A pair of office towers in Tulsa, Oklahoma, is giving commercialreal estate investors more reason to worry that the collapse in oilprices 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, accordingto a report from the firm that services a $45 million mortgage onthe buildings known as the Williams Center Towers. Samson, which ispreparing to file for bankruptcy protection this month, hasindicated it will abandon another floor next year, and its leasegives it the right to withdraw from more space after that,according to the report.

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