Next-day natural gas prices for Sept. 20 at the Waha hub in the Permian basin tumbled 60% to their lowest on record due to pipeline constraints limiting the amount of gas that can move out of the region.

Prices for Sept. 20 at the Waha hub fell almost a dollar to 66 cents per million British thermal units (MMBtu), according to SNL data going back to 1991.

That compares with an average of $2.25/MMBtu so far this year, $2.71 in 2017 and a five-year (2013-2017) average of $3.11.

The Permian is the biggest U.S. oil producing shale basin and since much of that oil comes out of the ground with associated gas, it is also the nation’s second biggest shale gas producing region, behind the Appalachian.

As the number of rigs seeking oil in the Permian rose this year to the highest since 2015, the amount of oil and associated gas produced has increased to record highs, constraining the region’s existing gas and oil pipes.

Those gas constraints have boosted the discount Waha trades at below the Henry Hub benchmark in Louisiana .

That spread rose to $2.45/MMBtu for Sept. 20, its widest since November 2008, according to SNL data available on Reuters.

That compares with an average discount of 70 cents so far in 2018, 27 cents in 2017 and a five-year (2013-2017) average of 14 cents.

Traders said Waha’s price drop was triggered by a pair of force majeures in Kinder Morgan Inc.’s Natural Gas Pipeline Co. of America (NGPL) system that are expected to restrict gas flows from West Texas to the Midwest.

A line strike on NGPL’s Amarillo line, a major route for Permian gas to flow to the Midwest, forced the company to declare a force majeure at Compressor Station 104 in Barton County, Kansas.

Flows through Segment 296, which spans Compressor Stations 103 to 106, have averaged 500 million cubic feet per day (MMcf/d), slightly more than half the previous 30-day average, according to S&P Global Platts.

NGPL said it expects the Kansas force majeure to continue through Sept. 24.

NGPL also declared a force majeure at Compressor Station 168 due to maintenance on the Permian Line in Bailey County, Texas. The company said work there began Sept. 19 and is scheduled to be done Sept. 21.

S&P Global Platts said the Texas force majeure reduced flows for NGPL Station 167 to Station 112 by 45 MMcf/d to 234 MMcf/d on Sept. 19.