CALGARY — Athabasca Oil Corp. says its president and chief executive officer will retire on Sept. 30.CEO Sveinung Svarte has been at the helm of the Calgary-based company since 2006. He will remain on the company’s board of directors as vice chairman and will also continue to be involved in business development for Athabasca.Board chairman Tom Buchanan will assume Svarte’s executive duties. “Since 2006, under his leadership Athabasca has grown into a $3 billion company with an extensive portfolio of world class resource assets in both light oil and thermal oil. We look forward to his continued involvement as a valued member of the board,” Buchanan said in a news release on Monday.Also, the company’s board of directors has approved an increase of $140 million to Athabasca’s 2014 capital budget, bringing the total to $667 million. Athabasca says it’s also maintaining its guidance range of 6,000 to 6,500 barrels of oil equivalent per day for the second half of 2014.Athabasca recently sold its stake in the Dover oilsands project to a Canadian unit of PetroChina, which will bring proceeds of $1.184 billion, $600 million of which has already been paid in cash. The company has said it will continue developing its holdings in Alberta’s promising Duvernay shale and the Hangingstone oil sands project.