Experimental:

Rock-Eval/TOC analysis provides fast and reliable characterization of the quantity and quality of sedimentary organic matter, as well as its level of thermal maturity. The previously published results (Snowdon, 1988) were generated using a Rock-Eval II instrument equipped with a TOC (total organic carbon) module. The new pyrolysis experiments were conducted using a Rock-Eval 6Turbo instrument. The data generated by Chevron were run using an Oil Show Analyzer, an instrument essentially similar to a Rock-Eval II but in which no S3 peak is measured and hence no Oxygen Index is available. The operating parameters for a Rock-Eval II (RE II) are provided in several GSC Open File reports (e.g. Snowdon, 1988). These are generally the default settings for this type of instrumentation and thus similar conditions likely will have been used by Chevron for the Oil Show Analyzer. The operating conditions for a Rock-Eval 6 (RE6) instrument are slightly different in some respects, although the measured or calculated parameters are nominally the same for the two instruments. The RE6 uses nitrogen as a pyrolysis gas rather than helium, resulting in a reduction in thermal gradients within the pyrolysis oven and a higher level of temperature control and precision. The default maximum pyrolysis temperature of a RE6 is 650 ºC rather than 600 ºC in a RE II. Thus the new instrument may yield somewhat higher S2 values especially for samples containing terrigenous (Type III) organic matter at high levels of thermal maturity. The RE6 instrument has infrared detectors that continuously monitor CO and CO2 production during the pyrolysis and oxidation steps of the analysis, and this provides a much more reliable determination of the S3 (and Oxygen Index = 100*S3/TOC) than the older machine. The RE6 detectors are more sensitive and thus the typical sample size used is 70 mg of rock rather than the 100 mg used in a RE II. The RE6 oxidation oven is programmed to heat to 850 ºC and thus TOC estimates tend to be more accurate than for the older technology, especially for high maturity samples. The temperature measured at the maximum of the S2 peak in a RE II is referred to as Tmax, whereas the equivalent temperature in a RE6 is termed Tpeak. There was an absolute error of about 40 ºC in the RE II instrument that is no longer present in the RE6. Tmax values reported from a RE6 instrument are calculated by subtracting ~40 ºC from the measured Tpeak value. Additional details on the RE6 instrument are available in Behar et al. (2001).

The determination of the quality of organic matter is based upon the calculation of Hydrogen (HI) and Oxygen (OI) indices (HI=S2/TOCx100, OI=S3/TOCx100) which are related to the atomic H/C and O/C ratios (Espitalié et al., 1977). The OI versus HI cross plots ("pseudo van Krevelen diagrams") can be used as an organic matter type indicator at low and moderate maturities. Results from the Oil Show Analyzer do not include the S3 and Oxygen Index parameter (plotted as zero values) and thus Hydrogen Index versus Tmax is commonly used as a surrogate for the pseudo van Krevelen diagram. The Tmax is an indicator of relative thermal maturity. According to Espitalié et al. (1985) the oil window is defined by the following Tmax ranges: 440–448 ºC (Type I), 430–455 ºC (Type II) and 430–470 ºC (Type III). A cross plot of Tmax versus HI is used to constrain estimations of organic matter type and its thermal maturity, while the Production Index (PI=S1/[S1+S2]) is used to indicate staining of a sample or as an additional maturity parameter.

While the RE6 instrument is much more stable and reliable than the older technology, interpretation of the results must be carried out with care. If TOC is very low (e.g. <0.1%) then all parameters have questionable significance and the experiment suggests no source rock potential. Similarly, if S1 and S2 are low (e.g. <0.2 mg HC/g rock), the analytical error may be large relative to the absolute measured value and the Production Index (PI = S1/(S1+S2)) ratio may be effectively undefined. Also it will be difficult to select a reliable peak top for a very low S2 value and thus the Tpeak (and hence Tmax) will be unreliable.

References:

Behar, F; Beaumont, V; Penteado, H L De B; 2001. Rock-Eval 6 Technology: Performances and developments. Revue de l’ Français du Pétrole, v.56/2, p. 111-134. http://ogst.ifp.fr/articles/ogst/pdf/2001/02/behar_v56n2.pdf

Espitalie, J., Laporte, J.L., Madec, M., Marquis, F., Leplat, P., Paulet, A. and Boutefeu, J., 1977. Methode rapide de characterisaion des roches meres de leur potential petrolier et de leur degre d'evolution. Revue de l'Institut Francais du Petrole vol. 32, p. 23-42.

Espitalié, J; Deroo, G; Marquis, F; 1985. Rock Eval Pyrolysis and Its Applications. Preprint; Institut Française 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; vol. 41, p. 73-89.

Snowdon, L R; 1988. Petroleum Source Rock Potential and Thermal Maturation Reconnaissance in Eagle Plain, Yukon Territory. Geological Survey of Canada, Open File 1720.



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