According to the NSA intercepts, Petro SE was devised by the Saudis because of their overriding fear that if an internal revolt or external attack threatened the survival of the House of Saud, the U.S. and other Western powers might abandon them as the Shah of Iran was abandoned in 1979. Only by having in place a system that threatened to create crippling oil price increases, political instability and economic recessions did the royal family believe it could coerce Western military powers to keep them in power.
Huffington Post provides extensive additional detail..,