US energy companies want to use their experience to help Brazil tap into its vast shale gas reserves, US Energy Secretary Ernest Moniz said on a visit to the Brazilian capital.

Studies show that one-tenth of the world's known shale gas reserves are in Brazil, and if it decides to exploit them the South American giant -- currently a gas importer -- could be the world's second natural gas producer.

Shale gas production could be viable only by 2023 assuming that investments begin now, a representative of the state-run National Petroleum Agency said in May.

The United States "clearly today has the most experience in this area," Energy Secretary Ernest Moniz said at a press conference Friday in Brasilia after a meeting with Brazilian industrialists.

Shale gas is extracted by a controversial technique known as hydraulic fracturing, or fracking.

Fracking has unlocked an energy boom in the United States, but has been banned in other countries over fears of environmental damage.

"It really has enormous impact and then when you come to Brazil, with its potential resource in gas, of course our companies are very interested in participating," Moniz said.

"Our government has made it clear that we think development of unconventional resources across the world could be a very good thing for the global hydrocarbon markets."

According to Moniz, shale gas production in the United States has been "a revolution, it has lowered carbon dioxide emissions by replacing coal, it has led to a revival of a lot of manufacturing."

In the fracking process, huge amounts of pressurized water mixed with chemicals are blasted into a well to crack open shale, a sedimentary rock, to release natural gas.

Environmentalists warn that the chemical-laced waste could contaminate freshwater resources, while many believe fracking causes minor earthquakes.

Fracking is practiced in the United States, Spain, Poland and Hungary, but it is banned in France and has sparked fierce debate in other countries with shale gas.

During the visit Moniz also met with Brazil's Minister of Energy and Mining Edison Lobao.