Japan Petroleum Exploration Co.'s test extraction of oil from deep underground shale rocks, a first for Japan, has raised hopes of improving energy security in a nation that relies on imports for almost all of its fossil fuel needs.

But newly tapped "unconventional" resources, such as shale oil and shale gas, require complicated methods of extraction that also raise environmental concerns.

Because it also entails relatively high drilling and extraction costs, it doesn't appear likely to become a primary source of energy in Japan, where the need for different resources has grown acute since the Fukushima crisis shut down nuclear reactors across the county.