For decades, Afghanistan has been ravaged by war. But beneath military boots and tanks are significant amounts of recoverable natural gas.

The Central Asian country was a producer of natural gas until 1990. Then output plummeted, falling from 104 billion cubic feet (bcf) to 11 bcf of gas within a year, according to limited industry data on the U.S. Energy Information Administration's website.

Since 1991 -- two years after the Soviet Union withdrew its occupying forces from Afghanistan and civil war broke out -- the production decline has continued. By 2009, years after the Taliban militia's rule was interrupted, the total was just over 1 bcf.

Gas production in Afghanistan has yet to rise, said Jack Medlin, a regional specialist with the U.S. Geological Survey who has helped conduct assessments of the country.

Medlin and his colleagues believe there could be the equivalent of 1.5 billion barrels of oil and 15 trillion cubic feet of natural gas in reservoirs in northern Afghanistan, near the borders with Uzbekistan and Turkmenistan. Their study was published in 2006.

That was well beyond any previous assessment, Medlin said.

If accurate, the amount would be enough oil and gas to satisfy the country's energy needs for years, Medlin said. Afghanistan's population is estimated at 30.4 million.

There could be even more oil and gas in basins near Pakistan and to the south. But Afghanistan's mostly rugged, mountainous terrain and lack of security mean that a thorough scientific survey would take far longer than the time during which crews could operate safely.

Safety concerns haven't deterred energy-hungry China, however. Its biggest oil and gas producer, state-owned China National Petroleum Corp., received approval in late December from the central government in Kabul to start exploring for oil -- Afghanistan's first such contract in decades.

The deal to develop three fields in the relatively peaceful north along the Amu Darya river -- deposits estimated at 87 million barrels of oil -- could earn Afghanistan $7 billion over 25 years, based on a $100-per-barrel price, mining minister Waheedullah Shahrani told reporters. CNPC's exploration isn't likely to begin until late this year, he said.

Since the U.S. military invasion of Afghanistan began in late 2001, the country's ailing oil industry has been slowly brought back to its feet, Medlin said. A few oil wells have been rehabilitated, and Afghan output is now about 800 barrels per day. The country is a very low 149th in the world in oil production and 87th in natural gas production, according to the Central Intelligence Agency.

Natural gas development, however, has fallen into disrepair since the days when the Soviet Union tapped into Afghanistan's gas fields. The Soviets started producing gas there in the 1950s and continued through the end of their 10-year occupation in 1989. After the Soviet Union's collapse in 1991, the flow of natural gas stopped, and old wells and infrastructure are now obsolete.

This has been one of the problems with Afghanistan, Medlin said.