EnCana Corporation and ConocoPhillips announced Thursday that they have agreed to create an integrated North American heavy-oil business. EnCana, Canada s biggest independent oil and natural-gas producer, is one of several developers of Canada s vast oil sands resources. The venture will be comprised of two 50-50 operating partnerships, one Canadian upstream partnership, and one U.S. downstream partnership. Over the next decade, the upstream partnership plans to invest $5.4 billion to increase bitumen production capacity in northeastern Alberta. By 2015, the downstream partnership plans to invest about $5.3 billion in U.S. refineries, the companies said.