HOUSTON, Aug 2 (Reuters) - Authorities on Thursday morning were investigating what caused a fire and a series of natural gas pipeline explosions on Wednesday in Midland County, Texas, which sent seven people to the hospital and shut down five lines before being extinguished late in the evening.
Workers and firefighters were responding to a leak when the blast occurred, Midland County Fire Marshal Dale Little said on Thursday.
Five workers with critical injuries were airlifted to University Medical Center in Lubbock, Texas, and were being treated at the center's burn unit, medical center spokesman Eric Finley said on Wednesday.
Also on Wednesday, two firefighters were also taken to hospital for treatment of burn injuries, said Elana Ladd, public information officer for the city of Midland.
Ladd said the pipeline explosions occurred just outside the city of Midland on a rural road, FM 1379, about 5 miles south of Highway 158. The cause was under investigation, authorities said on Thursday.
Kinder Morgan Inc's El Paso Natural Gas (EPNG) line was damaged by the blaze. It looked as if the problem started with a nearby third-party pipeline, and Kinder Morgan is evaluating damage to its pipeline, company spokeswoman Sara Hughes said in an email.
"There was a third-party pipeline involved that also experienced a failure, and preliminary indications are that the third-party line failure occurred before the EPNG line failure," Hughes said.
Oil and gas pipelines crisscross Midland County, which is located in the Permian Basin, the largest U.S. oilfield. The explosions affected five pipelines which share a transit channel and which were all shut in by operators, a Midland city official said on Wednesday.
Gas prices at the Waha NG-WAH-WTX-SN hub, in the Permian basin, increased by 13 cents, or about 6 percent, on Wednesday to $2.23 per million British thermal units, although much of the trade that day would have occurred before the fire, which started at around 11:14 a.m. CDT (1614 GMT).
Thomson Reuters data showed that as of Thursday, the explosions had not yet affected overall flows of natural gas in Midland County, including on Kinder Morgan's EPNG pipeline. (Reporting by Gary McWilliams in Houston and Scott DiSavino in New York; Editing by David Gregorio)