There’s more to come -- by one estimate, another $20 billion this year -- as executives at Occidental Petroleum Corp., Whiting Petroleum Corp., Penn Virginia Corp., Exco Resources Inc., Chesapeake Energy Corp. and Ultra Petroleum Corp. have all said in recent weeks that they are selling assets or exploring sales.

The surge shows how the industry’s two-pronged strategy for staying financially healthy since oil prices started tanking -- raising capital while tightening belts -- may not be enough, particularly for companies with a lot of outstanding debt. Bank regulators are getting nervous about the industry’s exposure to drillers, creating another incentive to sell assets.