What: In what's becoming a recurring theme this year, the price of oil fell again today, down by 3.6% to just over $28 per barrel. That slide pushed oil stocks firmly into the red, with Whiting Petroleum (NYSE: WLL), SM Energy (NYSE: SM), Encana (NYSE: ECA), RSP Permian (NYSE: RSPP), and QEP Resources (NYSE: QEP) among the dozens of energy stocks down double digits today.

So what: Not one of those companies had a single shred of noteworthy news, other than the fact that the price of oil was down. And, the only reason oil was down today was because sanctions against Iran were lifted over the weekend, which means it can begin exporting oil. This was something the market has been anticipating for quite some time, but now that it's a reality, the market is throwing another one of its fits.

Having said that, the fact that oil is now well below $30 a barrel is a problem for every single oil producer listed. Whiting Petroleum, for example, needs oil to average $50 a barrel in order to operate at cash flow breakeven. SM Energy needs $50 oil in order to earn an economic drilling return from its Three Forks wells. Encana had expected a $500 million shortfall between cash flow and capex this year, and that's based off the higher price of oil from December. In other words, that gap is widening with each drop in the price. Meanwhile, RSP Permian's capex plan for 2016 is flexible -- however, its low oil price scenario was based on $40 oil. Finally, QEP Resources, like everyone else, needs higher oil prices in order to earn an economic drilling return and match capex with cash flow.

Now what: Mid-tier oil producers spent most of 2015 gearing their companies to run at a lower oil price. Unfortunately, that price was more like $50 a barrel than the current sub-$30 oil price. That means each has a lot more work to do given that oil keeps falling, which is worrying investors because there's only so far these companies can be pushed before they reach their breaking point. 

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