The objective of this study has been to assess the potential applicability of Surface Enhanced Raman Spectroscopy (SERS) as a routine method for the quantification of source rock maturity. Despite decades of experience with the application of vitrinite reflectance and a host of proxy organic petrographic and geochemical methods (e.g. Rock-Eval Tmax VRE), the assessment of maturity is still challenging in many petroleum systems. Raman spectroscopy, particularly via the modified SERS approach, offers a possible solution to this maturity challenge. We have applied this technique, employing optimal laser intensity and analysis duration, on powders of coal, kerogen and shale. The powders are first blended with an aqueous nanoparticle solution containing micro-silver, to take advantage of the SERS Effect. The SERS maturity data show consistent variability with maturity, which is not susceptible to challenges such as vitrinite suppression. This procedure works well when performed on bulk kerogen samples, removing the requirement for subjective recognition of specific maceral constituents under the microscope. This study demonstrates the ‘proof-of-concept’ of this method for maturity determination. Research is ongoing to simplify the procedure and develop a consistent calibration system.
Reliable quantification of thermal maturity is essential for the effective assessment of source rock data, for both regional play calibration and well assessment. Accurate predictions of ultimate potential and fluid properties rely upon consistent and reliable maturity determinations, but which must also be cost-effective. In many unconventional plays, classic vitrinite material is often lacking. Currently, this often necessitates involved organic petrographic studies, potentially leading to challenges with consistency and cost management. The industry urgently needs a fast, simple and reliable method of maturity determination.
Raman spectroscopy quantifies the inelastic scattering resulting from the excitation of molecular systems by (typically) a laser light source. It is particularly useful for the quantification of mono-atomic molecular configurations, such as C-C (sp3), C=C (sp2) and C≡C (sp) carbon-carbon bonds. With increasing maturity, condensation and aromatization reactions lead to a progressive increase in the proportion of sp2 bonds concomitant with a decrease of sp3. These molecular shifts are reflected by changes in kerogen Raman spectra characteristics (Keleman and Fang, 2001). Figure 1 shows a series of Raman spectra for a sequence of coals varying in their level of maturity.