BY JAY F. MARKS Business Writer jmarks@opubco.comChesapeake Energy Corp. is facing a formal investigation by the U.S. Securities and Exchange Commission, the company confirmed Friday in a regulatory filing. The SEC's Fort Worth office initiated an informal inquiry of the Oklahoma City oil and natural gas company last year after Reuters reported on more than $1 billion in personal loans secured by CEO Aubrey McClendon, using his personal stake in Chesapeake wells as collateral. Chesapeake, which had been cooperating with the SEC, was notified Dec.