Canadian oil and gas company Encana Corp. will keep its U.S. headquarters in Denver, re-signing its lease in Colorado's tallest building through 2026. Encana signed a lease for 335,000 square feet in Republic Plaza, at 370 17th St., following a year that saw reductions in office space occupied by energy companies as mergers and consolidations in the oil and gas sector cut staffs. The company employs about 650 people in the Denver office.