UPDATE 1-Thai PTT group to invest up to $3 bln in Myanmar energy

* Preparing investment plan for Myanmar government

* Plans to set up PTT Myanmar to oversee investments

* Keen to invest in petrochem complex in Vietnam

BANGKOK, July 17 (Reuters) - PTT Pcl, Thailand's top energy firm, said on Tuesday it planned to invest $2 billion-$3 billion in projects in Myanmar, including a 150,000-barrel-per-day refinery, coal mines and a power plant, as part of its drive to expand in Southeast Asia.

State-controlled PTT, which aims to invest over $11 billion in 2012-2016, is also keen to invest in a petrochemical complex in Vietnam, Nattachat Charuchinda, Chief Operating Officer for PTT's petroleum business, told reporters.

"We are preparing the investment plans for the Myanmar government, which should involve around $2-3 billion," Nattachat said, adding that the planned refinery would help feed a new petrochemical plant to serve growing demand in Myanmar.

Myanmar is opening up more to investment after political and economic changes pushed through by the government that took over from a military junta in March 2011.

PTT, Thailand's most valuable company, planned to set up a subsidiary, PTT Myanmar, to oversee its investment in the country, where it also wants to have 60 petrol stations within five years, Nattachat said.

PTT aimed to boost the number of its petrol stations in Southeast Asia excluding Thailand to 220 over the next five years from 95 now, he added.

PTT, valued at $30 billion, runs Thailand's gas pipeline monopoly and controls more than 30 petroleum, gas exploration, petrochemical and refinery businesses. It is Asia Pacific's third-biggest oil and gas firm by market value behind PetroChina and Sinopec.

Its investment in Myanmar has been spearheaded by 65 percent owned PTT Exploration and Production Pcl (PTTEP), which already has five projects there.

On Monday, PTTEP won a battle against Royal Dutch/Shell to take over Cove Energy Plc, which will give it access to the booming East African gas sector.

OPENING UP

Western countries have eased sanctions against Myanmar that were imposed under the junta and big Western firms are keen to get in.

Companies from Thailand and most other Asian countries had no such constraints but they, too, are stepping up their interest because of the improved investment climate.

PTT's proposed investment is among the biggest to be announced and follows plans outlined to Reuters in June by PTTEP to invest more than $2 billion in the country, mostly in gas and deep-sea exploration.

The energy sector has attracted other huge projects, notably two pipelines to take oil and gas to China, due to be completed next year.

The junta gave out little information on that deal, which was signed in 2009, but total investment has been put at $6.5 billion, including $3.5 billion for the twin pipelines and $3 billion to develop offshore gas fields.

Natural gas from Myanmar accounts for about 30 percent of Thailand's consumption, mostly used in power generation.