By Ludwig Burger and Vera Eckert

FRANKFURT (Reuters) - BASF will grant Russian gas company Gazprom full control of their jointly run European gas trading and storage activities to gain more access to Siberian gas fields.

"Through the swap, BASF aims to further expand its production of oil and gas and to exit the gas trading and storage business," German chemicals group BASF said in a statement on Wednesday. It declined to provide a deal value.

"The traditional natural gas trading business,..., offers (BASF oil and gas subsidiary) Wintershall little possibility for differentiation," it added.

Pending regulatory approval, the no-cash transaction will be completed by the end of 2013.

Gazprom, for its part, has long been seeking better access to nearby downstream European markets where margins are still lucrative, even if pipeline gas from origins such as Russia has come under margin pressure as a result of cheap new shale gas output from North America.

State-controlled Gazprom's tighter grip on European energy markets could be a politically sensitive issue as governments are concerned about too much reliance on Russia.

Russia's wrangling with gas transit country Ukraine in 2006 and 2009 disrupted westbound gas supply in mid-winter.

During a few days last winter, Gazprom could not meet all of Italy's short-term gas orders, with the gas company saying more storage tanks were needed.

Apart from seeking gas assets in Europe, Russia has also been acquiring oil refinery stakes in Germany.

Russian oil company Rosneft now commands around a fifth of German refining capacity after buying a stake in German oil company Ruhr Oel from Venezuela's PDVSA.

But talks between Gazprom and German utility RWE about joint power production ventures in Europe were scrapped last December.

BASF's deeper involvement in Russian exploration comes despite Russian media reports last month that a proposed increase in the country's mineral extraction tax (MET) was deterring the German group.

BASF on Wednesday said the tax did affect its negotiation position. "We have taken the tax increase into consideration in the valuation of the assets," a spokeswoman said.

Two more blocks of the Achimov formation of the Siberian Urengoi gas field will be made available to Wintershall, which has long been a partner in the field's exploration with Gazprom. Continued...