--------

Most of Venezuela's output, especially from the Orinoco Belt, is extra-heavy, high-sulfur crude (roughly 8-16° API). Some lighter and medium grades exist, but heavy dominates. Raw Orinoco bitumen-like crude is so thick it has to be diluted with lighter hydrocarbons (naphtha or light crude) or upgraded before it can move through pipelines.

Why it's tricky for gasoline

Low natural gasoline yield. Heavy crude has a large fraction of long, heavy molecules (residuum) and relatively few light ones. A simple distillation of it yields much more fuel oil and asphalt than gasoline.

High sulfur and metals. Sulfur, nickel, and vanadium require hydrotreating and can poison catalysts, so refineries need more processing.

Conversion is required. To get gasoline out of heavy crude, refineries need "complex" units such as cokers, hydrocrackers, and fluid catalytic crackers (FCC). These break big molecules into smaller ones, and the output is then desulfurized and blended into finished gasoline.

https://i.imgur.com/q7dWcIv.png

(http://www.autoadmit.com/thread.php?thread_id=5908564&forum_id=2#50165475)