Japanese trading houses say they will push on with investments in North American shale oil and gas fields, despite writedowns in the sector on low gas prices and reduced reserve estimates.

The comments from executives at Mitsubishi Corp, Mitsui & Co, Itochu Corp and other trading houses are seen a vote of confidence in the long-term outlook for unconventional drilling in the U.S. and Canada, after they saw more than $600 million in writedowns over the last two years.

They also come as the appeal of North American shale has been dented by discoveries in other countries and the possibility that transport costs could rise due to new rates in the Panama Canal.

Utilities in Japan, which are supplied by the trading houses, have been looking to North American shale gas to diversify their fuel sources in the wake of the Fukushima nuclear disaster in 2011.

They also hope to use shale as a bargaining chip to get better prices from oil and gas sellers in the Middle East, Australia and Southeast Asia.

But Itochu last month marked down its investment in Samson Resources by 29 billion yen ($285 million) after the U.S. oil and gas producer re-evaluated some unproven gas and oil reserves.

That came on top of a 33 billion yen writedown recorded in 2013 on a company that has stakes in shale gas fields in several U.S. states.

Marubeni Corp also took so-called impairment charges in the business year that ended March 2013 after it decided to sell rights it had in the Niobrara shale project in the western U.S.

"Trading houses have been trying to ride the 'shale revolution' through a wide variety of types of investment, so it is hard to see them looking to exit the area anytime soon," said Akio Shibata, president of the Natural Resource Research Institute, and a former Marubeni official.

Japan's major trading houses, which also include Sumitomo Corp, are key players in global commodity markets.

They handle a large proportion of Japan's imports of liquefied natural gas (LNG), which account for about a third of world shipments.

A number of global oil companies, including BP and Shell, have also struggled after making investments in U.S. shale which have left them exposed to depressed gas prices, dragging on their profits.

BP last month wrote off over $500 million related to its decision not to proceed with a shale project in the Utica basin.

Japanese trading houses are also looking to take advantage of the anticipated expansion in U.S. industrial production and the larger economy that the shale boom is expected to bring.

"As shale production levels expand, there is the potential that shale-focused chemical goods, power production and pipeline sectors, midstream and downstream areas will expand with it," Mitsui President Masami Iijima said.