An oil spill in the Gulf of Mexico began to trickle deeper into Louisiana's coastland on Tuesday, and tar balls have begun to wash onshore in Texas.

Oil sheen and tar balls from the Deepwater Horizon undersea gusher have been spotted in Lake Pontchartrain, the huge lake forming the northern boundary of the city that was rescued in the 1990s from rampant pollution.

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Ronnie Rouse of T&T Marine Salvage, a company contracted to clean tar balls off Galveston beaches, picks up a tar ball on East Beach in Galveston, Texas, Tuesday. ((U.S. Coast Guard Petty Officer 2nd Class Prentice Danner/Associated Press))

In Texas, officials questioned whether almost 20 litres of the stuff came naturally on the currents or was dragged by a passing ship from elsewhere, but crews combed the beach and pledged to collect damages from BP.

Meanwhile out in the Gulf, stormy conditions have delayed the hookup of a new containment vessel, the Helix Producer, to the cap collecting oil from the gushing wellhead on the seafloor. Officials had originally hoped to connect it on Wednesday.

A new target date hasn't been announced.

The weather also delayed the arrival of a navy airship that will serve as a floating observation post above the Gulf. It is now expected to arrive in Mobile, Ala., on Friday.

The spill's Joint Incident Command announced Monday that sheen and tar balls had been spotted in the Rigolets, one of the waterways connecting Lake Pontchartrain to the Gulf, and in parts of the 1,632-square-kilometre lake itself.

Crews placed boom at a natural choke point with hopes of stopping more from entering the lake. Nineteen skimmers and four containment vessels were sent to the affected areas. As of 7:30 p.m. CT Monday, they had collected half a ton of tar balls and waste.

The main part of the lake remained open to fishing, but a chunk southeast of an old car bridge across the lake was closed.

Bad weather blamed

The oil could have been pushed deeper into southeastern Louisiana by the tropical weather that began last week with the Hurricane Alex, which made south of the Mexico-Texas border, and continued into this week with a low-level tropical system that brought high winds and heavy rain to the state.

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A work boat manoeuvres past sections of an oil retention boom system in the Perdido Pass in Orange Beach, Ala., that broke in choppy seas, on Tuesday. ((Dave Martin/Associated Press))

The weather has essentially stopped offshore skimming off Mississippi, Alabama and Florida, and curtailed it off Louisiana.

Skimming across the Gulf has scooped up about 90 million litres of oil-fouled water so far, but officials say it's impossible to know how much crude could have been sucked up in good weather because of the fluctuating number of boats at work and other variables.

The oil's arrival in Texas was predicted Friday by an analysis from the National Oceanic and Atmospheric Administration, which gave a 40 per cent chance of crude reaching the area.

"It was just a matter of time that some of the oil would find its way to Texas," said Hans Graber, a marine physicist at the University of Miami and co-director of the Center for Southeastern Tropical Advanced Remote Sensing.

Source of tar balls questioned

About 20 litres of tar balls were found Saturday on the Bolivar Peninsula northeast of Galveston, said Capt. Marcus Woodring, the U.S. Coast Guard commander for the Houston/Galveston sector. Seven more litres were found Sunday on the peninsula and Galveston Island.

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A sample jar of tar balls found on Crystal Beach and Galveston's East Beach during the holiday weekend is displayed during a news conference at the U.S. Coast Guard Marine Safety Unit in Texas City, Texas, on Monday. ((Michael Paulsen, Houston Chronicle/Associated Press))

Woodring said the consistency of the tar balls indicates they could have been spread to Texas water by ships that have worked out in the spill. But there's no way to confirm how they got there.

Galveston Mayor Joe Jaworski said he believed the tar balls were a fluke, rather than a sign of more to come.

"This is good news," he said. "The water looks good. We're cautiously optimistic this is an anomaly."

NOAA scientists are looking at local weather, Hurricane Alex and Gulf vessels as possible causes for the arrival of the tar balls, agency spokeswoman Monica Allen said Monday.

The distance between the western reach of the tar balls in Texas and the most eastern reports of oil in Florida is about 885 kilometres. Oil was first spotted on land near the mouth of the Mississippi River on April 29.