The number of large earthquakes to rattle Oklahoma has declined so far this year, but experts aren't sure how much of the decline should be attributed to actions taken by state regulators, operators, a drop in wastewater injection, or a combination of all of those factors.

Oklahoma Geological Survey Director Jeremy Boak told NGI's Shale Daily that at the end of last week, 403 earthquakes measuring 3.0 magnitude or higher on the Richter scale had struck the state so far this year. By comparison, there were between 475 and 480 such temblors during the first six months of 2015, meaning the number of earthquakes has declined 15.2-16%.

"We're nearly 100 [earthquakes] below where we were at this time last year," Boak said Tuesday. "It's interesting because the year started off with a bang, but then [earthquake activity] actually slowed down steadily from about February or early March onward. We crossed somewhere in May."

The "bang" started in January, after a series of earthquakes, including two that measured 4.4 and 4.8 in magnitude on the Richter scale, struck the Fairview, OK, area (see Shale DailyJan. 14). At the time, the Oklahoma Corporation Commission (OCC) said the quakes may have been triggered by a power outage created by a winter storm, and ordered operators of 27 wastewater injection wells to reduce their disposal volumes by about 18%.

Last February, the OCC's Oil and Gas Conservation Division (OGCD) unveiled its Western Regional Reduction Plan, which called for a nearly 500,000 b/d reduction in wastewater injection volumes (see Shale DailyFeb. 17). One month later, the agency released its Central Oklahoma Volume Reduction Plan, which called for a 300,000 b/d reduction in injection volumes (see Shale DailyMarch 7).

Boak said the drop in injection volumes was the primary reason for fewer earthquakes.

"We've got more than 1 million b/d less injection than we had in 2015," Boak said, adding that injection volumes actually crested in late 2014. "As far as I can tell, it's cresting ahead of the directives from the OCC. Some of [the reduction] must be market driven.

"At one point I thought I could show that roughly three-quarters of the reduction we have seen was market driven. But now that the OCC has incorporated large areas [into its area of interest, AOI] and has applied some larger cuts, I have a feeling it may harder to distinguish [where fewer earthquakes] came from. They may well be coming essentially on top of a market driven drop. We may have a hard time determining how much, but we may be able to at least say there's a reduction beyond what the OCC asked for."

Boak said if someone believes the increase in seismic activity is a direct result of increased wastewater injections, "then you have to accept that the decrease [in seismic activity] is due to the decrease in injection." But he added that there have also been operational changes.

"There's been a small reduction in crude oil production in the state, but keep in mind we've got other shale plays in the state coming into play," Boak said. "I don't think [operators are] trucking this water somewhere else because that's expensive. 70% of injection is still going on within the AOI, it's just that they've shut in a lot of production.

"Some piece of that is likely due to market forces even ahead of the request of the OCC. In essence, they may have gotten away with a substantial reduction that was already starting when the OCC asked for specific reductions."

Cliff Frohlich, associate director of the Institute for Geophysics at the University of Texas at Austin (UT), told NGI's Shale Daily that while he agreed the number of earthquakes of 3.0 magnitude or higher is down from 2015, he isn't sure of the reason.

"It could be because of the OCC actions, or because operators have modified their procedures, or simply because energy prices are low and development of fields is less aggressive this year," Frohlich said Tuesday.

Last May, a UT and Southern Methodist University study led by Frohlich found oil and gas activities over the past four decades are probably linked to at least 59% of induced earthquakes in Texas (see Shale DailyMay 18).

Chad Warmington, president of the Oklahoma Oil & Gas Association (OOGA), said the oil and gas industry supports and is complying with the OCC's directives.

"These directives are based on researchers' determinations for best practices as it relates to injection," Warmington said Tuesday. "Our industry has worked with researchers on this issue by providing costly and typically proprietary 3-D seismic data to help regulators in their study of Oklahoma's subsurface.

"Thanks to this sharing of data, the decisions of the Oklahoma Corporation Commission in addressing this issue have been based in science and research...Additionally, Oklahoma's oil and gas operators have also been extensively researching, testing and involved in government review of efforts to determine feasible options for recycling and reuse of produced water across the state."

According to Boak, earthquakes from injection wells in Oklahoma are not being triggered by water used to perform hydraulic fracturing, which is mostly freshwater (see Shale DailyJan. 5). Rather, they are being triggered by extremely salty water that is a byproduct of oil and gas production. Oil and gas drilling in Oklahoma also targets limestone formations -- like the Mississippian Lime and the Hunton Formation -- which contain more water.

"If [operators] can realize some savings, and if there are alternative horizons to inject into, it may still be possible to make these very wet plays productive," Boak said. "I get the feeling that, at least for a while, a lot of energy is going into other [resource] plays."

The most recent earthquake of 3.0 magnitude or higher struck Oklahoma on Tuesday. A temblor measuring 3.5 magnitude struck 25 kilometers (15.5 miles) west of Perry, OK, at 12:21 p.m. CDT. The epicenter of the quake was 8.2 kilometers (5.1 miles) below the surface.

"I'm inclined to say industry and the OCC deserve credit for a change in the injection volumes," Boak said. "We'll just have to wait and see. It will be very tricky [to analyze earthquake data] if oil prices come back up and there's some pressure to start producing more and therefore injecting more."