Supporters of a moratorium on oil company mergers were seeking last night to stop for a year and a half controversial takeovers such as Mobil Corp.'s proposed acquisition of Marathon Oil Co..

Not all the details had been nailed down as of last night, but the tentative plan is to offer a moratorium as a rider to an unrelated and less controversial bill to be taken up by the Senate, perhaps as soon as today.

The legislation then would be sent back to the House of Representatives, where supporters of the legislation would try to bring it up under suspension of the rules, a procedure that acquires approval by two-thirds of the members.

The reason for the attempt to speed up consideration of a major and highly controversial piece of legislation is to put the moratorium in place before a wave of mergers makes the issue moot, said supporters of the bill.

What is expected to be offered is a proposal that expands on earlier bills proposed by Rep. Clarence Brown (R-Ohio) and Sens. Robert Packwood (R-Ore.) and Howard Cannon (D-Nev.). The proposal under consideration would impose a year-and-a-half moratorium extending from Dec. 15 to June 30, 1983. During that time, the Federal Trade Commission would be required to study the impact of oil company mergers and to report to Congress on its findings.

What the bill would do is to prohibit the top 20 oil companies from acquiring more than 5 percent of any of the top 40 oil companies and also to bar the top 20 from acquiring more than $1 billion worth of crude oil reserves from the top 40 companies. That provision apparently would prevent Mobil from taking over U.S. Steel Corp., since U.S. Steel plans to acquire Marathon's oil and gas reserves valued at more than $2.8 billion.

The concern over oil company mergers was triggered by Mobil's attempt to acquire Conoco, which was followed by its bid for Marathon. Mobil has sought the smaller oil companies as a less costly way to acquire reserves because the stock market has priced their stock relatively low compared to the value of their reserves.

If Mobil succeeds, industry analysts have said they expect to see a spate of takeovers of other so-called "second tier" oil companies.

A House subcommittee on fossil and synthetic fuels is scheduled to consider a merger moratorium bill today.

Yesterday members of a congressional panel raked the FTC over the coals, accusing the agency of giving Mobil a blueprint for how to acquire Marathon. The FTC announced last week that it would try to enjoin Mobil's takeover bid "unless and until" a court-approved plan is produced that would spin off Marathon's marketing, storage and transportation activities to another viable company. Mobil and Amerada Hess have announced a plan for Amerada Hess to take over those functions if Mobil wins Marathon.