WITH A SHRUG, Congress is apparently prepared merely to split the difference between the House and Senate versions of the oil windfall tax. The House originally passed it in a mild but reasonable form. Then the Senate, in which the oil-producing states are more influential, drastically weakened it. House and Senate conferees, before departing for the holidays, tentatively agreed to something halfway between. But the matter has not been finally settled. The White House still has an opportunity to press for a tax better able to protect the public and the economy from the tide of wealth that will otherwise flow to the oil producers.

There have been claims that the compromise windfall tax would collect $227 billion over the next 10 years. But this is not the heavy burden it is made out to be. The $227 billion figure is a hypothetical estimate calculated by congressional staff on several assumptions. And one assumption is a price of $30 a barrel, rising 2 percent a year plus inflation. The price of decontrolled oil in this country is already well over $30, and this year the price has been rising about 8 percent a month.

But even under that assumption of a stable $30 price, decontrolling oil prices would bring the producers $1 trillion in additional revenues over the next 10 years. Think for a moment about that figure: it means additional income, over the next decade, averaging $100 billion a year for an industry whose total revenue in this highly profitable year will be around $48 billion. It is likely that the real price of oil will rise much faster than 2 percent a year in the 1980s. But even if it does not, under decontrol the gross revenues of the oil-producing industry would triple.

Present income taxes and royalties would take some of that huge increment -- perhaps 40 percent of it. How much more should the windfall tax take? At present oil prices, under the compromise bill, the industry's total revenues would roughly double.

Some of these gigantic new revenues would be poured into exploration. But there is a point of diminishing returns in looking for oil. Even in the present anxious circumstances, there is no point in spending $100 to find and produce one barrel of oil. But a low windfall tax encourages doing just that. Much of the new money would unquestionably go into expanding the oil-producing companies. This tide of money, poured into conglomeration, has the most sinister implications for the terms of competition throughout American industry.

In strengthening this tax bill, the tax rate on newly discovered oil is crucial. The heaviest rates are on oil already in production, but that oil will be rapidly depleted. As time goes on, the effective rate of the total windfall tax will increasingly be the tax on the newly discovered oil. Under the Senate bill, the windfall tax on that oil would be 95 cents on a $30 barrel.

Another critical point is the Senate's exemption of what it comically calls the small producer -- one who produces no more than 1,000 barrels a day, currently worth something over $10 million a year. Enacting this exemption would merely invite the industry to reorganize itself into partnerships.

The conferees will return to town in January. If they do not then produce a tax of far more substantial dimensions than the disgraceful compromise they sketched out before their recess, President Carter should veto the bill and -- appealing to the sensitivities of an election year -- demand that Congress start all over again.