BANGKOK, April 4 (Reuters) - Thailand's top oil and gas explorer, PTT Exploration and Production Pcl PTTEP.BK , said its subsidiary has signed an agreement with BG E&P Brasil Ltda (BG Brasil) for a 25 percent stake in four deepwater blocks in the South American country.

The signing of the deal is the first step for PTTEP to expand its investment in South America, one of the world's highest potential oil resources, PTTEP Chief Executive Tevin Vongvanich said in a statement.

The four blocks -- BAR-M-215, BAR-M-217, BAR-M-252 and BAR-M-254 -- cover an area of about 3,077 sq km in the Northeastern deep water of Barreirinhas basin in the Atlantic Ocean.

BG Brasil is a subsidiary of oil and gas producer BG Group

BG.L .

The deal is subject to approval from Brazil's National Agency of Petroleum, Natural Gas and Biofuels. PTTEP's subsidiary will hold 25 percent stake in the blocks, while BG Brasil, the operator, will hold 75 percent.

The joint-venture partners plan to start a seismic survey in the four blocks to assess their preliminary potential later this year, Tevin said.

PTTEP, the flagship of state-controlled PTT Pcl PTT.BK and ranked among Asia's top 10 explorers, is scouting for oil and gas assets to boost energy security in Thailand as Southeast Asia's second biggest economy relies on natural gas to generate almost 70 percent of its electricity.

(Reporting by Khettiya Jittapong; Editing by Sunil Nair)

((khettiya.jittapong@thomsonreuters.com)(66 2 648 9728)(Reuters Messaging: khettiya.jittapong.thomsonreuters.com@reuters.net))

Keywords: THAILAND PTTEP/BRAZIL