Eco Atlantic upgrades oil and gas resources in Guyana field

* First well expected to be drilled in June

* Parterns in process of approving second well

* Block neighbours Exxon's giant Guyana discovery (Adds details)

LONDON, March 18 (Reuters) - An oil and gas field off Guyana's coast and owned by Tullow, Total and Eco Atlantic has about 3.9 billion barrels of oil equivalent, a third more than announced in an estimate released in September, Eco said on Monday.

The Orinduik Block is located next to an Exxon Mobil field, which has an estimated 4 billion barrels of oil equivalent. The Exxon discovery made Guyana one of the world's most closely watched oil basins.

Tullow, the operator of the Orinduik Block has 60 percent stake, Total has 25 percent and Eco Atlantic Oil and Gas has 15 percent.

Eco said Orinduik Block contained 3.5 billion barrels of recoverable oil and 2.84 trillion cubic feet of associated gas, based on a metric called unrisked prospective resources P50 (Best). It said the upgrade followed additional 3D processing and interpretation of seismic data.

The company said first well would be drilled in June and partners in the project were in the process of approving a second well.

"We hope to confirm drilling plans for well number two in the near future to take advantage of the economics of our rig on the block," it said.

(Reporting by Shadia Nasralla Editing Louise Heavens and Edmund Blair)