Is it hard to turn heavy sulfur oil into gasoline and diesel fuel?
To the best of my knowledge, “not hard with the right refinery”.
Valero in particular had been customizing its refineries for high-sulfur crude.
Not really. Venezuela had plenty of refining capacity to meet domestic needs. Then came Hugo Chavez, nationalizing all the joint ventures with PdVSA. Then all employees were to pledge absolute loyalty to Chavez. Anyone perceived in the slightest not showing absolute loyalty was fired. Thus they lost nearly all of their qualified engineers, technicians, and operators. All that was left were incompetent sycophants. And now all the refineries are just about wrecked, including one I worked on 21 years ago.
Before Chavez, Venezuela was the richest per capita nation in South America.
About 35 years ago, I was a consultant at a refinery in Texas tasked to make a mass balance of the facility unit by unit. Regarding the crude oil into the refinery, I had the records of the crude oil shipments via tanker and pipeline from West Texas. I got a kick out of the tanker records that typically originated from the Middle East and Venezuela. Very few of the tankers sailed in a straight line from start to finish. Most zigzagged as the cargo was bought and sold as they transmitted the ocean and the destination port changed.
No, it is not hard to refine heavy, high sulfur crude oils into desirable gasoline, diesel fuel, aviation fuels, asphalt and high sulfur fuel oils providing the refinery vessels, pipes and storage facilities are built using the proper metallurgy. The sulfur is corrosive to the metals if not designed properly. Also, the high sulfur heavy crude oil produces much more heavy fuel oils and less gasoline than the lighter sweeter crudes. So the refining vessels have to be designed to take care of this fact as well. Refineries are built to accommodate the crude feed stock that will be available to the refinery.