Remarkable issues of Rock-Eval pyrolysis in the assessment of shale oil/gas

Rock-Eval pyrolysis has been widely used in assessing source rocks from the very beginning. Although this approach can evaluate oil content, hydrocarbon generation, as well as the abundance, type, and thermal maturity of organic matter in a simple and rapid way, it is noteworthy that this technique has some limitations in application, and improper interpretation of pyrolytic data may bring more risks to shale oil/gas exploration. This study summarizes three main pitfalls commonly seen in previous publications based on massive experimental results. First, the use of hydrogen index (HI), oxygen index (OI), the temperature of maximum pyrolysis yields (T_max), and the ratio of S₂/S₃ to discriminate kerogen of diverse types should target source rocks with maturity less than 1.35 % R_o; the feasibility of the technique to highly-to-overmature source rock samples is limited. Second, the validity of T_max depends on the area of S₂ and whether it is in normal distribution, and the accuracy of T_max relies on kerogen type and thermal maturity; moreover, residual hydrocarbon and pyrite content have some effects on the accuracy of T_max. To obtain accurate T_max values, the maturity of source rocks of types Ⅰ, Ⅱ, and Ⅲ should not be larger than 1.70 % R_o. Third, the oil saturation index (OSI) has been used to indicate the mobility of shale oil, and a value larger than 100 mg/g TOC suggests sweet spots of shale oil. However, it should be noted that OSI could not directly provide information on the saturation of oil in shale. OSI values are generally smaller than 100 if the rocks are very organic-rich, and a small TOC value could also lead to a large OSI value (more than 100 mg HC/g TOC). Besides, only a few shales bear OSI higher than 100 mg HC/g TOC, although many of the shales have been proven commercially successful. Therefore, the applicability of OSI larger than 100 mg HC/g TOC as a parameter for shale oil mobility merits further consideration. We suggest using individual OSI criteria for different types of sedimentary basins and shale formations. Moreover, the loss of light hydrocarbons during the storage and preparation of rock samples is strongly dependent on rock lithofacies, and thus, classified assessment should be adopted for shale oil reservoirs of multiple lithofacies.