SYDNEY/SINGAPORE, Aug 30 (Reuters) - The threat of work stoppages at two of Australia's largest liquefied natural gas (LNG) production facilities operated by Chevron Corp has roiled global gas markets.

Australia is the world's largest LNG exporter, and the facilities are some of its biggest.

Here is what the dispute is about and what's at stake:

WHAT ARE UNIONS PLANNING?

Starting Sept. 7, workers will down tools and stop performing certain tasks at the Gorgon and Wheatstone LNG facilities, the Offshore Alliance union group warned Chevron this week.

At Gorgon and the Wheatstone onshore facility, workers will stop work for seven hours, split across two time blocks, on the first day and escalate to 10 hours from Sept. 8 to Sept. 13. There will be a single day with an 11 hour stoppage on Sept. 9.

Smaller stoppages of three or four hours are planned at the Wheatstone offshore production platform between Sept. 7 and Sept. 14.

There will also be bans on certain tasks including the starting or restarting of platform wells and night shifts by maintenance staff.

Roughly 500 employees at the Chevron sites are represented by the unions.

A similar action by the same union alliance last year against Shell at its Prelude floating LNG site off northwest Australia cost the company about $1 billion in lost exports in the two months it took to reach a pay deal.

WHICH UNITS WILL BE AFFECTED?

On Barrow Island, Gorgon operates a three-train 15.6 million metric tons per year (tpy) LNG facility.

Other Gorgon shareholders include Exxon Mobil Corp, Shell, Osaka Gas, Tokyo Gas and JERA.

Term buyers of Gorgon gas include SK LNG Trading, Osaka Gas, Tokyo Gas, JERA, Kyushu Electric, JX Nippon Oil and Energy and GS Caltex.

Wheatstone, located 12 km (7.5 miles) west of Onslow on the Pilbara coast of Western Australia, includes two LNG trains with a combined capacity of 8.9 million tpy and a plant that produces natural gas for the local market. Other Wheatstone shareholders include Kuwait Foreign Petroleum Exploration Co, Woodside Energy, Kyushu Electric Power Co and JERA.

At Gorgon, 9.5 million tpy of LNG is contracted out to long-term buyers, while Wheatstone exports 8.05 million tpy of supplies to term buyers, according to an annual report from the International Group of Liquefied Natural Gas Importers.

Gorgon and Wheatstone are also capable of producing a combined 500 terajoules per day of domestic gas for the Western Australia market, about half of the state's gas consumption.

WHAT DO UNIONS WANT?

Higher pay and more control over rosters, promotions and other conditions.

The Offshore Alliance, which combines the Maritime Union of Australia and Australian Workers' Union, said on Wednesday pay at Chevron lags other operators.

In a deal reached last week that averted a strike at the North West Shelf offshore platforms operated by Woodside Energy , the union alliance said it locked in base annual pay between A$265,000 ($171,375) and A$365,000.

That deal is a potential blueprint for what unions want from Chevron.

The alliance said it also won more control over rosters, which can now only be changed by mutual agreement, and clear rules over promotions. There are also limits on labour hire contractors.

In a series of social media posts this week, the union alliance called out disagreements with Chevron over job security, rosters and promotions. Chevron has not commented on the negotiation beyond saying it continues to discuss matters with the workers.

WHAT ARE LIKELY IMPACTS ON SUPPLY AND PRICES?

Australia has kept its crown as the world's largest LNG producer, exporting 80.9 million metric tons in 2022 compared to 79 million tons in 2021, according to the International Gas Union.

The bulk of LNG exports from Gorgon and Wheatstone head to Japan, followed by South Korea, China and Taiwan.

As the northern hemisphere's year-end winter season approaches, potential strike action will add to a tightening market outlook amid a rebound in Chinese demand and unplanned outages in Norway, said National Australia Bank analyst Baden Moore.

"We see the price volatility as an indicator of increasing market tightness ahead of rising winter demand in Europe, and forecast global gas and coal prices to rebound strongly through 4Q23 on this basis."

Asia spot LNG prices jumped to a five-month high in mid-August after the risk of possible industrial action at the Chevron and Woodside LNG facilities in Australia raised concerns over supply.

($1 = 1.5463 Australian dollars)

(Reporting by Lewis Jackson in Sydney, Emily Chow and Florence Tan in Singapore; editing by Miral Fahmy)