With oil majors being given the cold shoulder in many developing countries, it is no mean feat that Exxon Mobil  managed to replace 100 percent of production last year with new reserves. Even so, not all barrels of oil are born equal. By necessity, the stuff Exxon is using to fill its pipeline will be harder to extract or of lower value to investors.

That said, Exxon deserves applause for replenishing its supplies. The 100 percent figure arguably understates Exxon’s achievement, since it is based on restrictive Securities and Exchange Commission assumptions about oil prices. The oil giant’s own figures give a replacement ratio of 133 percent — beating its own 10-year average of 112 percent.

And clearing the crucial 100 percent hurdle by SEC standards should be child’s play in 2010. With reserves of more than 2 billion barrels of oil equivalent, Exxon’s latest acquisition — XTO Energy — will replace more than a year of Exxon’s output at a stroke, and figures out on Wednesday showed the shale gas group’s reserves growing briskly.

The bigger concern for Exxon shareholders will be the nature of these new reserves. On the bright side Exxon is being forced to steer away from capricious developing nations — meaning fewer political headaches. Recent reserve additions could not have been in more politically placid areas — notably Canada’s Kearl oil sands in 2008 and Australia’s Gorgon liquefied natural gas project last year.

Sadly, great economic risks may offset fewer political ones. Discoveries of precious easy-to-extract-and-refine oil are failing to keep pace with output. The barrels taking their place may be tougher or more costly to access, and therefore less profitable.

Hopes are high that cleaner-burning natural gas will grow in popularity. Trouble is, gas still trades at a punishing discount to its liquid cousin crude — especially in the United States, where it is worth around half the price of a barrel of oil. Liquids, about 57 percent of Exxon’s reserves 10 years ago, are now just a half, according to IHS Herold. And the oil sands upon which Exxon increasingly relies tend to be costlier to extract — often only becoming economical once oil surpasses $60 a barrel. Conventional wells are generally profitable at just half this level.

Exxon remains ahead of the pack in replacing reserves. But investors would be wise to focus on the nature, as well as the number, of Exxon’s replacement barrels.