The Macquarie Group puts the chances of OPEC reaching a credible crude output agreement that boosts oil prices at 60 percent, but closely followed investor Dennis Gartman thinks those odds are too generous.

The Organization of the Petroleum Exporting Countries may produce a deal when it meets in Vienna on Nov. 30, but it's doubtful such an accord would achieve its aim of bolstering oil prices following a two-year downturn, The Gartman Letter's publisher told CNBC's "Power Lunch" on Thursday.