Abstract: The invention teaches a method proposing two new indices for evaluating thermal evolution in oils from different basins and organofacies. The first index is based on the distribution ratio of high molecular weight sulfur compounds, belonging to the DBE 6 (benzothiophene) and DBE 9 (dibenzothiophene) series. The second index, called TEI, was created from the profile of aromatic hydrocarbons and molecules containing N, O and S. Both parameters were obtained from the direct characterization of the oils, by using the photoionization at atmospheric pressure (APPI) technique combined with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS).