By Adam Wilmoth

Using some of its strongest language to date, the Oklahoma Geological Survey said Tuesday the state's ongoing earthquake swarm is "very unlikely to represent a naturally occurring process."

The state survey said the suspected source of triggered earthquakes is the use of wastewater disposal wells that dump large amounts of water produced along with oil production.

"The observed seismicity of greatest concentration, namely in central and north-central Oklahoma, can be observed to follow the oil and gas plays characterized by large amounts of produced water," the report stated. "Seismicity rates are observed to increase after a time-delay as injection volumes increase within these plays. In north central and north-central Oklahoma, this time-delay can be weeks to a year or more."

The statement comes after a growing number of reports have pointed to the possible connection between water disposal wells from oil and natural gas production and the ongoing earthquake swarm. In February, the U.S. Geological Survey published a paper co-written by Oklahoma Seismologist Austin Holland that said the increase in earthquake activity in Oklahoma and other areas not usually susceptible to earthquakes was from human-induced activities, not natural processes.

Representatives from the state's oil and natural gas industry downplayed the report Tuesday.

"I don't see that this is terribly shocking," said Kim Hatfield, chairman of the regulatory committee at the Oklahoma Independent Petroleum Association (OIPA). "This is something the Oklahoma Geological Survey, Oklahoma Corporation Commission and OIPA have been working on for well over a year. We knew this was a possibility."

Hatfield emphasized the cooperation and collaboration between the industry and regulators over the past few years.

"Oklahoma's oil and natural gas producers have a proven history of developing the state's oil and natural gas resources in a safe and effective manner. That longstanding commitment to Oklahoma will continue as we work to develop greater understanding of Oklahoma's seismic events," he said.

Oklahoma Oil and Gas Association President Chad Warmington said more research is needed to see how disposal wells can be operated safely in the seismic areas.

"There may be a link between earthquakes and disposal wells, but we -- industry, regulators, researchers, lawmakers or state residents -- still don't know enough about how wastewater injection impacts Oklahoma's underground faults," he said.

Tuesday's state report pointed out that most of the recent earthquakes the state has experienced "have not occurred in the typical foreshock-mainshock-aftershock sequences that are characteristic of naturally occurring earthquake sequences throughout the world in a variety of tectonic settings."

The report went on to state, however, that naturally occurring earthquake swarms do occur and have occurred within the region.

Oklahoma has had a seismically active history. The state is crossed with thousands of fault lines and has experienced moderate and large earthquakes in the past. But the intensity of earthquakes has ballooned over the past few years.

The seismicity rate in 2013 was 70 times greater than the rate before 2008 and rapidly grew to about 600 times greater today, the report stated, saying such a rapid increase is "very unlikely the result of a natural process."

The Oklahoma Corporation Commission over the past few years has increased its focus on water disposal wells, especially in the areas where the recent earthquakes have been concentrated. The commission last month expanded the areas that come under extra scrutiny for wastewater disposal wells.

Commission spokesman Matt Skinner on Tuesday praised the geological survey for its work on the state's seismic swarm.

"The findings discussed in the latest OGS statement have already served as key components in the Oklahoma Corporation Commission's evolving approach to the issue of triggered seismicity, including the requirement of a seismicity review for all proposed disposal wells, further scrutiny of those proposed wells even after review, limited permitting for wells in certain areas, and most recently, an expansion of our 'traffic light' approach that, among other things, requires hundreds of Arbuckle disposal well operators to take action to mitigate the risk of triggered seismicity," Skinner said.

The commission has focused on disposal wells drilled into and through the Arbuckle formation, the deepest layer of sedimentary rock and a favored location for wastewater injection. Below the Arbuckle lies the crystalline basement, which is the top of the earth's crust. Oklahoma has 969 disposal wells in the Arbuckle formation.

Well operators in the commission's expanded areas of interest must either prove their wells do not extend into the basement or plug the part of the well that extends too deep.

The new regulations have flowed from a better understanding of the science, said Michael Teague, Oklahoma's secretary of Energy and Environment.

"It's an evolution on the science side of this and not just from the Geological Survey but the USGS as well," Teague said. "You see the evolution on the science, and you're seeing the evolution on a regulatory level with what the Corporation Commission is doing. We're trying to make sure the public information keeps up with those changes. It really is unprecedented across the country."

The average oil well in Oklahoma produces about 10 barrels of saltwater for for every barrel of oil. That ratio can be much higher in parts of north-central Oklahoma. For at least 70 years, the oil and natural gas industry has pumped the produced water deep below ground through water disposal wells.

Some recent studies have attributed at least part of the state's ongoing earthquake swarm to particularly high-volume disposal wells.

The Oklahoma Geological Survey on Tuesday said the high volumes and the characteristics of the Arbuckle formation could allow pressure from the disposal wells to be transmitted several miles from an injection site.

"The high density of injection wells in central and north-central Oklahoma combined with the high permeabilities within the Arbuckle makes identifying relationships between specific wells and seismic activity difficult," the report stated.

(c)2015 The Oklahoman