Apparatus and methods for characterizing hydrocarbons in a subterranean formation include obtaining a sample of the subterranean formation; measuring, uphole, the porosity of the sample; using a nuclear magnetic resonance (NMR) tool downhole in the borehole, sending NMR pulse sequences configured for formation pore size and measuring NMR signals that characterize the formation at a location in the formation; analyzing the signals to find a gas porosity of the formation at the location; and determining a hydrogen index (HIg) of the subterranean formation from the gas porosity and from the porosity of the sample. The obtained HIg may then be used in conjunction with downhole NMR measurements to find corrected gas porosities at locations of the formation.