The thermal history of an oil affects its gross properties in predictable ways: API gravity, saturated hydrocarbon content, and saturate/aromatic ratio increase, while sulfur content, viscosity, wax content, and pour point decrease. However, these parameters are often affected by other factors, such as variations in source-rock facies, biodegradation, phase changes in the reservoir, and migration, and thus cannot always be used quantitatively to evaluate maturity levels of oils. Gas chromatography is slightly more sensitive, but also cannot provide highly quantitative answers. The most reliable way of quantitatively evaluating the maturity of most oils is biomarkers, which are obtained from GC-MS analysis. For extremely mature oils, where the standard biomarkers have been destroyed by high temperatures, diamondoids provide a useful alternative.
Unfortunately, some of the most-commonly used biomarker data are of limited value for oils. The mixture of R and S epimers of extended hopanes normally reaches equilibrium long before oil generation occurs, and thus is not useful at all. Similarly, the mixture of R and S epimers of steranes reaches equilibrium prior to peak generation, and thus often cannot be used to determine precise oil maturity. Aromatic indicators such as the Methylphenanthrene Index (MPI) are valid only for oils that are generated from Type III kerogen, a serious limitation that is often overlooked by interpreters (Matthias Radke, personal communication).
We believe that the most-useful indicators of oil maturity are the Tm/Ts ratio (also expressed as Ts/Tm or Ts/(Tm+Ts)), the sterane/triterpane (or sterane/hopane) ratio, and the absolute concentration of total hopanes (or simply of the C30 hopane) in the sample. None of these ratios is currently very popular, mainly because the values vary from one organofacies (or oil type) to another, and they therefore must be calibrated for each system. Nevertheless, we have had very good success employing these ratios – calibration is generally not difficult, and the results are very consistent and cover a very wide range of oil maturity.
The plot below shows Ts/Tm covarying with the absolute concentration of C30 hopane in a single family of oils. With appropriate calibration data, all these maturity parameters can be linked, at least approximately, to Transformation Ratios in source rocks, and to vitrinite reflectance values.