(Updates with Pemex predicting return to normal)

By Catherine Bremer

MEXICO CITY, Oct 30 (Reuters) - Mexico's storm-crippled crude oil sector should be operating as normal by Thursday morning, state-owned energy monopoly Pemex said on Tuesday as one of the country's three main oil ports reopened.

Pemex spokesman Carlos Ramirez said ships were being loaded up at Cayo Arcas port, on the Gulf of Mexico coast, which was already easing inventory build-up and meant closed production wells in the Gulf could soon reopen.

"The estimate is between 24 and 36 hours for everything to return to normal, to reestablish normal conditions in all of the Bay of Campeche," Ramirez said.

Mexico reopened Cayo Arcas on Tuesday afternoon, but stormy weather kept the other two oil ports, Pajaritos and Dos Bocas, shut and stopped tankers heading out to sea for a third night.

Port officials, who grounded ships on Sunday afternoon, said the weather was still rough and they were skeptical about whether any oil tankers would sail before Wednesday. Pemex said it hoped Pajaritos port would open in the morning.

"It's going to go little by little, but now the port is open, shipments can leave," Ramirez told Reuters, adding that some 20 oil tankers were on standby to load up and head out.

Sunday's closures grounded oil shipments to the U.S. east coast and Europe -- some 80 percent of what Pemex exports. Pemex also shut down 600,000 barrels per day (bpd) of offshore production, a fifth of its output, as inventory build-up at the ports caused a bottleneck in shifting crude from wells.

A port official at Cayo Arcas said the weather was still "bad outside," and Dos Bocas official Vladimir Romero said there were forecasts of another storm on its way. "It's still pretty strong. I don't know when it will die down," he said.

The transport ministry said conditions on the Gulf coast were still foggy and rainy with gusts of wind as strong as 50 mph (80 kph) and waves 5 meters (16 feet) high in places.

The storm flooded huge swathes of Tabasco state, and sent temperatures in faraway Mexico City plunging to rare lows.

STORMS, SPILLS, SLIDING RESERVES

Pemex, a top-three supplier of crude to the United States, said the bottleneck at ports after another storm hit exports last week should be quickly fixed once exports resume.

"Once we start moving the product from our inventory, then the production process will start up again, which should happen relatively fast," Ramirez said earlier on Tuesday.

The Pacific port of Salina Cruz, which ships 300,000 bpd of oil to the U.S. West coast and Asia, also reopened on Tuesday. The next update on Gulf ports was due on Wednesday morning.

Pemex, which normally produces 3.1 million bpd of oil and exports 1.7 million bpd, was going into its third day with its output down by a fifth and exports mostly halted.

It is still reeling from last week's storm which caused an offshore drilling platform to keel over and hit a rig, sparking oil and gas leaks and leaving 21 oil workers dead as they tried to flee in life rafts in heaving seas and strong winds.

The storms are a new headache as Pemex grapples with lower yields at its Cantarell oil field, bomb threats from a Marxist rebel group and a blow to August output from a hurricane.

Pemex, also under fire this week for spilling 10,000 barrels of fuel in Veracruz state which oozed down rivers into the sea, is battling to restore oil output to 2004 peaks while reversing a slide in proved reserves to nine years' worth.

Energy Minister Georgina Kessel said Mexico had plenty more oil, but reaching it would be tricky.

Early tests indicate Mexico could have 55 billion barrels of new deposits, mostly in deep water, which Kessel said could double total reserves to 100 billion, or 61 years' worth.

"The oil in Mexico has not run out, what we have to do is be very creative to make sure these prospective reserves are converted into proved reserves," she told a business forum in the northern city of Monterrey. (Additional reporting by Robin Emmott in Monterrey)

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