The light, sweet versus heavy, sour problem goes beyond production.  What about inventory?  No one differentiates inventories on the basis of quality.  Matt Simmons used the following analogy.  

It's as if when you asked what a car costs, they gave you the price of a 2005 Rolls Royce.  When you asked how many cars are in inventory, they counted rusted out 1960 Plymouths the same as a 2005 Rolls Royce.  In other words, the commonly tracked crude oil inventory numbers are virtually useless.  As you noted, the developing light, sweet crude oil shortage is showing up in the spread between light, sweet and heavy, sour.

Jeffrey Brown
Petroleum Geologist

Exactly, it's a shame that there are no reliable statistics on the crude oil stock quality coming from the EIA. The fact that gasoline stocks are falling   in the US is an indicator that the quality of the crude input to the refineries may have changed.