A pair of office towers in Tulsa, Oklahoma, is giving commercial real estate investors more reason to worry that the collapse in oil prices is starting to infect their market.

The biggest tenant, oil and gas producer Samson Resources Corp., vacated one of the more than a dozen floors it occupied, according to a report from the firm that services a $45 million mortgage on the buildings known as the Williams Center Towers. Samson, which is preparing to file for bankruptcy protection this month, has indicated it will abandon another floor next year, and its lease gives it the right to withdraw from more space after that, according to the report.

Samson's shrinking footprint is laying bare the risks faced in cities that boomed amid the U.S. shale revolution — and are now the most vulnerable to the commodities rout. That's increasing concern in the market for commercial mortgage bonds, where yield-hungry debt investors have helped fund everything from office space for oil executives to housing for rig workers.