Sinkhole is size of football field

• State is investigating whether a nearby salt cavern is to blame
• A relief well will be drilled into the cavern to check
• In the meantime, residents remain under a mandatory evacuation order
• The sinkhole is about the size of a football field

(CNN) -- Residents living near a sinkhole in a southern Louisiana bayou are not expected to be allowed to return home for at least another month, officials said Friday.

Authorities are investigating the cause of the slurry-filled hole, which is roughly the size of a football field.

Specifically, they are looking into whether a nearby salt cavern could be to blame and have ordered the company that is responsible for that cavern to drill a relief well.

The process is expected to take up to 40 days, said Assumption Parish President Martin Triche. Until the well is drilled, authorities do not anticipate lifting the evacuation order that affects approximately 150 homes.

"We won't feel a level of confidence to lift the evacuation until we have more answers from that well," Triche told reporters.

Louisiana Sinkhole Opens Up

Roughly half of the residents told to leave heeded the call, he said.

Louisiana officials have reached out to Texas Brine Company, urging it to provide affected residents with some type of assistance while it investigates whether a breach in its cavern might have caused the sinkhole 100 yards away.

The situation is made all the worrisome because the hole is believed to be close to a well containing 1.5 million barrels of liquid butane, a highly volatile liquid that turns into a highly flammable vapor upon release. A breach of that well, Assumption Parish Sheriff Mike Waguespack has said, could be catastrophic.

Texas Brine has until Monday to submit a permit to drill the well or face a $5,000 per day fine, said Stephen Chustz with the Louisiana Department of Natural Resources.

The purpose of the well is to determine the structural status of the cavern as well as the pressure of any gas that might be inside of it, he said, adding that the last mechanical integrity test performed on the cavern was done in late 2010.

In working to expand mining of the cavern, the company made a cut in the well casing several hundred feet above its top, said Chustz. Texas Brine reported a potential breach at that time and that information was reported in 2011, he said.

According to Texas Brine, the company is committed to getting to the bottom of the sinkhole mystery. The company is not saying it is responsible, nor is it saying it's not, said spokesman Sonny Cranch.

Measurements taken early this week showed the hole measured 324 feet in diameter and is 50 feet deep, but in one corner it goes down 422 feet, said John Boudreaux, director of the Office of Homeland Security in Assumption Parish, about 30 miles south of Baton Rouge. The hole, which has swallowed 100-foot-tall cypress trees, has since grown another 10 to 20 feet.

The sinkhole appeared August 3, more than two months after local residents started noticing bubbles in the water. The bubbles grew in number and frequency, and in some spots they made the bayou look like a boiling crawfish pot, said Dennis Landry, who owns guest cabins about half a mile from the hole.

Frustrations are rising among residents and officials alike.

"As parish leaders, it's time for us to stand together and stand strong and hold Texas Brine's feet to the fire. There are no more excuses. There will be no more delays," said Waguespack, the sheriff.

CNN's Ed Lavandera, Jason Morris, Melissa Gray and Joe Sutton contributed to this report.