The Federal Power Commission yesterday approved the sale of a large amount of Louisiana offshore natural gas to four interstate pipeline companies, requiring that the gas producers involved make detailed requests for any gas they want to reserve for their own uses.

The decision reversed a policy in place since 1963 which allowed a producer to reserve an arbitrary percentage of gas supplies for its feedstock and industrial needs.

The four pipeline companies involved are: Trunkline Gas Co., Southern Natural Gas Co., Tennessee Gas Pipeline Co., and United Gas Pipe Line Co.

The proved and probable gas reserves of Louisana's South Marsh Island area were, at year-end 1974, 366 billion cubic feet.

Over the past 14 years the so-called reserve gas incentive, which reflected federal government effort to provide an incentive for increased gas exploration and development, permitted producers to use natural gas for what the FPC, now considers inefficient uses such as boiler fuel.

The FPC rule will permit the producers to continue to use the reserve gas, but only for high-priority uses such as liquid hydrogen, ammonia and fertilizer production.

The FPC said the producers must file within 30 days of yesterday's order specifying the high-priority uses they plan for the gas.

The gas sales will be at the FPC's nation wide rate of $1.42.

The decision will not affect residential gas supplies, the FPC said.