(Adds investor and analyst comments, updates share price)
CALGARY, Alberta, June 20 (Reuters) - Cenovus Energy Inc said on Tuesday it will replace Chief Executive Brian Ferguson, who championed an unpopular purchase of western Canadian oil sands assets that doubled the size of the company but sent its stock plunging.
Canada's third-largest oil producer failed to name a successor to Ferguson, however, pushing its stock down another 10 percent on Tuesday.
The company said Ferguson will remain CEO until October while it searches for a new leader, then stay on in an advisory role until March 2018.
The delay in naming a successor upset investors looking for quick change at the top of Cenovus, whose stock has lost about half its value, wiping out nearly $7 billion, since it announced plans in March to buy oil sands and natural gas assets from ConocoPhillips.
"There's no natural heir, they have not done a very good job of succession planning," said Laura Lau, senior portfolio manager at Brompton Group, which owns Cenovus shares. "That's why he is there until October and unfortunately he is almost a lame duck."
Shareholders balked at the size of the $13.3 billion ConocoPhillips deal, which doubles down on the high-cost oil sands at a time when international energy firms are exiting that area and growth opportunities look scarce because of weak global crude prices.
Investors criticized Ferguson for buying ConocoPhillips natural gas operations, an area where the company has no experience. They also complained they were given no opportunity to vote on the deal, which saddled the company's pristine balance sheet with debt.
A renewed slump in oil prices, which on Tuesday hit a seven-month low below $43 a barrel, added to Cenovus' clouded outlook. The company's shares were down 10.5 percent at C$9.20 in Toronto on Tuesday afternoon.
"It has gone from bad to worse for these guys," said Brian Pow, analyst at Acumen Capital Partners in Calgary. "What they thought was a great acquisition three months ago is turning out to look like it was bought at the top of the market."
Cenovus on Tuesday also unveiled plans to ramp up divestitures and sell C$4 billion to C$5 billion in noncore assets to reduce debt. Previously the company said it would offload about C$3.6 billion of assets including Pelican Lake and Suffield.
Other assets the company may sell include part of the Deep Basin gas field it acquired from ConocoPhillips and Marten Hills heavy oil assets.
It is also looking to sell its Palliser and Weyburn conventional oil projects in a deal that could be announced in the fourth quarter, Ferguson said at an investors' meeting in Toronto on Tuesday. ($1 = 1.3279 Canadian dollars)
(Additional reporting by Yashaswini Swamynathan in Bengaluru, and; Fergal Smith and Solarina Ho in Toronto; Editing by Jim Finkle and Matthew Lewis)