DUBAI, June 30 (Reuters) - Qatar Petroleum (QP), one of the world's top liquefied natural gas (LNG) suppliers, sold $12.5 billion in a jumbo four-tranche bond deal on Wednesday, a document showed, its first public bond sale and the biggest issuance out of emerging markets this year.

QP tightened the spreads across all four tranches by 30 basis points (bps) after combined orders topped $41 billion, the document from one of the banks on the deal showed.

QP sold $1.5 billion in a five-year portion at 50 bps over U.S. Treasuries (UST), $3.5 billion in 10-year paper at 90 bps over UST, $3.5 billion in 20-year notes at 3.15% and $4 billion in 30-year Formosa bonds at 3.3%.

The bonds landed roughly 10-15 bps wider than the sovereign's curve, which investors likened to Saudi oil giant Aramco's debt compared with Saudi sovereign debt.

"The most obvious comparable to QP is Aramco which is roughly 10-15 bps wider than the sovereign. But QP contributes to 80% of Qatar's budget versus 60%" for Aramco's contribution to Saudi Arabia's budget, said Zeina Rizk, executive fixed income director at Arqaam Capital.

The Formosa bonds - sold in Taiwan by foreign borrowers and denominated in currencies other than the Taiwanese dollar - and the 10-year paper attracted the most demand, over $12 billion each.

QP, which supplies one in five LNG cargoes globally, will use the proceeds for operational and investment purposes, including for its North Field expansion project, the bonds' prospectus said.

QP signed a contract in February for the first phase of its North Field LNG expansion project, which aims to boost Qatar's LNG output to 126 million tonnes per annum (mtpa) by 2027 from 77 mtpa currently.

Capital expenditure by QP, its subsidiaries and joint ventures through 2025 is projected at 300 billion riyals, the prospectus said.

QP posted a profit of 18.1 billion riyals ($4.90 billion) in the first quarter of 2021, up from 13.3 billion riyals a year prior, a bonds prospectus reviewed by Reuters showed.

Fitch Ratings assigned QP a long-term rating of AA, in line with Qatar's sovereign rating, saying it was "constrained by that of the sole shareholder - Qatar".

Fitch said key constraints on QP's rating include completion risk for large capital expenditure projects related to increasing LNG production, as well as political risk.

Timothy Ash, senior EM sovereign strategist at Bluebay Asset Management in London, said demand was "driven by assumed lack of sovereign issuance given higher oil prices and improving credit fundamentals (low budget financing needs), and view that QP is cheap sovereign exposure given 100% state ownership."

Citi, JPMorgan, BofA Securities, Deutsche Bank, Goldman Sachs, HSBC, MUFG, QNB Capital and Credit Suisse arranged the deal.

(Reporting by Yousef Saba and Yousef Saba Editing by Marguerita Choy)