By JACK Z. SMITH The stock of Fort Worth-based Encore Acquisition Co. jumped 20.5 percent Monday after Plano-based Denbury Resources announced plans to acquire the oil and natural gas exploration and production firm in a deal valued at $4.5 billion.Denbury stock (ticker: DNR) fell 10.3 percent, however, and Moody’s Investors Service placed its credit ratings for the energy firm under review for possible downgrade.