It appears that the world is awash in more petro distillates than it
can consume or store. Ships in transit have become the storage of
last resort and that can't last forever. This will continue to
translate into lower prices at the pump but there is only so much
elasticity in demand as prices come down. Eventually the oil
producers will have to decommission some of their wells or pump less
agressively.

http://www.reuters.com/article/2015/10/26/oil-diesel-glut-idUSL8N12N36520151026

There is an interesting effect where diesel prices come down faster
than gasoline since diesel fuel gets produced as a byproduct of
gasoline distillation, but the demand for diesel increases more slowly
as prices come down. That creates a temporary production imbalance
and lower prices for diesel.