UPDATE 1-Nigeria's NNPC issues 2016 crude oil contracts worth $13.5 billion

(Adds background, table) By Julia Payne ABUJA, Dec 17 (Reuters) - The Nigerian National Petroleum Corporation (NNPC) has issued its 2016 crude oil term contracts to 21 companies, going directly to international refineries, trading houses and local downstream firms, according to a list obtained by Reuters. The contracts cover 991,000 barrels per day (bpd) of oil, worth $13.5 billion at current crude oil prices, which is roughly half of Nigeria's crude oil production of around 2 mi