The source rock maturity is one of the most fundamental parameters to assess the petroleum system. With a more accurate maturity assessment, a range of key parameters in a petroleum system can be better constrained and predicted, including the hydrocarbon generation/cracking, the original oil and gas in place, the gas-oil-ratio (GOR), API gravity, and gas wetness. However, maturity for many samples cannot be readily assessed by conventional methods such as vitrinite reflectance, due to lack of sufficient vitrinite materials and/or interference of lipids, and other uncertainties. We present the application of a novel surface-enhanced Raman spectroscopy (SERS) method to understand the fundamental structural changes in the kerogen and the saturates-aromatics shifting during the thermal maturation process. Cuttings and core chips samples were prepared without kerogen isolation, and treated with silver nanoparticles, which serve as antennas to enhance the Raman scattering signals, especially for the low maturity samples. The Raman bands were processed to investigate the kerogen structure, which is closely related to the thermal maturation it went through. Vitrinite reflectance equivalent values for the samples tested were converted from measured Raman bands information using kinetic-based calibrations. Source rock samples from various basins were tested with the proposed method to establish calibrations and the SERS responses of different kerogen types were discussed.
The thermal maturity of organic matter in coal petrography and petroleum geology is determined by vitrinite reflectance (Diessel, 1978), which requires considerable effort and expertise (Hackley, 2015). Unlike the traditional vitrinite reflectance method, Raman spectroscopy offers nondestructive and fast measurements, which only need a small amount of sample, to evaluate the maturity of organic matter (Beyssac, 2002; Quirico, 2005; Lahfid, 2010; Guedes, 2010; Hinrichs, 2014; Wilkins, 2014).
The Raman spectrum of organic matter consists of the disordered (D) band and the graphite (G) band. The G-band is associated with the in-plane vibration of carbon atoms in graphene sheets with E2g2 symmetry (Tuinstra, 1970; Jehlička, 1999). The D-band is related to structural defects and heteroatoms (Beny-Bassez, 1985). When the maturity of organic matter increases, aromatization and condensation reactions cause a continuous increase in the proportion of sp2 (i.e., C=C) bonds with decrease of sp3 (i.e., C-C) bonds. For instance, Figure 1 shows a set of Raman spectra for coal samples varying in their maturity. Increased sp2 leads to blue shift of the G peak, whereas decreased sp3 leads to red shift of the D peak. Therefore, the distance between the D-band and G-band (RBS) increases with growing maturity up to 3.0 %VRo. In disordered organic matter, extra peaks are identifiable as asymmetric bands and small bumps. However, the nomenclature and origin of these bands are not consistent during deconvolution procedures from different experiments (Beyssac, 2002; Romero-Sarmiento, 2014; Ferralis, 2016). Therefore, the RBS is still the most popular Raman parameter to estimate organic matter maturity (Henry, 2019).
