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Experimental:
Rock-Eval/TOC analysis provides fast and reliable characterization of the quantity and quality of sedimentary organic matter, as well as its thermal maturity. All samples were analyzed on a Rock-Eval 6 Turbo (RE 6) instrument equipped with a Total Organic Carbon analysis module. A typical RE 6 experiment is initiated with heating of a pulverized rock sample at 300ºC for 3 min in nitrogen atmosphere, when naturally occurring hydrocarbons (free and adsorbed) are volatilized. The oven temperature is then steadily increased to 650ºC at a rate of 25ºC/min and decomposition of kerogen occurs. The amount of hydrocarbons volatilized at 300ºC and evolved from kerogen during the ramped heating are quantitatively determined by a flame ionization detector, and recorded as the S1 and S2 peaks, respectively. The final stage involves oxidation and combustion of the residual organic matter at 850ºC. The temperature measured at the maximum of the S2 peak is referred to as Tpeak while Tmax is calculated by subtracting about 40ºC from Tpeak (the exact correction is determined using a manufacturer standard). The amounts of CO and CO2 are quantitatively determined by infrared detectors. Additional details on the RE6 instrument are available in Behar et al. (2001). The percentage of carbon in CO2 formed during oxidation and in the hydrocarbon peaks S1 and S2 is used to define the total organic carbon content (TOC), expressed as a weight percentage. The determination of the quality of organic matter is based upon the calculation of Hydrogen (HI) and Oxygen (OI) indices (HI=100xS2/TOC, OI=100xS3/TOC) 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. 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: 440º-448ºC (Type I), 430º-455ºC (Type II) and 430º-470ºC (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. Rock-Eval results correlate to other techniques (Espitalié et al., 1985; Tissot and Welte, 1978). Source rock potential is sensitive to lithology, TOC and S2 values. 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 reliable significance only above threshold TOC, S1 and S2 values. If TOC is less than or equal to 0.3% then all parameters have questionable significance and the experiment suggests no petroleum source potential. Oxygen Index (OI) 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 rock mineral composition. The mineral matrix may either retain hydrocarbon compounds, generally lowering the S1 or S2 peaks, while increasing Tmax, or liberate inorganic CO2 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. Additional guidelines on the interpretation of Rock-Eval data may be found in Peters (1986), Snowdon (1995) and Sykes and Snowdon (2002). 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.pdfEspitalié, 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. 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. Snowdon, L R; 1995. Rock-Eval Tmax suppression: documentation and amelioration. American Association of Petroleum Geologists Bulletin, v. 79, p. 1337-1348. Sykes, R; Snowdon, L R; 2002. Guidelines for assessing the petroleum potential of coaly source rocks using Rock-Eval pyrolysis. Organic Geochemistry v. 33/12, p. 1441-1455. Tissot, B. P. and Welte, D. H., 1978. Petroleum formation and occurrence; Springer-Verlag, Berlin, 538 p. |
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