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 misuse in exceptional cases which could lead to erroneous conclusions in using Rock-Eval pyrolysis to evaluate properties of the organic matter, a cross-plot of S2 (petroleum potential) versus TOC (total organic carbon) is an usable tool which can facilitate better interpretations. The graph can apply as a criterion accuracy of geochemistry parameters, and provides the correction for S2, HI (hydrogen index), kerogen type, and measuring the adsorption of hydrocarbon by the mineral matrix. In addition, this article demonstrates a manner based on the graph 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 geochemical study using Rock-Eval VI pyrolysis and Leco Carbon Analyzer has been conducted on samples from different outcrop sections in Lurestan province, Aligudarz region, from South-West of Iran, High Zagros. Plotting the data on a cross plot of S2 versus TOC, and determining the regression equation is the best method for determining the real values of S2 and HI parameters and measuring the contamination by bitumen or hydrocarbon. Hydrocarbon contaminations makes geochemistry data unreliable, as unrealistically increase S2 and HI, lower Tmax values, and reduce the thermal maturity level of organic matters or kerogen of source rock samples in two study locations. For skipping the effect of contamination and obtaining the real geochemistry parameters, the y-intercept of the graphs must be removed. The cause of contamination in organic rich facies of the Garau Formation is the adhesion of heavy bitumen to organic facies which is due to the covalent bonds between carbon and hydrogen ions.