DUBAI, Oct 20 (Reuters) - Saudi Arabia's international bond issue saw heavy grey market trading early on Thursday with the prices of all three tranches rising from the reoffer price, traders said.

Several said the premium to the reoffer price averaged more than 50 cents. Two said the five-year tranche was trading with a 50 cents premium and the 10-year tranche at about 40 cents over, while one said the 30-year tranche had gone as high as about 150 cents.

Saudi Arabia conducted the largest-ever emerging market bond sale on Wednesday, selling $17.5 billion of debt in the government's first international offer while attracting investor orders totalling almost four times that amount.

A $5.5 billion five-year tranche was launched at 135 basis points over U.S. Treasuries, a $5.5 billion 10-year tranche at 165 bps over, and a $6.5 billion 30-year tranche at 210 bps over. ID:nL8N1CP3CQ

(Reporting by Davide Barbuscia; Writing by Andrew Torchia) ((andrew.torchia@thomsonreuters.com; +9715 6681 7277; Reuters Messaging: andrew.torchia.thomsonreuters.com@reuters.net))