Feb. 18 (Bloomberg) -- Chevron Corp.'s Australian unit halted production at two oil fields off the northwest coast, bringing shutdowns triggered by a tropical cyclone to more than 40 percent of the nation's total crude-oil output.

Chevron Australia moved non-essential workers from the Barrow and Thevenard islands and shut down 9,000 barrels a day of production as Tropical Cyclone Nicholas nears the area, Scott Walker, a Perth-based spokesman, said yesterday in an e-mail.

BHP Billiton Ltd., Woodside Petroleum Ltd. and Apache Corp. are among oil producers that have shut down at least 220,000 barrels a day of oil production off Western Australia. The northwest coastline is the nation's most cyclone-prone region, with an average of five striking it between November and April each year, according to the Bureau of Meteorology.

The Bureau is warning of ``hurricane-force'' winds offshore and ``very high to phenomenal seas'' near the center of the storm.

Nicholas, categorized as a Severe Tropical Cyclone, was 265 kilometers (165 miles) north-northwest of Onslow at midday local time, the Bureau said on its Web site. It is moving southwest at 9 kilometers an hour and is expected to continue in that direction before turning more to the south later today, the bureau said.

Iron-Ore Exports

Rio Tinto Group, the world's second-largest iron-ore exporter, halted rail operations from mines in the Pilbara region in the northwest after the cyclone forced the closure of ports and brought hurricane-force winds. Rail shipments were suspended after the company stopped loading vessels on Feb. 16, Rio Tinto spokesman Gervase Greene said today by phone.

The loading of ships at Rio's two ports, Dampier and Cape Lambert, is yet to restart, while mining remains unaffected, Greene said.

BHP Billiton's iron ore operations in the Pilbara aren't affected, said Samantha Evans, a Melbourne-based spokeswoman.

``At this moment we are operating as normal,'' Evans said. The company is monitoring the path of the cyclone, she said. Fortescue Metals Group Ltd., developing a mine in the region, was also unaffected.

Port Hedland, the nation's largest iron-ore port, may resume full loading later today as conditions start to improve, Lindsay Copeman, Harbor Master for the Port Hedland Port Authority, said by phone. No ships have been brought in since Feb. 16, he said.

Stybarrow, Griffin

BHP yesterday said it suspended operations at the Stybarrow and Griffin offshore oil fields. Stybarrow, in which Woodside has a 50 percent stake, started production in November and reached 80,000 barrels a day in December. Griffin produces about 8,000 barrels a day of oil, as well as some natural gas.

Woodside, Australia's second-biggest oil and gas producer, halted production on the Nganhurra vessel used at the Enfield field, which was producing 30,000 barrels a day as at the end of December. On Feb. 15 Woodside halted production at the North West Shelf venture's Cossack Pioneer oil vessel, which produced an average of 74,000 barrels a day in the December quarter, Woodside said last month.

The Enfield and Cossack projects are still closed, Roger Martin, a spokesman for Perth-based Woodside, said today. Gas production on the North West Shelf is continuing.

Earlier in the week Apache Corp. halted production at the Stag and Legendre fields, while AED Oil Ltd. shut down the Puffin venture. Santos Ltd.'s Mutineer-Exeter field, which is also in the region affected by the cyclone, is already closed for maintenance, spokesman Matthew Doman said.

Australia's crude-oil production is forecast to increase by 4 percent to about 500,000 barrels a day in the year ending June 30, the Australian Bureau of Agricultural and Resource Economics said in its latest quarterly report.

To contact the reporter on this story: Angela Macdonald-Smith in Sydney at amacdonaldsm@bloomberg.net

Last Updated: February 18, 2008 00:14 EST