It's been going on for a couple of weeks. Look at BP. It was trading at about $53 and it closed today at $39 and change.
That chart doesn't reflect today's plunge which would take it completely off the chart.
These are enormous oil companies, and so we're talking tens of billions of dollars of market cap disappearing, daily!
There's no good reason for it in terms of the industry. Oil prices are pretty solid, and with the weakening dollar, they'll probably rise. I think they're simply being caught in the mutual fund redemptions that are going on. Investors are selling their mutual funds and the managers have to unload stocks to pay them.
It's not terribly sustainable, though. Most of the big oil stocks pay a dividend, and at these stock prices, the dividend is paying more than some CDs. When investors notice that, they'll prefer to buy these stocks instead of CDs. They pay more, plus they can appreciate while a CD can't.
That's where the bargains are in the stock market today. Look for a stock that's paying a dividend higher than what you would get if you cash out, as long as that dividend isn't likely to be cut.