RIYADH, June 23 (Reuters) - Saudi Arabia&aposs National Petrochemical Co 2002.SE (Petrochem) has completed a 1.2 billion riyal ($319.9 million) debut sukuk issue, it said in a bourse filing on Monday.The Islamic bond, which has a five year lifespan, was priced at 170 basis points over the six-month Saudi interbank offered rate (Saibor).Deutsche Bank&aposs Saudi Arabian arm DBKGn.DE and the investm