Characterizing maturity of reservoir pyrobitumen with strong anisotropy: A calibration between reflectance and laser Raman spectral parameters

Pyrobitumen reflectance is a proxy for evaluating the thermal maturity of ancient sedimentary rocks, particularly where vitrinite is absent. However, due to the strong optical anisotropy of pyrobitumen, it is challenging to accurately characterize the maturity. In this study, we investigated the petrography and spectroscopy of pyrobitumen in the Sinian (Neoproterozoic) Dengying Formation in the Sichuan Basin, China. Based on differences in the morphology and texture of pyrobitumen, maturity characterization parameters were optimized. Pyrobitumen in the Dengying Formation can be divided into six types of single forms and five types of multiple forms based on optical texture. The average reflectance of isotropic to slightly and finely mosaic-textured pyrobitumen, and that of dark areas of irregularly mosaic-, spherical-, flow-, and fibrous-textured pyrobitumen is relatively stable and correlates well with Raman spectral features. In contrast, the average reflectance of bright areas of irregularly mosaic-, spherical-, flow-, and fibrous-textured pyrobitumen is variable and anomalously high, and does not match the Raman spectral features. The difference in textures and reflectance is controlled by nanoscale deformation of the pyrobitumen surface, which may reflect differences in the composition of the residual pyrobitumen and its viscosity, suggesting variable organic matter precursors, secondary modification, and physical conditions of oil and pyrobitumen formation. The Raman spectra of all of the optical textures of pyrobitumen can overcome strong optical anisotropy and can be used as a thermal proxy. The Raman band separation, instead of the reflectance of pyrobitumen, is a good thermal proxy in all cases because the optical texture of pyrobitumen has little effect on Raman parameters but has a great influence on reflectance (thereby rendering the reflectance of the bright area inaccurate). The reflectance of the dark area of pyrobitumen is also a good proxy and corresponds well with Raman parameters. In contrast, the reflectance of the bright area of pyrobitumen is distorted due to graphitization and thus cannot serve as a maturity proxy. The Raman spectra have the greatest application in overmature, exhumed conventional oil reservoirs and overmature unconventional reservoirs.