Akhmetov had been Kazakh prime minister since June 2003 - he is a career politician and a Nazarbayev supporter.
Nazarbayev, the country's leader since Soviet times, reappointed him to the post last January.
The country is rich in natural gas and produces about a million barrels of oil a day, which it pumps mainly to the West.
It has ambitions to triple output by 2015, pushing it into the world's top 10 producers.
Western energy companies have poured money into Kazakhstan since the breakdown of the Soviet Union.
Oil companies in particular have complained of a change in the law which states that Kazakhstan's main state oil company must own half of all future investments, while foreign companies bear the burden of exploration costs - a change instigated by Akhmedov.