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By Alex Lawler and Dmitry Zhdannikov
LONDON, June 12 (Reuters) - Oil major BP cut itsestimates of global gas reserves steeply on Wednesday, revisingRussia's still classified reserves down sharply and puttingIran at the top of the world league table.
In its benchmark annual statistical review for 2012, BP putglobal proven gas reserves at 187.3 trillion cubic metres as ofthe end of 2012, enough for about 56 years worth of globalproduction at current rates.
BP's last report for 2011 estimated global gas reserves at208.4 trillion. The cut of 21 trillion equals roughly to sevenyears of global gas consumption.
Russia, the world's biggest gas reserves holder for manyyears, was responsible for the bulk of the reduction, with itsreserves estimate downgraded to 32.9 trillion from 44.6trillion.
BP's chief economist, Christoph Ruhl, said the companydecided this year to adjust its estimates for the former SovietUnion, including Russia, where data on reserves is classified.
"Traditionally countries of the former Soviet Union haddifferent criteria than used elsewhere. So we used a conversionfactor to convert that from those countries where we don't getdirect data," Ruhl said.
"In some countries, reserves are still a state secret, so wehave to rely on these data," he added.
The downgrade left Iran at the top of the table of theworld's largest gas reserves holders for the first time indecades, with its broadly unchanged reserves of 33.6 trillion.
BP also steeply downgraded Turkmenistan's reserves to 17.5trillion from 24.3 trillion as well as reserves in Kazakhstan,Uzbekistan and Azerbaijan.
OIL ESTIMATES
On the oil side, BP estimated global proven oil reserves at1,669 billion barrels at the end of 2012, up slightly from 1,654billion at the end of 2011 and enough to maintain current globalproduction levels for 53 years.
In its report a year earlier, BP had revised global oilreserves sharply higher as new technology made heavy crudegrades in Canada and Venezuela economically profitable toextract.
The United States, where the energy industry has beentransformed by shale oil and gas, oil reserves rose to 35billion barrels from 31 billion last year, more than 2 percentof global reserves.
BP upgraded reserves for both Iran and Iraq by severalbillion barrels and kept them at their No.3 and 4 global spots,respectively. Venezuela and Saudi Arabia kept their first andsecond places with no major revisions over the year.
Ecuador, Norway and China were included in upward reservesrevisions, while estimates were downgraded for South Sudan,Malaysia, Angola and Gabon.
