A preliminary investigation of the optical properties of asphaltene and their application to source rock evaluation

Organic petrological methods have been used to investigate optical properties of asphaltenes from source rocks, humic coals and crude oils. With respect to microscopic structure, fluorescence and reflectance, asphaltenes form a distinctive group. All asphaltenes exhibit brown fluorescence with relative intensity ranging from 0.15 to 0.35, the intensity decreasing gradually with increasing maturity of the source rocks from which the asphaltene is extracted. Asphaltene reflectance is an important geochemical parameter; the reflectance of asphaltenes in air medium (Ra%) varies in the range of 4.5–8.0% and is affected mainly by source rock type and maturity. In general, the richer the hydrogen content of a kerogen, the weaker the reflectance of its derived asphaltenes. The contribution of source matter type to the variation of asphaltene reflectance can be as much as 0.50% Ra in the low maturity range. There is a nearly linear relationship between asphaltene reflectance and vitrinite reflectance in oil (Ro%) and geochemical maturity indicators such as CPI, MPR and MPI. Asphaltene reflectance is a potentially valuable technique for the maturity determination of source rocks and crude oils.