"Activists are secretly buying up stakes now because they believe the markets have hit rock bottom," said Kai Haakon Liekefett, a partner at Vinson & Elkins LLP in Houston who heads the law firm's shareholder activism response team. He said hedge fund activity in the oil patch "may get hot" as early as this fall or winter.

Still, oil prices are down 50 percent from last June, leaving many energy company stocks looking cheap. A Reuters review of regulatory filings, as well as interviews with more than two dozen company executives, investment bankers, lawyers and investors, reveal that many hedge funds are seizing the opportunity to buy shares on the cheap.

It's a bet on a view that's contrary to some analysts: that oil prices are poised to rebound. This month Citigroup cut its crude price forecasts, saying West Texas Intermediate could go as low as $20 a barrel before recovering. West Texas crude is currently at about $51 a barrel, up from a low of $43.58 in January.

Elliot Management and Omega Advisors are among activist hedge funds buying stakes in North American oil and gas producers whose stock has fallen because of weaknesses exposed by the collapse of energy prices.

Activist investors aim to make money by buying shares of companies and then pushing them to change their strategy. This can include raising their dividend, launching a share buyback, spinning off a division, or an outright sale of company.

In the case of oil and gas companies, calls for a change in corporate strategy may not come straight away because some hedge funds will wait for energy prices to rise first, boosting the value of the company shares they acquired. That way, a hedge fund may cash out profitably even if its activist strategy fails, or if it decides against moving forward with it.

In disclosures this week, hedge funds known for their activist investing, including Elliot and Omega, said they bought stakes in Continental Resources, Marathon Oil, Laredo Petroleum and Sanchez Energy. They are obliged to disclose such stakes in quarterly regulatory filings with the U.S. Securities & Exchange Commission.

In the last six months, since June 19, Continental shares have fallen 36.9 percent, Marathon is down 25.2 percent, Laredo has slipped 52.5 percent and Sanchez has dropped by 60 percent.

While the hedge fund investors' immediate intentions are not known, funds are taking advantage of the companies' low valuations to build positions they can exploit later, industry sources said.

Read MoreOil may fall again … but buy energy stocks?

For example, Elliott, a hedge fund run by billionaire Paul Singer, took a 4.5 percent stake in oil and gas exploration and production company Hess in 2013 and got three seats on its board after a bitter proxy fight with the company's management. It then successfully pushed the company to carry out several asset sales over the next two years.

Overstretched oil and gas exploration and production companies with assets in multiple basins, inefficient production and low growth are most likely to fall prey to activists, according to industry executives.

For example, Resolute Energy, said last week it would cut down on spending, hampering the developments of its assets and potentially limiting its earnings growth. Resolute's assets are spread out in Utah's Aneth Field, Wyoming's Hilight Field and Texas' Permian Basin.

Resolute Energy as well as Laredo are vulnerable to activists because of their weak balance sheet. Their indebtedness, equivalent to 20.61 and 9.85 times of their annual cash flow, respectively, is much higher than the industry average of 5.84 times annual cash flow. This could make them susceptible to activist calls to bolster their finances through asset sales. Laredo and Resolute Energy did not respond to requests for comment.

Agitating now or later

It is unlikely activists will call for an oil and gas company to explore a sale in this environment, given the effect of low energy prices on company valuations, a view that would align with those of oil company executives. But activists may put pressure on them to shed assets now in a bid to strengthen their balance sheet, as Elliott did with Hess.



Activists are betting bet consolidation in the energy sector will occur once oil prices stabilize.Exxon Mobil, Chevron and other large exploration and production companies will ultimately use their cash piles to make some large acquisitions. Activists may hope companies they are buying into would be the targets.

Another strategy activists may demand is that mature oil producing assets be spun off into yield-generating vehicles known as a master limited partnerships (MLPs). This is because the remaining assets that stay with the company will have higher growth, and so the shares held by the hedge fund in that company are likely to get a boost.

To be sure, many hedge funds acknowledge that their strategy comes with significant risks. Their bets could be wiped out if the oil price turns against them.

"You can have a perfect thesis and argument for what a company should do to unlock shareholder value, but if the commodity price moves down and the share price drops, the value of the proposals could be completely destroyed," said Shreyas Gupta, a partner at activist investor Sandell Asset Management, a firm with a history of activist investing.