Two U.S. energy giants, Conoco Phillips and Exxon Mobil Corp., dropped out Tuesday as Venezuela struck agreements with four other foreign oil companies to take a major share in tapping a lucrative oil field.

Venezuela's state-run oil company Petroleos de Venezuela signed deals with another U.S. oil company Chevron Corp. as well as Norway's Statoil, Britain's BP Plc and France's Total to increase its average share to 78 percent from the current 39 percent in the exploration of the Orinoco heavy crude field.

"Exxon Mobil and Conoco Phillips are ending their cooperation inthe exploration activities," Venezuela's Energy Minister Rafael Ramirez said at the signing ceremony.

The drilling projects in Orinoco are valued above 30 billion U.S. dollars with an estimated daily production of 600,000 barrels.

Venezuela's President Hugo Chavez is in the midst of efforts to nationalize a wide range of sectors including energy.