Saudi Aramco is beginning to reap the rewards of developing two giant petrochemicals joint ventures with a combined price tag of almost $30bn – the $20bn Sadara complex built with US firm Dow Chemical and the $9.1bn Rabigh-2 expansion of the existing Petro Rabigh refining and petchems JV with Japan’s Sumitomo. Both are in the vanguard of the state petroleum giant’s push further downstream into petchems in search of bigger returns on produced crude oil.

While much of the...