All investors in the oil and gas space should be familiar with the concept of a company's reserves. This is because the oil and gas industry is by its very nature extractive. In order to sell either oil or gas, the company first has to pull it out of the ground. However, there are a finite quantity of resources in the ground and once they are extracted, they are gone forever. By looking at an oil companies reserves, we can see how much oil and gas the company has in the ground that it can extract. We can also get a good idea of its reserve life, which is how long the company's reserves will last if it keeps producing at its current level without discovering any new sources of oil. In general, you want both figures to be as high as possible in order for the company to be sustainable over the long term.

In this article, we will briefly discuss two companies that have exceptionally large reserves and long reserve lives which should ensure that these companies are very sustainable over the long term.

Total's global operations offer safety
The first company that we will look at today is French oil giant Total S.A. (TTE -1.50%). Total is one of the largest oil companies in the world and so, as you might expect, Total has operations in many different countries.

Source: Total S.A.

This worldwide presence also means that Total has reserves located in many different countries. In fact, some of its reserves would be located in any country in which Total has an operating oil field. This is just the nature of how reserves work.

Just the fact that Total has reserves located in many different countries gives it a certain amount of security. This is due to the very political nature of the oil and gas industry. For example, it is not uncommon for governments to seize control of oil fields from the companies that are currently operating them. Something like this would reduce Total's reserves. Therefore, by having its reserves located in several different countries, Total can ensure that the capricious actions of any given government will not have an outsized impact on the entire company.

Total's enormous reserves
As I already mentioned, Total's oil and gas reserves are enormous and the company managed to increase the size of them last year by discovering more resources than it extracted from the ground. This is actually a trend that has been going on for quite some time now. Total has, on average, been discovering more oil and gas than what it pulled out of the ground over both the 2008-2010 and 2011-2013 periods.

Source: Total S.A.

All of this success at exploration has given Total proved reserves of 11.5 billion barrels of oil equivalent. This is enough to last the company for thirteen years!

Source: Total S.A.

Canadian oil sands giant
The second company we'll look at today is Canada's Suncor Energy (SU -0.77%). Unlike Total, the majority of Suncor's reserves are located in Canada. This is because of Suncor's dominant position in the Athabasca oil sands, which is one of the largest deposits of unconventional oil in North America.

As can be expected, much of Suncor's reserves are actually in the form of bitumen, which is considerably more expensive to extract out of the ground. Therefore, a significant portion of the company's reserves may not be economically viable to extract at this time. But, Suncor still has a substantial reserve base.

Suncor's enormous reserves
Suncor has proved reserves of roughly 4.8 billion barrels of oil equivalent located in Canada and a few other countries around the world. This gives Suncor a proved reserve life of 23 years at its 2013 production level -- substantially higher than what Total has.

But, it gets better. Suncor also has an additional 2.9 billion barrels of probable resources, which are defined as reserves the company is 50% confident are actually present, but it is not certain if they are or not. The company also has vast quantities of resources that are not really economically viable to extract using current technologies. So, we won't worry about them. All in all, Suncor has a very long and sustainable future ahead of it just because of its proven reserves, which is exactly what we want to see.