Identifying independent parameters to bracket the oil window in sedimentary basins is of first importance for petroleum exploration. We investigate the magnetic assemblage of Cretaceous claystones collected from the western volcanic margin of Greenland. These claystones experienced low-grade burial ( 300 K) properties of isothermal remanent magnetization. The magnetic assemblage of immature claystones (lower Ro) resides in goethite, greigite, and some magnetite. The magnetic assemblage of immature to early mature claystones (higher Ro) resides in stoichiometric magnetite and fine-grained pyrrhotite. By comparing these results with those from claystones from the Paris Basin (France) and the Jura Mountains (Switzerland), we suggest the existence of similar mechanisms for the formation of magnetic minerals during burial. If confirmed by further studies, the magnetic assemblage of claystones may be diagnostic of oil windows.