DUBAI (Reuters) - Saudi Arabian oil group Aramco is expected to raise $6 billion on Wednesday as the company returned to the international debt markets with its first U.S. dollar-denominated Islamic bond sale comprising three tranches, a document showed. Pricing guidance on the Islamic bond or sukuk tightened to around 70 basis points (bps) over U.S. Treasuries (UST) for a three-year tranche, around 90 bps over UST for a five-year tranche and around 125 bps over UST for a 10-year tranche, all plus or minus 5 bps, the document from one of the banks on the deal showed. Orders on the sale, which is expected to close later on Wednesday, topped $55 billion. Initial guidance was around 105 bps over UST for the three-year bonds, around 125 bps over UST for the five-year notes and around 160 bps over UST for the 10-year tranche. Reporting by Yousef Saba. Editing by Jane Merriman Our Standards: The Thomson Reuters Trust Principles.