European Central Bank staff presented policy makers with models for buying as much as 500 billion euros ($591 billion) of investment-grade assets, according to a person who attended a meeting of the Governing Council.

A 500 billion-euro purchase program would take the ECB halfway toward its goal of boosting its balance sheet to avert a deflationary spiral in the euro area. The institution is also buying asset-backed securities and covered bonds, and government bond-buying would be part of fresh stimulus to be considered at the Governing Council’s Jan. 22 meeting.