that these features are more segmented than can be mapped with conventional subsurface data. Much of the recent seismicity in the south-central Delaware Basin is associated with these faults and InSAR surface deformation observations find that these faults also may be slipping aseismically.