(Corrects spelling of Navitas executive's name to Neskora from Keskora in 5th paragraph)
By Gary McWilliams and Scott DiSavino
HOUSTON, Aug 2 (Reuters) - Authorities on Thursday were investigating what caused a fire and series of natural gas pipeline explosions in Midland County, Texas, that sent seven people to hospital and shut down five lines before being extinguished.
Workers were conducting maintenance on a line when it caught fire and exploded on Wednesday, Midland County Sheriff Gary Painter said.
The blast affected lines that furnish gas processing plants in the area but outages were not significantly impacting pricing or supplies, analysts said.
Pipelines operated by Kinder Morgan Inc and Navitas Midstream Partners LLC were impacted, according to a spokeswoman for the Texas Railroad Commission, the state's energy regulator. Regulators were at the scene on Thursday but have not yet determined a cause.
Navitas declined to make an executive available to comment but in a statement Chief Operating Officer Bryan Neskora said company employees were among the injured. Investigators were expected at the site to determine the cause, he said.
Five workers with critical injuries were airlifted to University Medical Center in Lubbock, Texas, and were treated at the center's burn unit.
One man remained in critical condition and three others were upgraded to serious condition, all with burn injuries, medical center spokesman Eric Finley said on Thursday. A fifth worker was released after treatment.
Two firefighters who where at the site when the explosions occurred also were taken to hospital on Wednesday 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.
Marty Baeza, a Fort Stockton, Texas, oilfield worker who was working at a site about a half mile (0.8 km) from the explosion, said the blast shook the water-treatment unit where he was working.
"It felt like someone had bumped us," said Baeza. He went outside and saw a large fireball that lit up the sky for about five minutes. Firefighters arrived quickly, he said.
Kinder Morgan's El Paso Natural Gas (EPNG) line was damaged by the blaze, but service impacts were expected to be minimal, spokeswoman Sara Hughes said in an email. The company believes the problem started with a nearby pipeline.
"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 that share a transit channel and which were all shut in by operators, a Midland city official said on Wednesday.
The outage on the larger pipeline, Kinder Morgan's El Paso Natural Gas line, will impact less than 10 million cubic feet per day of flows, estimated Joseph Bernardi, research analyst, natural gas at energy data provider Genscape.
Gas prices at the Waha NG-WAH-WTX-SN hub, in the Permian Basin, increased by 31 cents, or about 14 percent, at midday on Thursday to $2.54 per million British thermal units.
Tyler Jubert, an energy analyst at S&P Global Platts, said the relative small volumes affected should have little influence on hub prices. Some of Thursday's gain could be attributed to strong demand in the West where temperatures are hotter than normal. (Reporting by Gary McWilliams in Houston and Scott DiSavino in New York; Editing by Jonathan Oatis and Tom Brown)