The Federal Energy Regulatory Commission said yesterday that it has launched a sweeping, industrywide probe of possible overcharges for natural gas by producers that also operate gas-processing plants.

At the same time, the commission announced that the investigation has already borne its first fruit -- nearly $65 million in refunds to be paid to the Tennessee Gas Pipeline Co., a subsidiary of Tenneco. Of that amount, Tennessee Gas already has received about $60.5 million and has passed the money on as refunds to its customers.

Tennessee Gas's principal service area is New England and the Middle Atlantic States. Washington Gas Light Co. receives part of its supply from Tennessee Gas. Because of the number of customers on the receiving end of the amount, which is small compared with Tennessee Gas's overall revenues, the refunds are likely to produce a negligible effect on end-use gas bills, a Tenneco spokesman said.

According to the FERC, Tennessee Gas bought natural gas at the wellhead from producers and moved it through the Tennessee pipeline to two Louisiana processing plants.

Processing plants remove liquifiable hydrocarbons, such as butane and propane, from natural gas, a process that results in loss of volume. According to the FERC, the producers that had processed the gas paid Tennessee improperly low prices for the gas volume that was lost.

The commission identified the producers that have paid refunds as Amoco Production Co., Arco Oil and Gas Co., Chevron U.S.A. Inc., Cities Service Co.'s Conoco Inc., Exxon Co. USA, Getty Oil Co., Mobil Oil Exploration and Producing Southeast Inc., Newmont Oil Co., Shell Oil Co., The Superior Oil Co., and Tenneco Oil Co. Tennessee also said that Gulf Oil Co. is considering whether it should pay refunds.

The FERC will continue its investigation to determine if similar mispricing occurred in any of the more than 800 natural gas processing plants in the United States, the commission said. The FERC offered an incentive for firms to come forward voluntarily with information about possible overcharges, saying that firms that do so within 60 days will be fined only 2.5 percent of the amount refunded. Producers that come forward in the following 60 days will pay fines equivalent to 5 percent of the refunds.