By Chen Aizhu and Trixie Yap SINGAPORE (Reuters) – Shell has shortlisted at least four companies including state-run China National Offshore Oil Corp (CNOOC) and top global energy trader Vitol as bidders for its Singapore refinery assets, sources with knowledge of the matter said. Two privately controlled Chinese chemical producers – Eversun Holdings in Fujian province and Shandong province-based Befar Group – were also shortlisted, sources said. The companies have been asked to submit formal bids by the end of February, two of the sources said. Two sources also said Shell aims to close a tran…