Assessment of shale hydrocarbon evaporative loss during exposure: Insights from online low-field nuclear magnetic resonance observations in the Qingshankou Formation, Songliao Basin, China

Understanding the evaporative loss of shale oil is critical for the shale oil resources assessment. However, previous studies have largely neglected the rapid evaporative loss of shale oil that occurs immediately after drill cores are retrieved to the surface at the well site. Thus, the factors influencing the evaporative loss of shale oil during thermal evolution remain inadequately explored. In this study, closed-system pyrolysis experiments were carried out to the artificially mature Qingshankou Formation shales. Subsequently, low-field nuclear magnetic resonance (LF-NMR) techniques were utilized to monitor the variations in oil content of the shales under different exposure times. Our experimental approach successfully reconstructed the entire evaporative loss process of shale oil, spanning from its original subsurface location to the well site, from the well site to the laboratory, and ultimately to long-term core storage. We find that the maximum loss of shale oil reaches approximately 10% within the first 10 h following the retrieval of drill cores from the subsurface to the ground, followed by a gradual deceleration, and the maximum loss ranges from 11% to 89%. As thermal maturity increases to a range of R_o = 0.89%–1.20%, the loss proportion of shale oil shows a decreasing trend, which can be attributed to the reduction in total organic carbon (TOC) content. It is worth mentioning that the loss proportion of shale oil exhibits a continuous increasing trend at the thermal range of 1.20%–1.75%. At this stage, the hydrocarbon composition (C₆–C₁₄/C₁₅₊ ratio) may be the mainly controlling factors for the shale oil loss proportion, while TOC content and nanopore volume serve as secondary factors. Our analyses reveal that hydrocarbon evaporative loss of shale oil is complex and involves various factors, especially the first 10 h during the transfer of the shale from its in-situ reservoir to the surface at the well site, which is critical to the understanding of shale oil occurrence and accurate resource assessment.