U.S. oil giant Exxon Mobil said on Monday it had started drilling an oil well of a depth of 6.8 km (4.25 miles) off Azerbaijan,the deepest yet in the oil exploration of the Caspian Sea, The move by the world\'s biggest oil firm came just weeks after Ilham Aliyev won an election to take over the country\'s presidency from his father, Haydar, sparking a bloody riot. Aliyev sealed the first dynastic succession in a former Soviet Union republic and reassured investors keen for continuity in the oil rich state. Exxon has invested around $1 billion in Azerbaijan since the collapse of the Soviet Union. The consortium led by Exxon said it would take 240 days to drill the well on the Zafar-Mashal oilfield, located 100 km (62.5 miles) offshore from the Azeri capital of Baku. The well\'s platform will be installed in waters as deep as 620 metres, also a record for Caspian exploration. An official for the international consortium, which also includes U.S. third largest oil firm ConocoPhillips, said the group hoped to discover two deposits during the drilling. Exxon was among the main investors in a $250-million oil drilling rig Lider, one of the world\'s most powerful. The rig was completed in July. (Reuters)