ALMATY, May 21 (Reuters) - Russia’s Rosneft has quit Eurasia, an early-stage oil exploration project in Kazakhstan, while Shell has joined it, an executive of the Kazakh company involved in the project said on Monday.

The Eurasia project focuses on exploring a geological area known as the Caspian Depression in western Kazakhstan by drilling ultra-deep wells of up to 15 kilometres.

Kazakhstan has estimated that the untapped lower levels of the geological structure could hold up to 60 billion tonnes of oil. The initial stage of exploration, including the drilling of the first well, could cost $500 million.

Six companies, including Kazakh national energy firm KazMunayGaz, Azerbaijan’s SOCAR, Italy’s ENI, China’s CNPC and U.S. geological services group NEOS, signed up for the project last year.

But Rosneft, which also signed the memorandum, told the Kazakh government it was quitting the project, Alexander Denyakin, chief executive of KMG-Eurasia, a subsidiary of KazMunayGaz, told Reuters.

Rosneft declined to comment on the matter on Monday.

At the same time, Shell is joining the project, which will remain open to new participants until the end of this year, Denyakin said. Shell’s Kazakh office could not be reached for comment on Monday.

Due to its technological complexity Eurasia could cost more than Kashagan, a giant offshore field in the Caspian on which investors have spent over $50 billion. (Reporting by Mariya Gordeyeva Additional reporting by Vladimir Soldatkin in Moscow Writing by Olzhas Auyezov Editing by Louise Heavens)