"It appears to be an accident rather than an act of sabotage," one diplomat said.
State oil and gas firm Aramco is undertaking projects to boost output at the natural gas liquids recovery plant in Hawiyah.
The project will provide petrochemical feedstock to the industrial cities of Jubail and Yanbu in the kingdom, the world's largest oil exporter.
The Hawiyah programme will produce 310,000 barrels of ethane and liquefied natural gas (LNG) products by 2008.
It was not clear if the fire would delay the project.
Japan's JGC Corp was awarded the contract for the Hawiyah's liquefied natural gas production facilities.
Italian oil and gas company, Eni, is carrying out work related to gas treatment and compression facilities at the plant.