Efforts to cap a massive oil well blow out in the Gulf of Mexico appear to have faltered, and the well may be pouring out far more crude than anyone previously thought, congressional and industry sources say.

In addition, a northward swing in gulf currents around Christmas means that any more spillage will become a horrible belated Christmas present next spring to the Texas Gulf shoreline.

The oil that blackened 140 miles of Texas beaches last summer represented only about two days' gush from the Ixtoc I well, 58 miles off Mexico's Yucatan coast in Campeche Bay, according to Dr. John Robinson, the National Oceanic and Atmospheric Administration (NOAA) science coordinator for the spill. That will pale by comparison to what Texas can expect if the monster well continues to spout very long after Christmas, he said.

The spill began June 3 with a violent explosion as the Mexican government oil company, Petroleos Mexicanos (Pemex), hit a massive new reservoir in what is already a major field in Campeche Bay. Pemex estimated then that 30,000 barrels of oil a day were spouting into the sea, but maintained that control efforts had reduced the flow to 2,000 barrels a day or less as of mid-October. A Pemex spokesman cited that figure again last week.

In testimony before the Senate energy resorces and materials subcommittee this month, however, Dr. Jerome Milgram of the Massachusetts Institute of Technology estimated that the well was pouring about 50,000 barrels of oil a day into the sea when he left the site in late October.

That figure could be half or twice as large because of problems involved in measuring flow underwater, Milgram said, but it is still far larger than any past or present estimate by Pemex. Milgram, a naval engineer, was a consultant in the October effort by Brown & Root Co. of Houston to cap the well with a conical steel lid, dubbed the sombrero.

But those efforts and others have so far been unable to stop what is far and away the world's worst oil spill, "and there's not a damn thing we can do about it," complained Sen. Lowell P. Weicker (R-Conn.), ranking minority member of the energy subcommittee.

He said Mexican government figures had proved unreliable at best.

"The truth is not in 'em," he said. "All the information they were giving us was not stating the case as it was . . . if it were not for being on our knees in front of anyone that has oil, we would have told them long ago to shape up."

A pemex spokesman, Ulysses Martinez de Hoyos, confirmed by telephone from Mexico City last week that the sombrero was not operating because of mechanical problems and bad weather and that it was being repaired. But he said the sombrero had never been intended as the final fix.

That, he said, will come from the second of two relief wells being drilled 11,000 feet below the ocean floor into the Ixtoc reservoir from points on either side of the broken well.

The first relief well "helped but has not reduced the flow as much as originally expected," according to unofficial Mexican sources. The idea, which has worked well on smaller blowouts, is to pump chemical "mud" into the reservoir under the blowout point to displace the oil flow and block the outlet. Concrete can then be poured to seal the well.

Martinez said the second well had reached the reservoir but had not begun operating because of heavy seas. He said 70 percent of the oil that reaches the surface of the bay was being contained by ships and barriers.

A Coast Guard reconnaisance flight over the area on Dec. 17 reported to Weicker's office, however, that the thick brown "chocolate mousse" mixture of oil and water appeared from the air to be a giant fan, two miles across near the source and about 10 miles wide 25 miles down current.

"Absolutely no cleanup effort was visible," said a Weicker aide. A continual brown upwelling and turbulence capped with about an acre of burning gas marks the site of the runaway well the aide said.

An estimated $370 million in lawsuits have been filed by the federal government, fishermen, shrimpers and tourist businesses against Mexican and American contractors involved in Ixotoc construction Weicker said he had asked the State Department, which has been handling his contacts with Pemex, to arrange an on-site inspection by NOAA and other U.S. environmental officials, but has received no response.

Tucker Scully of the State Department's oceans and environment office said Pemex had preferred to tackle the blowout with minimal U.S. aid.