Historically Brent crude was slightly cheaper than Light Sweet for the most part. Looking at the prices today, Brent is over $10 more than LS. Why the disparity? It can't all be currency related.
If WTI sells a too big a premium over brent, people will import brent and not buy WTI getting prices to be closer over time. When the opposite happens like right now, that mechanism doesnt exist because WTI from Cushing can only be sent to Chicago(people trying to take advantage of that in the UK, cant) and if there is no demand by the refiners there(and it should be the case given the recession) then the WTI just keeps being stock pilled more and more and driving the price down
The big difference right now is the lack of storage capacity in the US, so few can take physical delivery and store it. So it does not take a lot of sellers to push the price down. Also the Feb contract expires in a few days so people are unloading Feb. contracts.