The Senate yesterday approved a tax break for operators of low-volume oil wells and rejected an effort to bar the wealthy from qualifying for a proposed 1980 tax credit for oil royalty owners.

Both provisions have the effect of modifying the huge domestic crude oil tax that Congress enacted this year. They were attached to a $4.2 billion omnibus revenue package that is aimed at keeping down the projected budget deficit for next year.

The package, which relies mainly on increasing payments that corporations will make on their estimated taxes, is coupled with more than $6 billion in spending cuts that committees of both houses are recommending. Without both measures, the newly projected deficit for 1981 would approach $40 billion rather than $30 billion.

By 72 to 23, the Senate agreed to a proposal by Henry Bellmon (R-Okla.) to exempt from the crude oil tax the first two barrels a day from "stripper" wells, low-volume wells that produce no more than 10 barrels a day.

Proponents argued that the exemption would provide the margin between profit and loss and is needed to keep the wells operating. The estimated $1.4 billion annual cost to the Treasury would be offset under Bellmon's amendment by increasing taxes on other producers.

Earlier, voting 24 to 73, the Senate turned thumbs down on a proposal by Bill Bradley (D-N.J.) and Daniel P. Moynihan (D-N.Y.) to exempt taxpayers with incomes in excess of $40,000 a year from a royalty tax credit that the Senate Finance Committee tacked on to the revenue bill last month.

The Finance Committee's proposal, which the Senate approved as part of the omnibus package, provides as credit equal to the first $1,000 of crude oil tax liability for the 1980 tax year. Its estimated $190 million cost would be offset by increasing taxes on oil producers.

The Bradley-Moynihan amendment would have given the full credit on incomes of up to $30,000, phasing it out as incomes approached $40,000.

The Senate had approved such a credit as part of its oil tax bill last winter, but the House refused to go along and it was dropped in conference.

The credit must still pass the hurdle of a House-Senate conference. A comparable revenue package proposed by the House Ways and Means Committee does not include the stripper-well exemption or the royalty tax credit.