The trend took root in the United States last year where activist shareholders targeted sizeable companies including Hess Corp (>> Hess Corp.). It now has three new directors and is selling assets in Asia after the intervention of Elliot Management.

The UK-listed companies in the sights of activists are smaller but given the worsening outlook for oil and gas prices and disappointment with some more marginal energy prospects of the boom years they may look for more and bigger targets.

Some UK companies have ended up with significant assets, including cash, relative to their shrunken stock market value.

"It's a very simple model," said a London investment banker who specialises in oil and gas and did not want to be named.

"You don't have to take a view on the value of the actual assets or know anything about oil and gas. You just know the cash is there for the taking."

One target is 3Legs Resources Plc (>> 3Legs Resources plc), which is exploring for shale gas in Poland. Polish shale was once seen as a red-hot opportunity, but enthusiasm waned quickly last year when the government downgraded resource estimates and top global player Exxon Mobil (>> Exxon Mobil Corporation) pulled out.

The stock, which values the company at about 24 million pounds, trades below the cash position it is expected to have in the middle of next year, the company's broker, Jefferies, said in a June 20 research note.

Boston-based activist investor Weiss Asset Management's stake in 3Legs topped 15 percent in April, according to a disclosure by 3Legs. Another activist, Guernsey-based Damille Investments, acquired a similar-sized stake in February.

In the past six months the company has twice faced down attempts to install a new board and put the business up for sale, defeating the activists in votes at extraordinary shareholder meetings and insisting its commitment to Polish shale will pay off.

Another example is Northern Petroleum (>> Northern Petroleum Plc), a company now worth about 30 million pounds but with a disappointing stock performance, and in which Damille emerged as a 5 percent shareholder in December. Since Damille took a stake it has made a number of board changes.

Weiss declined to comment on specific investments. Damille's parent, Nimrod Capital, did not respond to phone calls and an email requesting comment.

Another company trying to fend off shareholder activism is JKX Oil and Gas (>> JKX Oil & Gas Plc). Earlier this year, CEO Paul Davies narrowly survived a shareholder revolt led by Eclairs Group - not a traditional activist fund but an investment vehicle of Ukrainian billionaire Igor Kolomoisky, who has a 27 percent stake.

FERTILE GROUND

Mainstream activist fund strategy is to buy enough stock to force a shareholder vote, or buy a more modest holding and recruit other shareholders to a cause - usually selling assets or ousting directors seen as weak, misguided or overpaid.

Investment trusts and property, where valuation is easy and assets are relatively liquid, are common activist targets, but last year and this year, the energy sector has drawn them in.

This is partly because lenders trying to find good returns in a low interest rate world offered funds to energy companies set up amid excitement over the potential for unconventional oil and gas reserves to create riches.

In a note on activism in the U.S. oil and gas sector, debt rating agency Moody's described this atmosphere as "an entrepreneurial culture which can lead to weaker checks and balances on corporate leadership and less discipline around executive compensation practices".

The energy sector activism trend also reflects the absence of the predatory takeover bidders that have emerged in past oil price cycle downturns. "There are no aggressive cash buyers out there," said the banker.

Other U.S. examples alongside Hess Corp include Sandridge Energy (>> SandRidge Energy Inc.) which has four directors on its board appointed by hedge fund TPG-Axon. It is under pressure to sack its CEO, and is looking at making asset sales too while Devon Energy (>> Devon Energy Corp) is seen to be pre-empting activist interest by putting assets up for sale.

Moody's said the surge in shareholder activism in the North American energy sector "will persist into 2014".

This year, it said, the total number of activist shareholder campaigns in the U.S. and Canada looks likely to exceed 2012's record levels, "due to record levels of cash on corporate balance sheets, a ready supply of inexpensive debt and activists' own success in attracting investment capital".

(Editing by Anna Willard)

By Andrew Callus