Oklahoma City-based Kerr-McGee Corp. announced plans Wednesday to buy Denver-based Westport Resources Corp. in a $2.5 billion stock deal that will significantly expand Kerr-McGee’s oil and natural gas holdings in Texas and the Rockies. The newly-acquired properties will increase Kerr-McGee’s daily production volume by more than a third, and will extend Kerr-McGee’s current Gulf Coast property portfolio into the Rocky Mountain area. The deal is expected to close in the third quarter of this year.