A process analyzes gases emitted during drilling of a borehole using oil based mud. The process comprises (a) using mass spectrometry, analyzing the gas recovered during drilling through a hydrocarbon-poor zone at spaced apart locations to provide a mass spectrum for the gas emitted at each of the locations; (b) using mass spectrometry, analyzing the gas recovered during drilling through a hydrocarbon-rich zone at one location to provide a mass spectrum for the gas emitted at the location; (c) using spectra from the measurements in (a) to extrapolate and predict a peak in a mass spectrum caused by compounds in the mud at a time when the drilling is proceeding through the hydrocarbon-rich zone; and (d) comparing the spectrum obtained in (b) with the predicted spectrum obtained in (c) to further predict at least one of the quantity and identity of the formation gases emitted from the hydrocarbon-rich zone.