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 S_g, 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 hydrocarbons thermally desorbed at three temperature stages using gas chromatography (GC) and X-ray microcomputed 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, C_1-5 and C_1-7 hydrocarbons constitute almost all of S_g and S₀, respectively. S_g and S₀ are predominantly composed of C_1-3 gaseous hydrocarbons, with a maximum proportion of 42.93%. In contrast, S₁^* contains a substantial amount of C_16-31 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 ℃, the pore volume significantly increases, particularly between 90 and 300 ℃. 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 waxing, to minimize volatile loss.