Steep downward revisions to oil and gas reserves at the end of this year are likely to increase scrutiny of how energy companies tally future barrels — a process that has become more opaque with the rise of shale drilling.

The revisions due in December will reflect a deep plunge in crude prices and should not come as a surprise for investors who have been pouring billions of dollars into U.S. oil companies betting that crude prices will recover.

But investors may not fully appreciate other risks stemming from the wide variety of methods companies use to estimate and vet their reserves or economically recoverable oil and gas.

Reserves have long underpinned company stock prices. Reserve growth is used at companies, including ConocoPhillips as a component of the chief executive's ­compensation.

Yet there is plenty of uncertainty in the industry because measuring unconventional resources such as shale oil is a young science and there is no uniform process for reserve reporting, geologists, investors and lawyers say.

In contrast to financial reporting, the U.S. Securities Exchange Commision does not require outside auditing for reserves. That leaves the process "a little opaque" according to Stewart Glickman, director of energy research at S&P Capital IQ in New York.

A Reuters analysis of public filings by companies in the Dow Jones U.S. Oil and Gas index reveals a wide variety of practices used to report them.

Of the 41 exploration and production companies in the Dow Jones Oil and Gas Producers Index, 15 companies had independent petroleum engineers generate their reserves estimates for them. Another 18 companies came up with the estimates themselves and had an independent engineering firm audit those results, Securities and Exchange Commission filings show.

Four of the firms generated their own results and had consultants review the procedures and methods used to prepare the estimate, but not review the underlying data.