By Selina Williams 

LONDON--Europe's biggest independent oil companies are flush with energy assets, but battered by the collapse of crude prices.

Investors are now betting on which one could be the next big acquisition target, following Royal Dutch Shell PLC's deal earlier this month to buy BG Group PLC for $70 billion.

Many analysts have been predicting a wave of deals in the wake of that blockbuster and after last year's sharp fall in the price of oil. Europe's bevy of exploration and production companies sits at the top of the list of potential targets.

Those include Tullow Oil PLC, the U.K.'s largest independent oil explorer; Genel Energy PLC, which produces crude in Kurdistan; and Sweden's Lundin Petroleum AB, which has a stake in a huge new North Sea development.

Investors are expecting deals in the U.S. as well. Possible prey include some of the relatively small but highly leveraged shale oil-and-gas producers that have played a key role in quickly raising U.S. production, a big factor in the current price weakness. But analysts caution that the gap between buyers' and sellers' expectations on price may still be too wide to immediately spark a big wave of deal-making there.

In Europe, share prices have soared amid near-daily speculation among analysts and investors about who might be next. Tullow stock, for instance, is up around 40% since the Shell offer, significantly outperforming the index of the FTSE's top 10 oil and gas companies over the same period.

"Companies that are unloved by the market but big in strategic resource themes--U.S. tight oil, East Africa liquefied-natural gas, deep water or frontier exploration--will be the focus of their attention," said oil and gas consultancy Wood Mackenzie in a note.

Energy acquisitions were expected to pick up after oil prices dropped from a high of $115 a barrel for Brent crude, the global benchmark, last summer to a low of $45 in January. Few deals were made, though, as the market's continued volatility made it hard for buyers and sellers to value assets and agree on a price.

Shell's offer for BG sent a jolt through the industry, triggering speculation over what might be next. Possible acquirers include Exxon Mobil Corp., which has expressed interest in deal making this year. State-run oil companies in Asia, like China's Cnooc Ltd. and India's Oil & Natural Gas Corp., may also see opportunity.

Exxon declined to comment on the subject. Chairman and Chief Executive Rex Tillerson told analysts in March: "No question, there are some good opportunities in front of us right now." Cnooc declined to comment. A spokesman for ONGC said its international arm, ONGC Videsh, was always looking at opportunities, but declined to give any details.

Goldman Sachs said in a recent report that big companies could buy smaller explorers and scrap their own expensive projects. A potential 5 million to 10 million barrels a day of low-cost projects could move from small companies to the majors and state-run oil companies, the bank said.

Obstacles remain to any new deals--including a big question mark over whether some of the largest companies are willing to make a move. Executives at BP PLC of the U.K. and Total SA of France said last week that acquisitions may not be in the cards. Both had borrowed money in recent weeks, fueling speculation they were on the hunt.

As for potential prey, investors are betting Tullow could be next. Its assets include an already-producing Ghana oil field, a second one starting up next year and some undeveloped oil discoveries onshore in Kenya and Uganda. The company has said that pumping oil from the two Ghana developments will cost around $8 a barrel--the kind of cheap production that would be tempting for a bigger oil company trying to boost production and save money while oil prices are around $60 a barrel.

Although Tullow's share price has rebounded sharply since the Shell announcement, the company's current market capitalization of about $5.44 billion is still less than a quarter of its $21.8 billion stock-market value at its peak in 2012.

Top management has been publicly opposed to a sale. Tullow founder and Chief Executive Aidan Heavey has said the company isn't for sale. He declined to comment for this article.

Genel CEO Tony Hayward, the former boss of BP, told investors Tuesday that the company was looking at acquisition opportunities itself.

"We're actively engaged both in getting together with larger companies and smaller companies in Kurdistan," Mr. Hayward said. "Whether anything will come of it only time will tell," he said.

Genel's production costs are low--it costs the company around $2 a barrel to pump oil from its Kurdish fields. But it still hasn't been fully paid by the regional government for its oil exports and last year reported a net loss.

Shares in Sweden's Lundin Petroleum and Det Norske ASA have also risen since the Shell-BG deal. Both companies have stakes in Johan Sverdrup, a huge oil field in Norway's North Sea, which is expected to start producing in 2019. Statoil ASA, which operates the field, has said the field would be profitable at an oil price above $40 a barrel. Both companies declined to comment.

Write to Selina Williams at selina.williams@wsj.com

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