HOUSTON, March 19 (Reuters) - Chevron was surprised when Exxon Mobil filed for arbitration over Chevron's plans to buy Hess's Guyana oilfield stake, Chevron CEO Michael Wirth said on Tuesday.

Chevron announced plans to buy Hess last year, and the Guyana stake is Hess's most valuable asset.

Chevron had done extensive due diligence on the operating agreement between Exxon and Hess in Guyana and has extensive experience in those types of agreement around the world, Wirth said.

Chevron had been in talks with Exxon over the matter, he said. Exxon, which operates the Guyana oilfield, says it has the right of first refusal for Hess's stake.

"We were surprised when they a couple of weeks ago abruptly ended those discussions and publicly announced... they had filed for arbitration." (Reporting by Sabrina Valle; Editing by Simon Webb)