HARRISBURG — A natural gas drilling company failed to use a proper backup pressure-control system last month when hooking up a well to a pipeline, leading to a major blowout in Pennsylvania that spewed gas and wastewater for 16 hours, a state investigation has found.

EOG Resources of Houston, which operates nearly 300 wells in Pennsylvania, cut corners by not using a second set of pressure-control devices, a consultant hired by the state concluded in a report issued Tuesday.

EOG took similar safety shortcuts on at least some of its other wells in Pennsylvania, where about half of its drilling operations are in the gas-rich Marcellus Shale reserve, a lucrative source of natural gas that has drawn scores of companies to the state.

"I don't know any company that would cut corners like this on this kind of well," said consultant John Vittitow, a Texas-based petroleum engineer. "This was just a bad decision and it caught up with them."

In signed papers released Tuesday, EOG and its contractor, C.C. Forbes of Texas agreed to maximum fines of more than $400,000 combined and to take corrective actions. But they also were allowed to resume all activities in Pennsylvania after the state had shut down some operations since the June 3 blowout.

State regulators also sent a letter ordering all drilling operators to adhere to a set of safety standards designed to prevent another such incident.

In a statement Tuesday, EOG said it regretted the blowout.

"We will be implementing the new operational procedures as defined in the letter to all gas well operators and look forward to resuming our activities in the commonwealth," said EOG vice president and general manager, Gary Smith.

For 16 hours, explosive gas and briny wastewater shot into the air before specialists called to the site brought it under control. No one was hurt and the gas did not ignite, although about 35,000 gallons of polluted water was collected after two nearby creeks were polluted.

State Environmental Protection Secretary John Hanger said state regulations did not allow EOG's practices, and he warned that another such preventable failure could mean the end of EOG's business in Pennsylvania.

The blowout occurred on the grounds of a hunting club in a heavily forested section of Clearfield County, near Interstate 80.

It happened about a week after the crew had finished the hydraulic fracturing process.

The C.C. Forbes crew was carrying out the well-finishing work, such as drilling out plugs inserted during hydraulic fracturing, and preparing to connect the well to a pipeline when it lost control of the well.

The crew damaged the lone rubber pressure-control seal in the well and, without a backup system, highly pressurized fluid and gas began spraying out.

Vittitow said he couldn't explain why EOG would take such a risk. Cost and time savings might have been a consideration, as well as the company's use of similar practices at low-pressure wells in other regions where it is standard, he said.

Hanger said his agency would redouble its inspection activity with more emphasis on well-finishing work, but said it was a "fair question" as to whether an agency inspector should have been on site during that stage.