The source of their wealth is Saudi Aramco
Facts
Saudi Aramco , the Saudi Arabian Oil Company (formerly Arabian-American Oil Company), is a Saudi Arabian public petroleum and natural gas company based in Dhahran.
Saudi Aramco has both the world's second-largest proven crude oil reserves, at more than 270 billion barrels (43 billion cubic metres), and largest daily oil production of all oil producing companies.
Saudi Aramco operates the world's largest single hydrocarbon network, the Master Gas System.
It manages over one hundred oil and gas fields in Saudi Arabia, including 288.4 trillion standard cubic feet (scf) of natural gas reserves.
Saudi Aramco operates the Ghawar Field, the world's largest onshore oil field, and the Safaniya Field, the world's largest offshore oil field.
The company has a market capitalisation of about US$2 trillion
Saudi Aramco was ranked as the 5th-largest public company in the world.