Oil Minister Ali al-Naimi said oil and gas output was unaffected by the "terrorist attempt" - the first direct strike on a Saudi oil target since al Qaeda militants launched attacks aimed at toppling Saudi Arabia's pro-Western monarchy in 2003.
"Security forces foiled an attempted suicide attack at the Abqaiq refinery using at least two cars," an official said.
Oil prices jumped $2 a barrel on the news of the attack in the world's largest oil exporter, which came a year after Saudi-born Osama bin Laden urged his supporters to hit Gulf oil targets.
Saudi Security Adviser Nawaf Obaid said security forces fired on three cars at the outer gates of the Abqaiq facility, 1.5km from the main entrance.
One car was carrying gunmen and two others, packed with explosives, rammed the gates, he said. It was not clear how many militants were involved in the attack, but all had been killed.
Security sources in Riyadh said four militants and two security officers died and two other officers were wounded.
Interior Ministry Spokesman Mansour al-Turki told the state television earlier that six workers were also slightly injured. Security forces were combing the site for evidence.
"We have yet to determine the identity of the attackers. We are currently checking DNA samples," Turki said. Dubai-based Al Arabiya television said the attackers used cars carrying the logo of Saudi state-owned oil company Aramco.
Oil Minister Naimi, quoted by the Saudi Press Agency, said a small fire was quickly brought under control after the incident which he said took place at 3:10 pm.
Friday's attack was the first major strike by militants in Saudi Arabia since suicide bombers tried to storm the Interior Ministry in Riyadh in December 2004.
The prospect of a direct attack on Saudi crude facilities has been a doomsday scenario for oil consumer nations heavily reliant on Saudi oil. The kingdom accounts for around a sixth of the world's oil exports, supplying 7.5 million barrels a day.
Former Middle East CIA field officer Robert Baer has described Abqaiq as "the most vulnerable point and most spectacular target in the Saudi oil system."
But Aramco says it has the tightest security at all its oil plants, including helicopters, cameras, motion detectors and thousands of armed guards. "The security measures at the oil facilities are better than those at the royal palaces," said al Qaeda expert Fares bin Houzam. "The attack makes clear their (militants) ignorance of Aramco's facilities".
"It's not clear what damage there is but Abqaiq is the world's most important oil facility," said Gary Ross, CEO at PIRA Energy consultancy in New York.
"This just emphasises fears over global oil supply security when we're already facing major ongoing risks in Nigeria, Iran and Iraq."
"There had been concern that even though their capabilities had diminished they still had the intent to launch attacks in the kingdom," a US counter-terrorism official said.