Russian gas giant Gazprom and German partner BASF, the world's biggest chemicals company, are jointly to tap and develop the Yuzhno-Russkoye gas field in western Siberia, a spokesman for the German group said on Monday. "BASF and Gazprom want to strengthen cooperation and develop a gas field in western Siberia," the spokesman said, confirming a corresponding report in the business daily Handelsblatt. The deal was to be signed by Gazprom and BASF's oil and gas unit Wintershall at the Hanover Messe trade fair on Monday, the spokesman added. Further details would be revealed at a news conference. The reserves of the Yuzhno-Russkoye gas field are estimated to be big enough to cover all of Germany's gas needs for a period of seven years, newspaper reports here have suggested. The deal will come as a heavy blow to Germany's leading power supplier, E.ON, which had also been in the running to secure the contract. E.ON holds a 6.5-percent stake in Gazprom. BASF and Gazprom have already cooperated for many years via the joint gas trading and transport company, Wingas. The two partners also plan to participate in a billion-dollar project for a a new gas pipeline in the Baltic Sea.