The problem is the following: An export surplus is unsustainable over the long term. Achieving balanced trade is a good thing. However, if the entirety of your wealth is built on oil exports, it also means it is very cheap to import everything that isn't oil as people must sell you their (non oil) things in exchange for your oil.

You must diversify your economy to become less dependent on oil. However, it is easy for the government to take control over oil. The president can become a multi billionaire while ignoring the rest of your country.