A flaming cauldron of crude oil - millions of gallons a week - is spreading off the coast of southern Mexico at the site of the world's worst offshore oil well blowout.

Swirling into the Gulf of Mexico, the rust-brown slick has been growing, shifting and splitting apart for more than five weeks, since Mexico's Ixtoc I exploratory well blew out of control 50 miles off this island and its shrimp port.

The threats its poses to Mexico's $100 million-a-year shrimp industry and the shorelines of Mexico and Texas are subjects of debate.

Mexican oil officials say that about 30,000 barrels - 1.26 million gallons - of high-quality crude oil is gushing from the ruptured well each day.

The spill caused by the June 3 explosion has already far exceeded the 1977 blowout in the North Sea, in which 200,000 barrels were lost. The oil lost so far in the Mexican blowout is close to the worst off-shore oil disaster, the breakup of the super-tanker Amoco Cadiz off France in March 1978, in which 1.28 million barrels of crude were spilled.

The oil revenue loss from the Mexican blowout is estimated at $5 million a week. In addition, Mexican fishermen stand to lose income from a ban by the Ministry of the Environment on the consumption of fish and shellfish from the polluted areas, which include the ports of Tampico, Vera Cruz and Campeche.

Jose Luis Garcia Luna, a senior engineer with the state oil monopoly Pemex, said the slick is split into narrow ribbons less than a mile wide and stretching northward less than 90 miles.

In the United States, the National Oceanographic and Atmospheric Administration says the oil has divided into more than a half-dozen large blotches than trail off more than 300 miles from Ixtoc I.

None of the experts in Ciudad del Carmen is predicting how long it will take to plug the well.

Most engineers do not expect to tame the blowout until a pair of relief wells are completed in about two months.

"They've already tried just about everything they can think up and nothing's worked so far," said one engineer who asked not to be identified.

The well has frustrated veteran oil field troubleshooters. Paul "Red" Adair of Houston, famous for controlling a 1977 blowout in the North Sea off Norway and a number of oil field fires, attacked Ixtoc I a few weeks ago.

He left the area at least 10 days ago. Pemex spokesmen refused to say why and Adair was not available for comment.

Two Democratic congressmen from Texas, Reps. Joe Wyatt and Eligio (Kika) de la Garza, flew over the well Saturday aboard a Coast Guard plane.

"Everything that can or should be done at this time is being done," said de la Garza. "I'm really quite impressed with the effort being made by the national response team with the availability of resources."

A number of oilmen involved in the effort to cap the well said the problem stems from damaged emergency shutoff valves at the wellhead.

They said last week that when a stream of gas shot up the well on June 3 and exploded, the $22 million floating platform was so badly damaged that tons of drilling equipment toppled, falling into the 165-foot deep water and apparently damaging the valves. No one was injured in the explosion.

Garcia Luna, the senior Pemex engineer, said in an interview that about half the gushing oil is consumed by a ring of fire burning directly above the blowout. Ships and barges scoop up thousands of barrels each day, he said, and the winds and sea evaporate, disperse or degrade most of the rest. What remains is a thick, reddish-brown substance.

The U.S. oceanographic agency said that while favorable currents have so far kept the spill from shorelines, a change in prevailing windscould drive it onto land. CAPTION: Map, no caption, By Dave Cook-The Washington Post