RAS TANURA, SAUDI ARABIA -- On the radar screen in the port director's shed overlooking the oil piers jutting out into the Persian Gulf, every steel-hulled ship or aluminum rowboat within 24 miles appears as a bright amber glow.

Saudi Arabian coast guard cutters and ships of half a dozen friendly navies patrol the bright green waters, protecting the world's biggest oil-loading port from seaborne attack.

On the landward side, between the loading piers and the country's biggest refinery, missile and antiaircraft batteries point skyward. Policemen carrying sidearms and M-16 rifles scrutinize every visitor and patrol the perimeter of this sprawling facility, checking for breaches in the razor wire-and-chain link fence.

Ras Tanura is generally the first name that comes up in discussions of the security of Saudi Arabia's oil installations. The producing fields are mostly invisible and far from the scene of any confrontation with Iraq, but Ras Tanura is a mammoth, stationary potential target on the Persian Gulf shore, about 100 miles from the Kuwaiti border.

It has enough berths to fill up to 12 tankers at a time, its storage tanks hold tens of millions of barrels of crude oil and gasoline, and its 530,000 barrel-a-day refinery supplies almost all of Saudi Arabia's domestic demand for gasoline and jet fuel, freeing the rest of the country's output for export.

Some Saudi offshore wells are closer to Iraqi positions in occupied Kuwait, but they could be shut down with little impact on supply. Ras Tanura is another story, symbolically and in fact. Oil analysts say any disruption of loading or refining operations here would send oil prices skyrocketing again.

Saudi Arabia has other means of delivering crude oil, including a pipeline across the country to the Red Sea port of Yanbu that can carry 3.2 million barrels a day, about half of what is currently flowing through Ras Tanura.

But oil analysts in the United States and Saudi officials here learned from the experience of a 1977 fire and explosion in a pumping station at Abqaiq that any perception of Saudi vulnerability traumatizes world oil markets. In the current environment, they say, an incident at Ras Tanura might have an incendiary effect. Some Saudi officials believe such an incident might even provide the provocation that could start a war in the region.

After Iraqi President Saddam Hussein threatened last month to blow up the region's oil facilities, a former security chief of Saudi Aramco, the state-owned oil company, said that "they haven't done as much as they should" to protect oil facilities. Other experts said that while Ras Tanura is out of Iraqi artillery range and a military attack appears unlikely, Saudi facilities could be vulnerable to terrorists or commando actions.

Behind their security screen, Aramco officials said today they are satisfied that the country's single most important oil center is safe from either full-scale military attack or penetration by terrorists.

"We're in good hands. The oil flow will continue," said Ali Saleh Ghamdi, vice president for operations. "We've got our own security force, friendly navies all over the place, mobile units. It's difficult to imagine infiltrators coming in from the water."

"The Saudi military and friendly forces here tell us we are protected, and we take them at their word," Nasr Ajmi, executive vice president of Saudi Aramco.

Still, a facility such as this cannot be sealed off entirely. The nautical radar will not pick up small boats made of wood or rubber, such as those sometimes used by Palestinian guerrillas in efforts to infiltrate Israel. The refinery and crude oil tanks are fed by pipelines easily accessible from an unguarded highway, raising the possibility of an attack that would probably have a minor impact on oil supply but a major one on market psychology.

Most of Saudi Aramco's 43,000 employees received unexpected 15 percent pay increases Oct. 1, after Saddam's threat to the oil facilities. But Ghamdi said this was not an attempt to buy their loyalty, which he said the company already has. "If I didn't believe that, I wouldn't be able to sleep," he said.