NEW YORK (AP) — Chesapeake Energy, a shale drilling pioneer which was once one of the largest natural gas producers in the U.S., has filed for bankruptcy protection. The oil and gas company was a leader in the shale boom. But that boom was fueled largely by debt, and many of those bills are coming due in the next few years. More than 200 oil producers filed for bankruptcy protection in the past five years, a trend that's expected to continue, especially among smaller companies which may not be able to repay or refinance those loans. By CATHY BUSSEWITZ and TALI ARBEL AP Business Writers