According to this spread chart, current price relation OIL/SPX and price movement are nothing new, they resemble late 90`s - like many other things (e.g. dollar strength, biotech bubble = dotcom bubble, or check http://blogs.barrons.com/stockstowatchtoday/2014/12/08/look-on-the-bright-side-dow-drops-100-points-small-caps-tumble-as-oil-craters/
The peaks corelate with turning points in Gulf War and Iraq War.
The bottom of the spread could be distant another 10% at minimum, although now the spread could bounce from intermediate support and retrace last huge drop. However, both OIL
have lot of time till their trends will turn.