The massive nation of Russia is one that few of the world's largest oil companies are ignoring, and for good reason. The country has some of the largest oil reserves in the world. While estimates vary widely, Russia is believed to have proven reserves of 60 billion-200 billion barrels of oil, with the Energy Information Administration putting the total at 80 billion barrels.

The country also has enormous amounts of shale oil, with the Bazhenov shale alone believed to contain nearly eighty times the amount of oil that is in North America's Bakken shale. Thus, it is little surprise that companies such as ExxonMobil (XOM -0.17%) and BP (BP 0.40%) have been ignoring the U.S.-imposed sanctions on Russia and have been expanding their operations in the country. It is likely that these companies are hoping to make a discovery similar to the one that Spain's Repsol just announced.

Repsol makes major discovery in Russia
On June 23, Repsol (REPYY 1.33%) reported that it made two discoveries in the Karabashsky 1 and 2 blocks located in Western Siberia. These two finds could contain a total of 240 million barrels of oil, making them two of the largest discoveries in recent history. They are also the largest discoveries in Russia in two years. In addition to making these two discoveries, Repsol has significantly increased its production of hydrocarbons in the nation this year. In 2013, Repsol's Russia operations produced an average of 14,600 barrels of oil equivalent per day. This average has increased by 20.8% so far in 2014 to 17,690 barrels of oil equivalent per day.

As should be immediately obvious, this is an impressive growth rate and a very large find. The hope of results like this is almost certainly one of the reasons why ExxonMobil and BP (among others) were willing to take the risk of defying the sanctions against the country.

Let's have a look at the activities of BP and others in Russia, to illustrate how significant the presence that these companies have in the country is.

BP's Russian operations
BP first began operating in Russia during the early 1990's and eventually acquired a 25% stake in what was, at the time, the fourth largest oil producer in Russia. Obviously, this represents quite a substantial investment. In 2003, the company merged its Russian operations with TNK to create TNK-BP, a joint venture with more than 50,000 employees and more than $5 billion in net income annually.

TNK-BP was eventually sold to Rosneft, easily one of the largest energy companies in the world. This sale netted BP a 19.75% stake in Rosneft, which actually produces more oil than any other publicly traded company in the world.

ExxonMobil's partnership with Rosneft
ExxonMobil has also been aggressively expanding into Russia. The company has operated in the world's largest nation for twenty years, but it was only recently that the company really moved toward building very large operations there.

In 2011, ExxonMobil entered into an agreement with Rosneft to explore and produce oil jointly in Russia, the United States, and elsewhere. This was expanded further in 2012, when the two companies agreed to engage in several multi-billion dollar exploration projects and development projects together. Clearly, this represents an enormous investment on the part of ExxonMobil, but it is one that has already begun to pay off.

ExxonMobil and Rosneft to develop four huge fields
The investment is, fortunately, already beginning to pay off for ExxonMobil. The two companies recently announced that they will be jointly developing four very oil-rich reservoirs in the Russian Arctic. These four reservoirs are the Anisinsk-Novosibirsk and Ust-Oleneksk reservoirs in the Laptev Sea and North Wrangel-2 and South Chukchi Shelf reservoirs in the Chukchi Sea. These projects certainly have the potential to deliver impressive results for ExxonMobil from its Russian operations like Repsol has seen from its own.

Foolish takeaways
In conclusion, Russia offers significant opportunities for oil and gas producers. These opportunities are so significant that several companies have been ignoring the sanctions that the United States and Western Europe have put on Russia. This could certainly prove to be rewarding for shareholders in these companies as these opportunities begin to pay off.