ROCK-EVAL/TOC GUIDELINES FOR INTERPRETATION

 Rock-Eval results correlate to other techniques (Espitalie et al., 1985; Tissot and Welte, 1978). Source rock potential is sensitive to lithology, TOC and S2 values (Table 1). It is common practice to rate carbonate rocks with lower TOC comparable with richer clastic rocks. Extractable HC yields from leaner carbonate rocks are comparable to richer clastic rocks (Tissot and Welte, 1978, p. 430; Gehman, 1962). The organic matter associated with carbonate rocks is often more hydrogen-rich and thermally labile than that in fine-grained clastic rocks. As a result, more TOC in carbonate rocks may be transformed into bitumen compared with average clastic source rocks of comparable maturity.

Rock-Eval/TOC parameters have significance only above threshold TOC, S1 and S2 values. If TOC is less than less than or equal to 0.3% then all parameters have questionable significance and the experiment suggests no potential. Oxygen Index (OI), S3/TOC, has questionable significance if TOC is less than or equal to 0.5%. Both Tmax and Production Index (PI = S1/(S1+S2)), have questionable significance if S1 and S2 values are less than or equal to about 0.2 mg HC/g rock. Results can be affected by mineral matrix effects. These either retain generated compounds, generally lowering the S1 or S2 peaks, while increasing Tmax, or by liberating inorganic CO2 and increasing S3 and OI. These effects are important if TOC, S1 and S2 are low, an effect not significant where sources have TOC values greater than 5%. OI values greater than 150 mg/g TOC suggest either low TOC or a mineral matrix CO2 contribution during pyrolysis. Generally, a TOC content of at least 2% is needed for a source rock. Note that TOC and Hydrogen Index decrease with increasing thermal maturity due to hydrocarbon generation.


References for this section of text.
Espitalie, J., Deroo, G. and Marquis, F. 1985. Rock Eval Pyrolysis and Its Applications. Preprint; Institut Francais du Petrole, Geologie No. 27299, 72 p. English translation of, La pyrolyse Rock-Eval et ses applications, Premiere, Deuxieme et Troisieme Parties, in Revue de l'Institut Francais du Petrole, v. 40, p. 563-579 and 755-784; v. 41, p. 73-89.

Gehman, H. M. Jr., 1962. Organic matter in limestones; Geochimica et Cosmochimica Acta, v. 26, p. 885-897.

Peters, K. E., 1986. Guidelines for evaluating source rock using programmed pyrolysis; Bulletin of the American Association of Petroleum Geologists, v. 70, p. 318-329.

Tissot, B. P., and Welte, D. H., 1978. Petroleum formation and occurrence; Springer-Verlag, Berlin, 538 p.

Return to index