Source-Rock Evaluation Using the Rock-Eval Technique

Abstract

The basic pyrolysis techniques used for source-rock geochemical analysis are generally simple and can be interpreted in a straightforward way. However, some of the data generated can be misleading and lead to confused interpretations if not properly assessed. This chapter discusses the different aspects of source-rock evaluation using the commonly used Rock-Eval technique on a step-by-step basis. Here, the impact for several factors on key Rock-Eval derived measurements, viz. particle crush-size, FID signal, S2 pyrogram shape, FID linearity, S4CO2 oxidation graphics, are addressed. Careful monitoring of these key parameters enables analysts/interpreters to conduct meaningful source-rock assessment. The shape of S2 pyrograms helps to predict the type of kerogen(s) present within a sample and can be indicative of their level of thermal maturity. While type I kerogen bearing JR-1 standard and type II kerogen-mimicking IFP160000 synthetic shale standard, show tighter Gaussian shaped S2 peak shapes in their pyrograms, type III kerogen bearing shales typically show a right-side tailed effect. Further, owing to their extremely high hydrocarbon generation potential, even at lower sample weights, type I kerogens show higher FID signals than other kerogen types. Type III-IV kerogen bearing shales show least FID signals even at higher sample weights owing to their lower petroleum generation potential. For type I kerogen bearing shales FID signals can be very high; if they rise beyond the Rock-Eval equipment’s FID detection limits, the resulting pyrograms are likely to be erroneous. Migrated hydrocarbons in the samples tested are likely to have an impact on the Rock-Eval pyrograms they yield. Particle crush-sizes of the samples analyzed are potentially more significant for organic-rich shales compared to organic-lean shales. Sample weights on S4CO2 oxidation graphics are shown to be potentially significant. For carbonate-free shales, with increasing sample weights, increasing portions of the CO2 from the organic-matter (represented by S4CO2 graphics) tends to be undercounted. This result in an underestimation of the residual carbon (RC) and TOC content, and erroneous estimation of carbonate mineral content. To obtain reliable Rock-Eval results it is necessary to conduct simultaneous monitoring of FID signals, S2 pyrograms shapes, and S4CO2 oxidation graphics. For organic-lean shales, the S2 signals may be too low, below the FID detection limits of the Rock-Eval equipment, generating erroneous thermal maturity data.