Shale oil resource evaluation with an improved understanding of free hydrocarbons: Insights from three-step hydrocarbon thermal desorption

The pyrolysis parameter S1, which indicates the amount of free hydrocarbons present in shale, is often underestimated due to hydrocarbon loss during sample handling and measurement processes. To remedy this issue, we strongly recommend an immediate three-step hydrocarbon thermal desorption (HTD) approach to be conducted on oil shale at the drilling site. This approach measures Sg, S0, and S1*, which refer to gaseous, light, and free hydrocarbons, respectively. The new shale oil content value, calculated from the total of these three parameters, is far more precise and reliable than traditional pyrolysis S1. Moreover, we thoroughly investigated the components and microscopic occurrence features of hydrocar-bons thermally desorbed at three temperature stages using gas chromatography (GC) and X-ray micro-computed tomography (CT). For example, we selected Chang 73 mud shale. Our experimental results irrefutably indicate that the ultimate shale oil content of poor resource rocks is significantly impacted by evaporative loss, with this effect being greater when the total organic carbon (TOC) is lower. Additionally, C1-5 and C1-7 hydrocarbons constitute almost all of Sg and S0, respectively. Sg and S0 are pre-dominantly composed of C1-3 gaseous hydrocarbons, with a maximum proportion of 42.93%. In contrast, S1* contains a substantial amount of C16-31 hydrocarbons. A three-dimensional reconstruction model of an X-ray micro-CT scan shows that while the amount of shale organic matter greatly decreases from the frozen state to 300 & DEG;C, the pore volume significantly increases, particularly between 90 and 300 & DEG;C. The increased pore volume is mainly due to macropores and fractures. It is imperative to note that the shale oil triple-division boundaries must be adjusted based on more accurate oil content, although this would not affect the resource zones to which the samples already belong (ineffective, low-efficient, and enriched resources). In conclusion, we strongly advise conducting an immediate well-site analysis or utilizing preservation procedures, such as deep freezing or plastic film wrapping followed by core wax-ing, to minimize volatile loss.& COPY; 2023 China University of Geosciences (Beijing) and Peking University. Published by Elsevier B.V. on behalf of China University of Geosciences (Beijing). This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).