On Thursday, Royal Dutch Shell announced it was investigating a potential leak in the Gulf of Mexico. On the same day, it reported a more than sevenfold increase in the amount of crude spilled over the last year in Nigeria, one of its major host countries. But only one of these events is believed to be the catalyst for investor fright.

http://blogs.wsj.com/source/2012/04/13/a-tale-of-two-oil-slicks/?mod=google_news_blog