The House Ways and Means Committee completed most of its basic decisions yesterday on President Carter's purposed "windfall profits" tax on oil, but it still faces possible snags on secondary issues in a wrap-up session next week.

The panel spent most of yesterday working on definitions and other details of the bill. And it rejected a spate of amendments by oil-state legislators designed to ease the measure's tax bite on specific segments of the industry.

However, the committee stopped short of a controversial vote on whether to approve a companion plan by Carter to establish a special trust fund to channel the "windfall" tax monies to energy research and mass transit grants.

That proposal is expected to be considered Tuesday, when the committee is scheduled to hold a final session to report out the bill. The measure is scheduled to go to the House floor before the July 4 recess.

The major vote yesterday was to approve a plan by Rep. Richard Gephardt (D-Mo.) to define which categories of newly discovered oil qualify for special tax treatment that the committee voted on Wednesday to spur new exploration.

Meanwhile, new revenue extimates compiled yesterday showed that while the committee's bill is significantly tougher than the president's in the short-run, it would fall behind Carter's in terms of revenue after 1985.

The panel's amendments, as reported, basically stiffened Carter's original proposal by increasing the tax bite on the "windfall" that the oil companies are expected to reap from already discovered oil.

At the same time, however, the panel also voted to ease the tax on newly discovered oil - a move that is designed to encourage more new drilling, but which will reduce the measure's tax take in the latter 1980s.

The effect is to give the administration increased revenues between now and 1985 and greater production of new oil in the latter part of the next decade - by most standards, an improvement over the Carter bill.

Yesterday's estimates showed the committee's bill would bring in a cumulative $28.5 billion between 1980 and 1984 - or some $6.7 billion over a revised $21.8 billion tax take attributed to the Carter measure.

There were no estimates available comparing the two bills for 1985 and beyond, but staffers conceded the committee version of the measure would bring in less revenue then, in exchange for greater oil production.

Revised Energy Department estimates predicted the committee bill would increase the number of new barrels of oil spurred by the Carter proposal by 150,000 barrels a day in 1985 and more than 500,000 barrels a day by 1990.

The Gephardt proposal that the panel approved yesterday would disqualify tracts of land from preferential treatment under the "windfall profits" tax if they were part of a larger property being drilled and severed for tax benefits.

It also would withhold special breaks from oil drawn from properties which previously had yielded oil. New reservoirs on old properties would be treated as newly discovered oil only if no drilling had occurred there before.

The preferential treatment to which the Gephardt amendment applies would lighten the tax bite on newly discovered oil by holding the tax levied on it to 50 percent and reducing the portion of earnings subject to the tax.

The panel also approved yesterday a proposal by Rep. Charles A. Vanik (D-ohio) that would require the Treasury to compile information on the profits, taxes paid and sales of the nation's largest oil companies. CAPTION: Picture, Working on the "windfall profits" tax bill are, from left, Reps. William R. Cotter; James R. Jones; Bernard Shapiro, chief of staff of the Joint Committee on Taxation; Reps. Richard Gephardt; Joseph L. Fisher and Al Ullman. By James K.W. Atherton - The Washington Post