On the Depth of Earthquakes in the Delaware Basin: A Case Study along the Reeves–Pecos County Line

Yixiao Sheng,Karissa S Pepin,William L Ellsworth

doi:10.1785/0320210048

Abstract

Abstract Increased rates of seismicity in the Delaware basin, Texas, accompanying unconventional petroleum development have created intensive interest in determining their cause. Detailed and accurate spatial distribution of seismicity and focal mechanisms are critical components for understanding the underlying industrial processes responsible for inducing seismicity. We focus on a highly seismically active area straddling the Reeves–Pecos County line where two TexNet stations sit atop the seismicity, which includes 21 ML 3+ events from 2017 to 2020 (Advanced National Seismic System Comprehensive Earthquake Catalog). Short epicentral distance enables us to reliably estimate the hypocentral depth using seismic phase picks and standard location methods. We use a deep-learning-based method to detect earthquakes and time the phase arrivals. Hypocentral locations computed in a velocity model constrained by local well data reveal that the seismicity concentrates between 1.5 and 2.5 km below ground in the Delaware Mountain Group, the primary wastewater disposal zone at this location. Waveform inversions for the moment tensor and focal depth independently confirm the shallow depths. The moment tensor solutions define critically stressed high-angle normal faults, suggesting a causal connection between injection and seismicity.

Highlights

The Delaware basin of west Texas and southwestern New Mexico comprises the western portion of the Permian basin, and contains some of the deepest sedimentary deposits in the world
It is commonly accepted that the earthquakes are induced by oilfield activity, the mechanisms responsible for Delaware basin seismicity are under debate
The study area contains multiple horizontal production wells (∼3.6 km below the ground surface), all located below the depth of seismicity, we found no evidence of changes in seismicity coincident with the hydraulic fracturing of the eight wells completed during the study period

Summary

Introduction

The Delaware basin of west Texas and southwestern New Mexico comprises the western portion of the Permian basin, and contains some of the deepest sedimentary deposits in the world It is one of the largest oil and gas fields in the world and, as of 2020, was producing over 4 million barrels per day of oil (U.S Energy Information Administration Drilling Productivity Report 2/2021). It is commonly accepted that the earthquakes are induced by oilfield activity, the mechanisms responsible for Delaware basin seismicity are under debate Both hydraulic fracturing during well stimulation and wastewater injection have been proposed (Lomax and Savvaidis, 2019; Deng et al, 2020; Skoumal et al, 2020; Dvory and Zoback, 2021; Zhai et al, 2021)

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