Tumbling crude prices will trigger a flood of oilfield writedowns starting this month after industry returns slumped to a 16-year low, calling into question half a decade of exploration. With crude prices down more than 50 percent from their 2014 peak, fields as far-flung as Kazakhstan and Australia are no longer worth pumping, said a team of Citigroup Inc. (C) analysts led by Alastair Syme. Companies on the hook for risky, high-cost projects that don’t make sense in a $50-a-barrel market include international titans such as Royal Dutch Shell Plc and small wildcatters like Sanchez Energy Corp. The impending writedowns represent the latest blow to an industry rocked by a combination of faltering demand growth and booming supplies from North American shale fields. The downturn threatens to wipe out more than $1.6 trillion in earnings for producing companies and nations this year. Oil explorers already are canceling drilling plans […]