Methods for improving the quality of hydrocarbon fluids produced by in situ pyrolysis or mobilization of organic-rich rock, such as oil shale, coal, or heavy oil. The methods involve reducing the content of olefins, which can lead to precipitation and sludge formation in pipelines and during storage of produced oils. The olefin content is reduced by arranging wells and controlling well pressures such that hydrocarbon fluids generated in situ are caused to pass through and contact pyrolyzed zones in which coke has been left. This contacting chemically hydrogenates a portion of the olefins in the pyrolysis oil by reducing the hydrogen content of the coke.