Vienna, Nov 8 (IANS): The share of the Organisation of Petroleum Exporting Countries (OPEC) in the world oil market would decline by almost 8 percent till 2018 due to the rise of shale energy, a publication by the organisation said Friday.

It is the first time for OPEC, with the headquarters in Vienna, Austria, to make such an acknowledgment about shale energy's growing impact in the world oil map, Xinhua cited from the annual report World Oil Outlook of the group.

The report said that OPEC sees less demand of its crude in the next five years, and the demand for crude of the group, which produces about 40 percent of the world's oil output, will fall by 1.1 million barrels a day to 29.2 million barrels a day between 2013 and 2018.

Oil production from shale formations in the US and Canada is estimated to climb to 4.9 million barrels a day in 2018, compared with the estimate of 1.7 million barrels a day in last year's report.

OPEC, however, doesn't seem to worry about the pressure from shale oil, as it estimates that the output of the shale is expected to decline after the year 2020, citing challenges including a rapid output decline from wells, the environmental concerns and rising costs.

Apart from shale, growing non-OPEC crude supplies would also serve as a factor to the organisation's weakened impact in the world oil map.