The oil and gas industry’s earnings season is barely under way, and already there’s been $6.5 billion in writedowns.

On Thursday, Freeport-McMoRan Inc. reported a $3.7 billion charge for the third quarter, while Southwestern Energy Co. — which has a market value of $4.5 billion — recorded $2.8 billion. And that’s just the beginning. Barclays Plc estimated in an Oct. 21 analysis that there could be $20 billion in charges among just six companies. Southwestern’s writedown was double Barclays’ forecast.

“We also received many scars in the past tough times, and we’ll have many more before the cycle is finished,” said Steven Mueller, Southwestern chief executive officer, during a conference call with analysts on Friday.

Oil prices have tumbled 44 percent in the past year, and natural gas is down 35 percent, making the write-offs a foregone conclusion from an accounting standpoint. The companies use an accounting method that requires them to recognize a charge when estimates of future cash flow from their properties falls below what the companies spent buying and developing the acreage. The predictions of future cash flow have fallen along with prices.

Since it’s no secret oil and gas prices have plunged, “the majority of write-offs are typically non-events,” said Barclays’ analysts led by Thomas R. Driscoll in the report.

Share Plunge

Southwestern’s shares declined 5 percent today to $11.07, 67 percent lower than a year ago. Phoenix-based Freeport-McMoRan’s are down 62 percent in the past 12 months.

Barclays predicted ceiling-test impairments for Apache Corp., Chesapeake Energy Corp., Devon Energy Corp., Encana Corp. and Newfield Exploration Co. All five companies are scheduled to report third-quarter results in November.

“Many companies will have writedowns as the price of oil is about half of where it once was and gas is also down,” said Timothy Parker, a Baltimore-based fund manager at T. Rowe Price Group Inc.. “However, it won’t generally hurt the companies because very few have debt covenants that are linked to book value, which the writedowns affect.”