* 85 pct of U.S. offshore oil could close-Weather Insight
* Storm's westward shift brings greater US energy impacts
* BP shuts all Gulf of Mexico operations on storm threat
By Kristen Hays
HOUSTON, Aug 26 (Reuters) - Gulf of Mexico oil and gas producers were bracing for the first hurricane of the 2012 season to affect the offshore U.S. oilpatch, which could also disrupt the majority of U.S. offshore oil output.
After initially calling for Tropical Storm Isaac to pass well east of the U.S. offshore production zone, forecasters on Sunday were predicting a more westward track that could bring a powerful Category 2 Hurricane Isaac over the heart of the U.S. offshore oilpatch, which produces about 23 percent of U.S. oil output and 7 percent of its natural gas output.
Isaac could be the biggest test to U.S. energy infrastructure since 2008, when Hurricanes Gustav and Ike disrupted offshore oil output for months and damaged onshore natural gas processing plants, pipelines and some refineries.
Isaac has so far been responsible for shutting 24.19 percent of offshore oil output and 8.24 percent of natural gas output, according to U.S. government figures released on Sunday.
(For a FACTBOX on oil, gas operations affected by the storm, see: )
Outage figures will rise in the days ahead, according to forecasters at Weather Insight, an arm of Thomson Reuters, who predict the storm will spur short-term shutdowns of 85 percent of U.S. offshore oil capacity and 68 percent of its natural gas output. Isaac has a 95 percent chance of entering the heart of the oil and gas producing region, Weather Insight said on Sunday.
London-based BP Plc, the region's biggest oil producer, on Sunday evacuated all of its Gulf of Mexico platforms. The company had earlier shut and evacuated four of its seven facilities, including its giant Thunder Horse platform, the world's largest, which can process 250,000 barrels of oil and 200 million cubic of natural gas per day (mmcfd), and three others.
BP's evacuations and shutdowns initiated Sunday included Atlantis, the world's second-largest offshore platform, which can produce up to 200,000 bpd of oil and 189 mmcf per day of gas.
Royal Dutch/Shell, also a major offshore producer, said it planned to shut its platforms in the east-central Gulf. The company did not specify which structures would shut, but its platforms in that area include the Mars, Ursa and Brutus platforms.
Anadarko Petroleum Corp, the biggest natural gas producer in the Gulf, said it will shut six production platforms including its Independence Hub, which can produce up to 1 billion cubic feet per day of gas.
Isaac could also have an impact on the U.S. refinery row along the Gulf Coast, which stretches from Mississippi to south Texas and accounts for more than 40 percent of U.S. fuel output. Refineries in Louisiana alone are responsible for 18.7 percent of the nation's refining capacity.
Chevron Corp.'s 330,000 barrel per day (bpd) Pascagoula, Mississippi, refinery is closest to the storm's expected landfall.
Chevron said the company was "closely monitoring" Isaac's path and was following procedures for hurricane preparations at the plant.
Other New Orleans-area refineries that could be affected include Phillips 66's 247,000 bpd Alliance refinery in Belle Chasse, Louisiana, and Exxon Mobil Corp's 502.500 bpd Baton Rouge plant, the nation's third-largest.
The Louisiana Offshore Oil Port (LOOP), which can offload about 1 million barrels per day of foreign crude for delivery to Gulf Coast refiners, remained in operation on Sunday, a spokeswoman said.