* Lifts 2017 production guidance to 75,000-80,000 boepd
* Raises Zama field oil estimate to 1.2-1.8 billion barrels
* H1 cashflow nearly trebles to $292 million
* Zama breakeven may be as low as $28 a barrel -CEO (Adds details, background, CEO, analyst)
Aug 24 (Reuters) - Britain's Premier Oil on Thursday lifted its full-year production guidance as well as estimated resources at a huge oilfield it discovered off the coast of Mexico.
Premier Oil, the operations of which stretch from Indonesia to the Falkland Islands, now expects 2017 production in the range of 75,000-80,000 barrels of oil equivalent per day (boepd), up from the previous forecast of 75,000 boepd, thanks to its strongly performing North Sea fields.
The oil producer, turning a corner after concluding a refinancing deal to survive the oil market downturn, also upgraded its oil-in-place estimates at the Mexican Zama field to 1.2-1.8 billion barrels, compared with a less specific previous forecast of more than 1 billion barrels.
"This looks a positive set of results driven by a strong operational performance ... Cashflow almost tripled and deleveraging has started," Panmure Gordon analysts said.
Premier Oil's cashflow from operations rose to $292 million in the first half of the year, up from $109 million a year earlier, while net debt has fallen to $2.74 billion from $2.77 billion at the end of 2016.
The company's shares traded up to 3.5 percent higher on Thursday but fell later in the session in line with its peers, pressured by weaker oil prices.
After taking a strong financial blow from the oil downturn that coincided with hefty spending commitments on new fields, Premier Oil is now working on further reducing debt, to be accelerated by the start-up of its North Sea Catcher field by the end of the year.
The discovery of the Zama field, which Wood Mackenzie analysts called one of the 20 largest shallow-water finds in two decades, has provided further momentum for the company.
"Initial scoping estimates are that Zama has very low breakeven, perhaps as low as $28 a barrel. It looks like a world-class asset that will fit well into our portfolio," Premier's chief executive, Tony Durrant, told Reuters.
The oil producer, which last year bought E.On's North Sea assets for $120 million, has also reduced development costs at its Tolmount gas field in the North Sea by $175 million through the sale of its stake in the infrastructure part of the project.
It said on Thursday that it had agreed that field partner Dana Petroleum and Antin Infrastructure Partners will build and operate the platform and pipeline of the project in exchange for a yearly usage tariff.
Premier Oil retains its 50 percent interest in the field. (Reporting by Karolin Schaps; Editing by David Goodman)