Document Type : Research Paper


1 M.S. Student, School of Geology, College of Science, University of Tehran, Tehran, Iran.

2 Assistant Professor, Chemical and Petroleum Engineering Department, Ilam University, P.O. Box 69315/516, Ilam, Iran.


The Rock-Eval pyrolysis is a thermal method that is widely used by the petroleum geologist for evaluation of source rock characteristics and obtain geochemistry parameters. However, there are misconceptions and misuses in exceptional cases that could lead to erroneous conclusions after using the Rock-Eval pyrolysis data to evaluate the properties of organic matter. However, a cross-plot of S2 (petroleum potential) versus TOC (total organic carbon) is the usable tool to solve issues and applied for checking the accuracy of the geochemistry parameters. The graph provides the correction criteria for the S2, HI (hydrogen index), and kerogen type. As well as, the graph measures the adsorption of hydrocarbon by the mineral matrix. In addition, this article demonstrates a manner based on the data plot of S2 versus TOC to detect bitumen or hydrocarbon contaminations. Based on our knowledge about the Garau Formation as a possible source rock in petroleum geology of Iran, a geochemistry study by Rock-Eval VI pyrolysis and Leco Carbon Analyzer has been conducted on many rock samples collected from different outcrops in the Lurestan province, Aligudarz region, from South-West of Iran, High Zagros. Plotting the data on a cross plot of S2 versus TOC, drawing the regression line, and finding the regression equation are the best method for determining the real values of S2 and HI parameters and bitumen/hydrocarbon contamination. Contamination creates a y-intercept in the graph of S2 versus TOC which makes geochemistry data unreliable in two study location. As, led to the S2 and HI data unrealistically increased, while the Tmax values went down and reduced the thermal maturity of the organic matters from its real status. For skipping the effect of contamination and obtaining the real geochemistry parameters, the y-intercept of the graphs removed and the corresponding values subtracted from the HI and S2. The cause of contamination in the Garau Formation is the adhesion of heavy bitumen to organic facies due to the covalent bonds between carbon and hydrogen ions


  • Application of the S2versus TOC graph in the detection of in-situ bitumen contamination.
  • Evaluating the accuracy of geochemistry data obtained from the Rock-Eval pyrolysis apparatus.
  • The y-intercept in the graph of S2 versus TOC.
  • Determining depositional system tracts using the graph of S2 versus TOC.


Main Subjects

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