UPDATE 1-Gazprom seeks more money from E.ON, BASF for Siberian field
* Gazprom says Yuzhno Russkoye field bigger than expected
* All parties to reassess size of reserves
* E.ON says process could be lengthy (Adds detail, background, shares)
FRANKFURT, Oct 11 (Reuters) - Russia's Gazprom is seeking more money from Germany's E.ON and BASF for their stakes in a western Siberian gas field because it believes the field is larger than initially assumed.
"Gazprom is in talks with German partners now over the reevaluation of Yuzhno Russkoye reserves," a spokesman for the state-owned Russian firm said in Moscow, declining to comment on the level of the compensation sought.
An E.ON spokesman confirmed Gazprom was demanding more money for its 25 percent shareholding, but declined to specify the possible sum in question.
BASF, which also has a 25 percent stake in the field, declined comment on the question of compensation but gave an estimate of over 600 billion cubic metres (bcm) for the size of the field.
Media reports have said this figure, agreed at the time the German firms bought their stakes last decade, has now been raised to 790 bcm and that Gazprom is seeking an extra 1 billion euros ($1.4 billion) from its partners.
At 0940 GMT, E.ON shares were down over 3 percent, while BASF's were flat, compared with a 0.4 percent rise in Germany's blue-chip DAX index.
E.ON chief executive Johannes Teyssen said at a meeting in Brussels he did not expect the issue to be quickly resolved as new studies had to be taken into account.
"We are far away from reassessing the value of that field," Teyssen said during a gathering of utility chiefs.
"Much of it is wishful thinking," he added.
E.ON's head of investor relations, Marc Spieker, said in a conference call with analysts on Thursday that a possible revaluation was part of the agreement between the parties.
Output from the field helps fill the Nord Stream pipeline under the Baltic Sea that brings Russian gas directly into Germany where BASF, jointly with Gazprom subsidiaries, and E.ON are big local operators.
The deal with BASF's Wintershall was agreed in 2007 and that with E.ON in 2009.