By Christopher Bjork 

MADRID--After assuming full executive powers at Repsol SA this week, Chief Executive Josu Jon Imaz now faces the challenge of completing the Spanish oil giant's biggest acquisition in years--while pushing the steep cost-cutting needed to survive today's crude-price rout.

Mr. Imaz, a former Spanish politician who rose through Repsol's ranks by squeezing profit from its refinery business, became CEO a year ago. However, since then the 51-year-old Mr. Imaz had been serving in the shadow of Chairman Antonio Brufau, who relinquished his executive duties Thursday, after a tumultuous decade at the helm. Mr. Brufau, will stay on at Repsol as a nonexecutive chairman.

Late last year, Mr. Brufau orchestrated Repsol's $8.3 billion purchase of Canadian oil and natural-gas producer Talisman Energy Inc. The purchase is a chance to restore Repsol's oil and natural-gas producing ambitions more than two years after Argentina expropriated a large chunk of its fields.

The Talisman deal is expected to close May 8. As Mr. Imaz takes full responsibility for executing the integration, analysts are already second guessing the price Repsol paid--as oil prices remain low. Mr. Imaz said he'll apply lessons learned from his days running Repsol's refining operations to squeeze out costs in the company's newly expanded exploration and production business.

Oil firms in Europe have closed 20 money-losing refineries in recent years. Repsol went a different route, assigning Mr. Imaz to oversee a EUR4 billion ($4.45 billion) upgrade of its Spanish refineries so they could operate at a lower cost.

"We got used to working very efficiently, squeezing out profits when margins were very tight," Mr. Imaz told The Wall Street Journal ahead of this week's transfer of power at the oil firm. "That's the equivalent of operating with low oil prices."

Mr. Imaz said Repsol's expansion will help squeeze the cost of producing oil across its assets around the world--including Talisman's large, untapped shale resources in North America, and its oil and gas fields in the North Sea and off Southeast Asia.

Repsol now spends an average of $12 producing every barrel of oil. Mr. Imaz said he can cut that to between $6 and $7 a barrel by drilling more selectively in the company's expanded portfolio of reserves.

"For the past two decades, the sector was in an environment where adding barrels and growing mattered most," he said. "We all put efficiency second."

With oil prices now far lower than Repsol had envisioned as part of the calculus behind the Talisman deal, Mr. Imaz is under pressure to show results quickly.

Brent crude was trading just below $80 a barrel in late November when Repsol set its final valuation for Talisman. By the time the acquisition was announced in December, oil had fallen below $65 a barrel. Early Friday, Brent was trading just over $66 a barrel.

In justifying the deal's price, Repsol said it expects Brent oil to rise to $85 a barrel this year and to $99 within three years. Repsol said the deal makes sense even if prices remain between $70 and $80 through 2017.

"It looks like they bought a bit too early," said Pascal Menges, a fund manager at Lombard Odier. The fund doesn't own any Repsol shares.

Mr. Imaz said he expects an oil price recovery. "I don't know if it will take three or six months, a year or a year and a half," he said. "But I expect oil prices to return to $80 a barrel."

Still, Mr. Imaz doesn't see any more big acquisitions for the time being. With Talisman, Repsol has an optimal size, he said: "Our growth story has ended, for now."

Mr. Imaz has some cushion--Repsol's substantial refining business. In the fourth quarter, Repsol's so-called downstream division, which includes refining, made $370 million in profit, compared with $4 million profit in the company's upstream division, which includes oil and gas exploration and production.

With a Ph.D. in chemistry and no upstream experience, Mr. Imaz has scrambled to get up to speed on Repsol's drilling business, which will grow significantly with the Talisman purchase.

In a November meeting with one of the company's 20 largest institutional investors, he relied on other executives to answer detailed questions about Repsol's drilling activities, according to a portfolio manager who was present. At a third meeting, a few months later, Mr. Imaz held his own.

"You could ask him pretty much whatever you wanted to," said the portfolio manager.

Mr. Imaz rose from middle-class origins in the town of Zumárraga and eventually led the Basque Nationalist Party, the largest political force in northern Spain's heavily industrialized Basque region. He quit politics in 2007 after clashing with a faction of his party that wanted a referendum on the region's independence from Spain, a move he opposed.

Mr. Brufau, hired him a year later. The two had met when Mr. Imaz was serving as the regional industry minister.

He started running one of Repsol's Spanish refineries, then took over all five. In recent years, big shareholders have pressured Mr. Brufau to relinquish some of his control of the company. In the wake of the dispute with Argentina, Repsol began organizing a transition of power, including separating the roles of chief executive and chairman. Repsol's board chose Mr. Imaz as chief executive over two other candidates.

At Repsol's annual shareholder meeting Thursday, Mr. Brufau said the company needed a "youthful" leader.

Write to Christopher Bjork at christopher.bjork@wsj.com

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