Late last week, the region’s largest earthquake on record rattled the heart of South Texas oil and gas country, shaking the ground 47 miles southeast of San Antonio in Atascosa County and sending mild tremors as far north as Burnet. But the 4.8 magnitude earthquake also stirred up questions over oilfield activity and whether new processes involved could spark significant, damaging quakes in the future.
From New York State to Texas, critics of natural-gas drilling in oil-rich shale formations, using a process known as hydraulic fracturing or “fracking,” have claimed the method, which injects massive amounts of chemical-laden water thousands of feet below the surface, contaminates local groundwater, damages the health of local communities, and may even cause increased seismic activity. While industry insists fracking hasn’t been linked to any instances of groundwater contamination (even though a laundry list of cases test the claim), a growing number of reports link the process to minor earthquakes.
Within hours of Thursday’s South Texas quake, British researchers announced two minor earthquakes in northwest England with magnitudes of 2.3 and 1.5 “appeared to correlate closely” with fracking in the region. The Army Corps of Engineers has even requested a 3,000-foot buffer zone for drilling near dams, worried that fracking near fault lines could case earthquakes and geologic shifts that could weaken dams.
Central Arkansas, teeming with oil and gas exploration, saw an outbreak of roughly 1,200 earthquakes near an active fault line in 2010 and 2011 — including one magnitude 4.7 — although University of Memphis seismologist Stephen Horton, who studied the Arkansas quakes, said fracking wasn’t to blame, not directly. Rather, he found the injection of chemical-infused fracking wastewater, also known as fracking mud, deep underground was stressing an already active fault. “We saw the earthquakes getting larger and more intense, the seismic activity there was growing,” he said. Some companies voluntarily shut down disposal wells nearest the fault in March 2011 amid fears that more activity could cause larger, more damaging quakes. The state’s oil and gas commission outright banned such disposal wells last month. Still, Horton said, the vast majority of the Arkansas quakes weren’t large enough to be widely felt, let alone cause damage.
Another recent study by scientists with Southern Methodist University and the University of Texas at Austin linked fracking wastewater wells with an uptick in small earthquakes rattling the Barnett Shale region near Dallas-Fort Worth. Cliff Frohlich, a UT geophysicist who coauthored the study, said there needs to be more research and monitoring of seismic activity in and around Texas’ oil-shale plays. “When an earthquake big enough or in an urban area like Dallas happens, people feel it and it causes this fracas. But we need to look more systematically to see how common this phenomenon is and where it’s happening,” he said. “It’s in the interest of the public and also the industry – public concern can shut down an industry, even if it’s not warranted.”
Following oil and gas exploration that began in the 1950s, long before fracking of the Eagle Ford got underway, the area southeast of San Antonio started to see minor earthquakes as early as the 1970s, occurring mostly within natural gas fields, Frohlich said.
Julie Dutton, a geophysicist with the U.S. Geological Survey, said last week the area has seen more than a dozen earthquakes over the past two decades. “At a 4.8, though, this is of course the largest that we’ve seen,” she said.
Still, even before the Eagle Ford boom spiked drilling permits across the region, there have been number of known gas fields buzzing with activity in South Texas for decades, Frohlich said. Frohlich coauthored a study after a magnitude 4.3 earthquake, the region’s previous record-holder, rattled the region in 1993 and suggested the quake was tied to natural gas production. Both the 1993 earthquake and last week’s occurred across the Fashing gas field, just west of Karnes City, which has been actively exploited since the late 1950s — Houston-based Momentum Oil & Gas most recently announced buying up a swath of new property and wells in the Fashing in May.
“It’s funny, the analogy I make is like smoking,” Frohlich said. “If your grandfather dies of lung cancer, it’s hard to prove smoking caused it, but on the other hand, you can run statistics for hundreds of people and say, ‘Yea, smokers are much more likely to have lung cancer.’ It’s a lot like that.” Looking southeast of San Antonio, historically most of the area’s earthquakes have occurred in active gas fields, Frohlich said. “We’re unaware of any earthquakes that occurred before gas development began in the 50s, and so a logical person would probably conclude it’s related to that,” he said.
Of last week’s earthquake, Frohlich said, “My intuition is that there is a relationship, but it’s hard to prove.” •