Geochemical evaluation of pre-salt crude oils from the Santos Basin using light hydrocarbon parameters by GC×GC-TOFMS

Light hydrocarbons (up to C₁₃) comprise a significant fraction of most crude oils, particularly light oils such as those from the pre-salt of the Santos Basin, which are known for their high quality and API gravity of around 28. Therefore, interpreting molecular parameters based on light hydrocarbons is critical for assessing information regarding source rock, thermal maturity levels, and potential secondary alteration processes. This study aimed to evaluate the application of light hydrocarbons, analyzed via comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC×GC-TOFMS), for the geochemical characterization of 32 crude oils from a field located at the pre-salt of the Santos Basin. The crude oils, provided by Petrobras, were prepared by dissolving approximately 2.00 mg in 200 µL of carbon disulfide containing internal standards. The analyses were performed using a GC×GC-TOFMS Pegasus 4D system (Leco, St. Joseph, MI, USA The analytical conditions were set according to Coutinho et al. (2025), and GC×GC data were processed using ChromaTOF software version 4.51, applying a signal-to-noise ratio of 50:1 and a peak table approach. Classical geochemical parameters based on light hydrocarbons were subsequently evaluated. The Mango K1 parameter, calculated from the relative distribution of C₇ hydrocarbons, showed a strong linear correlation among the samples, indicating that the crude oils likely originated from the same family. The Thompson H–I diagram (heptane vs isoheptane ratio) indicated that the oils predominantly exhibit a normal maturity level (heptane values ranging from 18 to 22 and isoheptane values between 0.8 and 1.2), suggesting that the samples did not undergo significant thermal alteration after oil generation. Moreover, the relative distribution of C₇ light hydrocarbons, according to Hu (1990), enabled the determination of oil origin based on the nature of the precursor organic matter. The studied oils were positioned within the sapropelic kerogen field, which exhibits a high potential for petroleum generation due to its greater abundance of long-chain, saturated aliphatic compounds that are more susceptible to thermal cracking processes. The consistent results obtained from the different molecular parameters indicate the preservation of the original geochemical signature of the crude oils and contribute to the understanding of the petroleum system in the Santos Basin. The study demonstrates that GC×GC-TOFMS is a promising tool for the geochemical characterization of light hydrocarbons in crude oils, as its high chromatographic resolution allows the separation and identification of compounds that may coelute in conventional gas chromatography, providing more accurate molecular information for petroleum-system evaluation.