NEW YORK (MONEY) -- Last week I cautioned investors against loading up on energy stocks when oil prices dip.

The long-term outlook for energy stocks remains quite positive. But history suggests that the price of oil often pulls back for several years after a major runup.

That dip can be far larger than investors expect - oil traded for less than $20 a barrel as recently as the late 1990s.

There is, however, one type of energy stock that will likely prosper no matter what happens to oil prices during the next few years. Whatever the swings in supply and demand for energy, exploration and drilling will continue.

The greatest beneficiaries should be oil-service companies, particularly giant global firms like Schlumberger.

Demand for oil could grow more slowly than expected because of high energy costs and a softer economy. But energy companies know that oil and gas will be needed over the coming decades, so they will continue to explore and develop new fields.

Moreover, oil services will account for a growing share of the cost of future fields. Just consider Chevron's (Charts) most recent discoveries in the Gulf of Mexico. That field may hold the equivalent of billions of barrels of oil (mostly in the form of natural gas). But to reach it, drillers will have to work in 7,000 feet of water and go down nearly four miles. Development of these reserves could take seven years.

The bottom line: There are lots of energy reserves to develop, but they will require massive efforts lasting years -- and the global giant oil-services companies will get the business.

Schlumberger (Charts) is the premium stock in the sector -- and it looks quite attractive today purely on a value basis.

From January 2003 through early 2006, the share price rose steadily from $20 to almost $75. Since then, the price has dropped to $58.84, chiefly because of recent slippage in oil prices.

Historically, Schlumberger has usually carried a sizable premium over the rest of the market. But because of the stock's recent decline, the shares currently trade at 20 times this year's estimated earnings and less than 16 times next year's.

Earnings per share have grown at a compound 16 percent annual rate over the past five years. And analysts project growth for the next five years as high as 25 percent annually. Anything close to that would make Schlumberger one of the fastest-growing stocks in the Sivy 70.

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More energy stocks in the Sivy 70: ExxonMobil (Charts), Anadarko Petroleum (Charts) and ConocoPhillips (Charts)

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