This was an awful week for the market, which is now off to its worst start...ever. Among the weights pulling down the market was the price of crude, which plunged below $30 per barrel for the first time since 2004. That led to a very steep drop in oil stocks, including those of Williams Companies (NYSE:WMB), Seadrill (NYSE:SDRL), Chesapeake Energy (NYSE:CHK), Denbury Resources (NYSE:DNR), and Whiting Petroleum (NYSE:WLL)

While the price of crude was the main catalyst sending these stocks down, each was burdened with an additional weight that led to a truly ugly showing this week. Williams Companies was battered by not one, but two credit rating downgrades as well as concerns that its merger with Energy Transfer Equity will fall apart. One of the biggest concerns from its rating agencies is Williams Companies' exposure to Chesapeake Energy. Nearly 20% of its gathering and processing revenue is from its contracts with Chesapeake, which has credit issues of its own. 

Speaking of credit concerns, Denbury Resources is trying to address its own credit concerns by proposing a bond exchange. Bondholders, however, don't like the terms. This forced the company to sweeten the deal, which its investors didn't like.

Analysts, meanwhile, continued to throw in the towel on oil stocks, with Whiting Petroleum, Chesapeake Energy, and Seadrill all either being downgraded or having their price targets slashed. Analysts were particularly bearish on Whiting Petroleum: It receiving two downgrades, and another analyst completely gave up coverage of oil stocks, including Whiting. 

To learn more about why these stocks moved so sharply this week, check out the following slideshow.