For many oil and oil stock investors, it was scary news. That was the revelation in early September that a trio of companies had made a huge discovery in the Gulf of Mexico — 15 billion barrels of oil and gas reserves, an amount equal to half of America’s current reserves.

Not unexpectedly, oil prices, trading at the time of the announcement in the $60s-a-barrel range and down from their July high of $78.40, proceeded to fall even further. As might be expected, oil stocks, also reacting to the fear of a major new supply, joined in the slide as worried investors rushed to take profits in the hottest market sector of the past three years.

To some energy experts, though, the heavy profit taking was another example of unwarranted investment fear, a case of frightened, impressionable investors running for cover without fully grasping all the nuances.

Franklin Delano Roosevelt may have aptly put it best in his inaugural address in March 1933, when he said, “We have nothing to fear, but fear itself.”

Some energy industry trackers say the negative reaction to the discovery by the three companies — Chevron, Devon Energy, and Norwegian oil giant Statoil — is still prevalent among many oil and oil stock investors. And to some degree, it continues to pressure the price of both the commodity and the shares.

Not so, though, in the case of money manager Anthony Gartha of Los Angeles-based MCR Associates, who tells me he used the selling for what he regarded as “a good buying opportunity” to fatten his energy holdings, particularly in Chevron, ConocoPhillips, Transocean, and Schlumberger.

“The selling reaction to the find was dumb,” he says. “You’re talking about a big maybe, something that may never come to pass.”

That’s also the thinking of investment adviser Stephen Leeb of Leeb Capital Management, who views the big find as nothing more than “hype.” In effect, he suggests the discovery may simply be another one of those Wall Street pipe dreams.

The devil is in the details, and for an oil discovery a key detail is how much it will cost to pump oil out, bring it to a refinery, and convert it into gasoline and heating oil. “We think the costs will be daunting,” he says.

He also took note of the caution expressed by Statoil, which noted that another exploratory well will have to be drilled next year to determine whether development can actually proceed. If indeed development does proceed, commercial production, according to Statoil, isn’t expected from the well before 2013.

Mr. Leeb is also quick to note that the test well on which the discovery was based was the deepest ever drilled in the Gulf of Mexico (down 28,715 feet). No one knows much about rock formations at that depth, he points out, which is why more exploratory wells are needed. Nor, he adds, is it known at what price oil production would be economical. The most optimistic reports, Mr. Leeb notes, put the figure at $40 a barrel. But preliminary figures, he says, suggest $80 is more likely, and one geologist thinks a range of between $300 and $400 is more realistic to make the well worth developing. In sum, he argues, it’s too soon to add the discovery to current oil reserves.

Noting that the supply/demand balance in the oil industry remains razor thin, Mr. Leeb says once the discovery hype has run its course, he expects a healthy rebound in oil prices and energy stocks.

Among his top energy picks are Chevron, Exelon, Baker Hughes, Halliburton, Petro-Canada, Schumberger, Suncor Energy, and Transocean.

Another skeptic raising questions about the discovery is Bob Berke, an energy adviser to liquidity tracker TrimTabs Research of Santa Rosa, Calif. “Right now, it’s all very much of a question mark,” he says. Pointing, in particular, to such uncertainties as drilling pressures, heat levels, and the question of whether drills will hold up when you eclipse 25,000-feet depths.

He’s also dubious as to whether oil could be brought out of the ground at a price to make the discovery commercially feasible. “The thing to keep in mind,” he says, “is that the cost to do so could be outrageous.”

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