Sterne Agee CRT says that, among stocks in its E&P coverage universe, only Ultra (UPL), Gastar (GST) and Chesapeake Energy (CHK) have "potential credit issues before 2018." The firm expects Ultra to violate its leverage covenant by early May, and it notes that holders of the company's $1.35B in bonds must agree to relax the covenants. Gastar faces risk late in 2017, since its revolving credit facility expires in November of that year, while Chesapeake, which has debt maturities of $785M in 2017, could face "balance sheet stress in mid 2017," according to the firm. The firm says that Oasis (OAS) and Whiting (WLL) have strong liquidity. It keeps Neutral ratings on Chesapeake, Ultra and Gastar, and Buy ratings on Whiting and Oasis.