Occidental Petroleum Corp. announced yesterday that it has proposed a merger with Zapata Corp., a lucrative Houston-based offshore-oil-rig operator.

The offer was made during the weekend to Zapata officials, with Occidental proposing a stock swap that would amount to approximately $760 million. Zapata Chairman B. John Mackin said that Zapata is studying the unsolicited offer.

The proposed merger would bring to Occidental, the 20th-largest industrial corporation in the nation, a company that is a major operator and supplier of offshore drilling rigs. Occidental expects to spend approximately $2 billion on offshore drilling over the next five years, said Occidental's chairman and chief executive officer, Armand Hammer. With some 23 rigs, Zapata is is the second-largest operator of offshore rigs -- which Hammer said figured largely in Occidental's interest in the company.

Its other attractions include tax credits, oil exploration in the Gulf of Mexico, where Occidental is also active, and Zapata's coal properties, Hammer said. Zapata had revenues of $637 million and net income of $43.6 million during the fiscal year ended Sept. 30, 1980.

Occidental, a company that has grown from revenues of less than $1 million in the 1950s to $12.5 billion for the fiscal year ended Dec. 31, 1980. Net income for that period was $710.8 million.

Hammer said Occidental is interested only in acquiring Zapata in a friendly merger. Occidental was badly bruised in an unfriendly takeover fight with Mead Corp. several years ago.

Hammer made his overtures to Zapata in meetings with Mackin and the executive committee of the board of directors over the weekend. He said that the oil company offered Zapata a 30-day period to consider the offer but that Zapata asked that the period be extended to 45 days.

The offer calls for Occidental to issue 0.6904 share of common stock and 0.1804 share of a new series of voting nonconvertible preferred stock with a redemption value of $100 and an annual dividend rate of $14.63. These securities would be exchanged for each share of Zapata's 21.2 million outstanding shares after Zapata had redeemed its $2 noncumulative convertible preference stock and its $6 cumulative preferred stock.

Occidental described its offer as amounting to more than $36 a share for Zapata's stock. But analysts said the offer was somewhat lower, in a range between $33 and $36 a share. The year's high for Zapata was 39 7/8, but the stock closed at 27 3/4 on Friday.

If the merger is completed, Zapata would continue to operate independently under existing management and would retain its headquarters in Houston, Occidental said.

A figure in the merger negotiations with Zapata was David H. Murdock, a director and the largest shareholder in Zapata. Murdock was also the largest shareholder in Iowa Beef Processors Inc., which Occidental acquired last month.

Analysts said yesterday that it isn't clear whether Occidental's merger proposal would kick off a round of competing offers. "I think it's cheap," said James Horan, who follows Zapata for Bache Halsey Stuart Shields. Because they are heavy cash generators and because of the prospects for increased offshore drilling, Horan predicted that the Zapata offer might be the first in a string of acquisitions of offshore firms by energy companies looking for reserves.

"What Occidental is doing now is an extension of what it has been doing -- growth by skillful acquisitions," said Eugene Nowak, an analyst for Dean Witter Reynolds who follows Occidental's stock. Nowak said the proposed merger should produce synergistic benefits from Occidental, which is active in several areas that overlap with Zapata.