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.