DUBAI, Sept 3 (Reuters) - Saudi Basic Industries Corp , the world's fourth-biggest petrochemicals firm, started marketing a $1 billion dual-tranche bond offering on Thursday consisting of 10-year bonds and 30-year Formosa bonds, a document showed.

SABIC will sell $500 million in each tranche. It gave initial price guidance of around 190 basis points (bps) over midswaps for the 10-year tranche and around 3.375% for the 30-year Formosa notes, according to a document from one of the banks arranging the deal, which is expected to close later on Thursday. (Reporting by Yousef Saba; Editing by Mark Potter)