Addax Petroleum Corp., Calgary, (Toronto: AXC) has increased its participating interest in Block 4 of the Nigeria-Sao Tome and Principe joint development zone from 33.3% to 38.3% through the acquisition of a 5% participating interest from Overt Ventures Ltd. for US$15 million. Addax will pay US$10 million to Overt and US$5 million to the joint development authority. Block 4 covers a gross area of 857 square kilometers (211,773 acres) and is approximately 200 kilometers offshore Nigeria.