Unidentified gunmen ambushed a truck carrying soldiers and civilians in Sudan's Unity State, an oil-producing region whose fields are operated by the Greater Nile Petroleum Operating Co., a consortium led by China National Petroleum Corp.The attack, which killed 23 people, added to rapidly growing concerns that continued violence could unsettle the semiautonomous region ahead of a vote on independence to be held in January 2011.Despite such intimidation, however, most observers believe southern Sudanese will vote for secession on Jan. 9 after fighting the north for more than 50 years over issues including religion, ethnicity, ideology—and oil.Indeed, the distribution of oil resource...