(Adds comment by fire marshal, byline, NEW YORK dateline)
By Gary McWilliams and Scott DiSavino
HOUSTON/NEW YORK, Aug 2 (Reuters) - Authorities on Thursday were investigating what caused a fire and a 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 from two companies with adjacent pipelines were sent to the area Wednesday morning because of an underground gas leak when one pipeline caught fire and exploded, said Midland County Fire Marshal Dale Little, who is conducting an investigation.
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 both suffered damage, said Little. No cause for the blast has yet been determined.
Navitas Chief Operating Officer Bryan Neskora said in a statement that company employees were among the injured.
Five workers with critical injuries were treated at the burns unit of the University Medical Center in Lubbock, Texas.
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 were at the site when the explosions occurred also were taken to hospital on Wednesday for treatment of burns, said Elana Ladd, public information officer for the city of Midland.
Ladd said the explosions occurred just outside Midland on a rural road.
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.
Marty Baeza, a Fort Stockton, Texas, oilfield worker who was 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. A large fireball lit up the sky for about five minutes, and firefighters arrived quickly, he said.
Kinder Morgan's El Paso Natural Gas (EPNG) line was damaged 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.
The outage on the EPNG line will affect 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 relatively 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 Tom Brown and James Dalgleish)