On Thursday, OPEC announced that it would not curb production to combat the decline in oil prices, which have been blamed in part on a global supply glut.

And now that oil prices have fallen more than 30% in just the last six or so months, everyone wants to know how low prices can go before oil projects start shutting down, particularly US shale projects.

In a note last week, Citi's Ed Morse highlighted this chart, showing that for most US shale plays, costs are below $80 a barrel.