LONDON, Sept 12 (Reuters) -

* Total says has exercised option to buy a 25 pct interest in Orinduik exploration block offshore Guyana.

* Orinduik operated by Tullow Oil off Guyana contains an estimated 3 billion barrels of oil and gas, its partner Eco Atlantic said on Tuesday, as the two firms prepare to drill in the Latin American oil basin.

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* Tullow plans to begin drilling in the block in the third quarter of 2019, with Eco indicating it will begin "early" in the quarter.

* Guyana became one of the world's most closely watched oil basins after Exxon Mobil discovered more than 4 billion barrels of oil equivalent in its Stabroek licence in recent years. (Reporting By Ron Bousso, writing by Shadia Nasralla)