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Petroleum resource assessment obtained from laboratory tests on old core samples shelved in the open environment tend to underestimate in situ resources. Therefore, hydrocarbon loss and its restoration need to be investigated. The hydrocarbon loss was studied via the difference of programmed pyrolysis parameter (Rock–Eval derived S1) between preserved/fresh cores and their replicas that had been long-term exposed to ambient conditions. However, the in situ hydrocarbon potential is still seemed to be underestimated, because the effect of sample crushing and crucible waiting on pristine S1 detection did not receive enough attention. To solve this issue, 50 preserved shale core samples with R o from 1.00% to 1.51% were collected from the K1qn1 Formation of northern Songliao Basin, Eastern China. All the samples were subjected to four parallel experiments: Dean–Stark extraction and nuclear magnetic resonance (NMR) T1–T2 mapping, using large rock pieces 3–5 cm in size, and thermal desorption gas chromatography (TD–GC) and Rock–Eval analysis, using 60-mesh powdered samples. In addition, a time series of TD–GC and Rock–Eval analyses were conducted on a preserved shale sample to investigate hydrocarbon loss during sample crushing, Rock–Eval crucible waiting process, and elapse-time exposure to the open environment. Rock–Eval pyrolysis was also conducted on one month-exposed shale cores and solvent-extracted samples. The results show that the total hydrocarbon content of large rock pieces is significantly greater than that of powdered samples. Sample crushing or grinding leads C 8− light hydrocarbons to evaporate, and results in a loss of approximately 35% of S1. Rock–Eval crucible waiting process causes C 14− hydrocarbons to evaporate, and results in an additional 25% loss of S1. To compensate for the sample crush induced S1 loss, an exponential distribution model was proposed based on the C 8+ hydrocarbons in the TD–GC S1 fingerprints. Sample maturity, pristine light hydrocarbon (C 14−) proportions, organic matter richness, and oil-bearing pore size primarily control the hydrocarbon loss of old shale samples. The higher the proportion of C 14− and the lower the abundance of organic matter, the higher the amount of hydrocarbon loss. Finally, S1 restoration coefficient charts for large rock pieces and powdered samples with R o ranging from 1.00% to 1.42% are provided. It highlights that previous studies have severely underestimated the in situ S1 when using the Rock–Eval method. This research has great implication for estimating in situ hydrocarbon potential using conventional coring samples. • In situ oil content can be determined by Dean-Stark and NMR T1-T2 mapping using large preserved shale rock pieces. • The C 8- hydrocarbons was restored based on the TD–GC S1 fingerprints using an exponential model. • Factors influencing hydrocarbon loss of old shale samples were discussed. • S1 restoration coefficients charts of large rock pieces and powdered samples were plotted. [ABSTRACT FROM AUTHOR]

The shale of the Permian Lucaogou Formation in the Jimusar sag, Junggar Basin (abbreviated as the Lucaogou shale) is a typical lacustrine type-II source rock, and is among the most representative target zones for shale oil exploration in China. However, little attention has been paid to the dynamic changes of the shale decomposition; hydrocarbon generation, retention, and expulsion; physico-chemical properties; and producibility of shale oil at different stages of maturation in this shale. Semi-open pyrolysis experiments were performed on the Lucaogou shale; products including expelled gas, oil (expelled oil), bitumen (retained oil), and residual rock were obtained, and their quantities and chemical and physical compositions were comprehensively characterized. The results show that the density and molecular weight of the oil are lower than those of the bitumen, whereas the API gravity of the oil is higher than that of the bitumen, indicating the preferential expulsion of light hydrocarbons and short chains. The saturate, aromatic, resin and asphaltene (SARA) percentages of oils under the current semi-open pyrolysis experiments were similar to those of natural oils recovered from the Lucaogou Formation, suggesting that semi-open pyrolysis can achieve the same purpose as hydrous pyrolysis to simulate natural oil composition. The measured vitrinite reflectance (Ro) of the pyrolyzed shale was lower than the calculated Easy%Ro, which is in accord with findings from the Chang 7 shale (another typical lacustrine type-II shale in China) but distinct from previously reported type-III coals and marine type-II shales. Similar to the Chang 7 shale, the decomposition process of the Lucaogou shale did not follow the sequential reaction model described as kerogen → bitumen → oil or kerogen → NSOs → HCs. The generation of HCs (oil) and NSOs (bitumen) was independent and followed the "alternate pathway" mechanism under semi-open pyrolysis conditions. In the early oil window (Ro ˂ 1.0%), the hydrocarbon expulsion efficiency was relatively low, meaning that the shale simultaneously retained considerable amounts of oil during peak oil generation. The producibility of shale oil reached its maximum at the end of the oil window (1.0–1.2 %Ro), and this interval is considered the most suitable for fracturing technology. • Hydrocarbon generation, retention, and expulsion process of lacustrine type-II shale was systematically explored. • Semi-open pyrolysis system was applied to simulate natural maturation of shale. • Both chemical and physical properties of oil and bitumen were analyzed. [ABSTRACT FROM AUTHOR]

At present, there is a lack of research on the structural changes of lacustrine Type-II kerogen, which is regionally distributed but has significant potential for hydrocarbon resources. In our previous work, a series of non-hydrous, temperature-based semi-open pyrolysis experiments have been performed on the Chang 7 oil shale to simulate the thermal maturation (from 0.53 %Ro to 1.68 %Ro) of lacustrine Type-II kerogen. In that work, the parallel reaction pathway for kerogen decomposition was supported. The aim of this work is to reveal the structural changes of the post-pyrolysis kerogen. Integrated analysis including elemental analysis, programmed pyrolysis (Rock-Eval), Fourier transform infrared (FT-IR) spectroscopy, and solid-state 13C nuclear magnetic resonance (13C NMR) spectroscopy were conducted on the native unheated and post-pyrolysis rocks and kerogens. Based on the compositional features of the hydrocarbon products, structural changes of the post-pyrolysis kerogen were investigated, and insights on the initial chemical structure of the kerogen were also obtained. Before the peak hydrocarbon-generating stage (R o < 0.6%), defunctionalization of oxygen-containing functional groups, which leads to the generation of water, CO 2 and NSOs, occurs simultaneously with the detachment of short aliphatic side chains and small aromatic rings, which leads to the generation of volatile fractions (C 6 –C 14 , C 15+ sat and C 15+ aro). This process suggests that the Chang7 kerogen is highly branched. During the peak hydrocarbon-generating stage (0.6–1.09 % R o), the concurrent breakage of weak cross-link bonds and long aliphatic chains lead to the simultaneous peak generation of liquid hydrocarbons and NSOs. This reflects the highly cross-linked features of the Chang7 kerogen. After the peak hydrocarbon-generating stage (R o > 1.09%), the reactions in the previous two stages produce a highly aromatic kerogen. The increased aromaticity of the highly matured kerogen is most likely to be caused by aromatization or dehydrogenation of hydroaromatics and coking of aromatic bitumen, not by fusing or condensation of aromatic rings. • Chemical structural evolution of Chang7 lacustrine Type-II kerogen was characterized for the first time. • Kerogen reached artificial maturation under non-hydrous, temperature-based semi-open pyrolysis experiments. • Parallel bond breaking mechanism of unusual lacustrine Type-II kerogen was supported from chemical structure changes. [ABSTRACT FROM AUTHOR]