Privately held Chaparral Energy Inc., Oklahoma City, plans to acquire Tulsa, Okla.-based Calumet Oil Co. and certain affiliates for approximately $510 million in cash. Calumet owns properties principally in Oklahoma and Texas that are complementary to Chaparral's existing core areas, including the North Burbank Unit that it operates with 99% working interest. Current production is 28 million cubic feet of gas equivalent per day, raising Chaparral's production to 115 million cubic feet equivalent. Proved reserves are approximately 309 billion cubic feet equivalent (93% oil).