Patterns of Induced Seismicity in Central and Northwest Oklahoma

Abstract

Oklahoma experienced an average of 1.6 earthquakes of Magnitude 3 or greater (M3.0+) from the 1980s through 2008. Since that time, seismicity has increased to 907 M3.0+ earthquakes in 2015. More than 95% of these earthquakes occur over only ~17% of the area of Oklahoma. This pattern is generally attributed to increased injection of saline formation water co-produced along with oil and gas into the underpressured and relatively permeable Arbuckle Group, which lies directly on top of Precambrian crystalline basement (for example, Walsh and Zoback, 2015). Pressure communication from the Arbuckle to faults in the basement is interpreted to have reduced effective normal stress on the faults. This stress reduction allows faults aligned favorably with respect to the stress field in Oklahoma (SHMax = N 85° E) to move. The rise in seismicity began in about 2009, with 20 M3.0+ earthquakes, rose to 41 in 2010, and to 67 in 2011 (including the M5.7 Prague earthquake). The number dropped to 35 in 2012, then rose to 109 in 2013, to 579 in 2014, and to 907 in 2015. Oil price drops in 2014 and 2015 appear to have lead to a decrease in injection of co-produced formation water, beginning in late 2014. This in turn led to a decrease in earthquake frequency in 2016, with 508 M3.0+ earthquakes as of September 2016 (suggesting a year end value at or below 650). In the meantime, actions by the Oklahoma Corporation Commission have called for cuts in injection equal to most of the reductions initially driven by economic forces. On the other hand, in 2016, two M5.0+ earthquakes (Galena Township M5.1; Pawnee M5.8 – the largest quake in Oklahoma history) have shaken the state. These two ensure that seismic energy released in Oklahoma will be larger in 2016 than in any other year on record. This paper will discuss the evolution of this seismicity, the regulatory actions taken to reduce seismicity by reducing deep injection, and the importance of declining oil price in reducing injected volumes in advance of full implementation of these regulatory directives. It will also discuss regional variations within the earthquake area, and some aspects of frequency-magnitude relationships in the earthquake catalog.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017