Oil Earnings Plummet : Energy: Arco suffers a $156-million loss because of reorganization costs. Unocal’s income skids 93% due to lower profit margins and depressed prices.

Suffering from an industrywide malaise--including slackening demand for chemicals and petroleum products brought on by the recession--Atlantic Richfield Co. on Monday reported its first quarterly loss in six years and Unocal Corp. said its third-quarter net earnings were down 93%.

Los Angeles-based Arco reported a third-quarter loss of $156 million, including $340 million in charges from a corporate reorganization. That contrasted with net income of $462 million in the same period in 1990.

Across town, Unocal reported third-quarter net income of $9 million, down from $121 million a year ago, mainly because of lower refining and marketing profit margins and depressed prices.

Oil and gas prices are off from a year ago, when the Iraqi invasion of Kuwait spurred a rapid run-up.

“It’s an industry phenomenon,” said Frank P. Knuettel, an analyst with Prudential Securities in New York. “The dismal situation in refining and marketing has bettered a little, but . . . it’s still far from acceptable.”

Arco’s 1991 third-quarter earnings included $340 million in after-tax charges related to layoffs, property sales and writedowns associated with a corporate reorganization. It was Arco’s first quarterly loss since the second quarter of 1985, after a previous corporate restructuring.

As part of its restructuring to cut costs and eliminate non-strategic assets, Arco announced the sale Monday of about 1,100 oil and natural gas producing properties in California, Texas, Oklahoma, New Mexico, Kansas, Wyoming, Arkansas and the Gulf of Mexico. The lands accounted for about 2% of Arco’s oil and natural gas production last year.

The third quarter’s one-time charges included $190 million for personnel reductions of 2,100 employees--higher than the previously announced 1,500--and reorganization of Arco’s operations in the Lower 48 states. The charges also included about $135 million of losses on the sale of properties in the Lower 48.

In refining and marketing, Arco’s profit margins were lower and its operating costs higher. Both Arco and Unocal are heavily represented on the West Coast, where analysts say competition has been especially fierce and margins especially slim.

Arco’s third-quarter revenue totaled $4.38 billion, down 9.2%.

At Unocal, Chairman Richard J. Stegemeier blamed poor profits on lower refining and marketing profit margins and depressed natural gas prices, as well as crude oil prices that were substantially lower than those during the third quarter of 1990.

Despite higher oil and natural gas production, exploration and production, net income was down 49% to $78 million from $152 million in 1990. Unocal’s average worldwide oil price in the quarter was $16.04 a barrel, compared to $20.22 a barrel a year ago. Unocal’s average worldwide natural gas prices were down to $1.72 a thousand cubic feet from $1.83.

In refining, marketing and transportation, lower product sales and tighter margins contributed to net income of $14 million, down from $34 million a year ago. In chemicals, Unocal’s profit margins were hurt and sales slack on its mainstay agricultural chemicals, partly because of seasonal fluctuation, analysts said.

Unocal’s total third-quarter revenue slipped to $2.68 billion from $3.07 billion in the year-earlier quarter.