Direct constraints on the absolute timing of shale oil accumulation and microscale migration within shale systems remain limited. Here we investigate ostracod limestone interbeds, calcite veins, and barite veins from the Qingshankou Formation in the Songliao Basin, integrating fluid inclusion petrography, diagenetic phase analysis of host minerals, and in situ U–Pb dating to reconstruct the timing and evolution of shale oil accumulation in Member 1 of the Qingshankou Formation. Hydrocarbon inclusions within the veins are dominated by yellow-green, blue-white, and predominantly blue fluorescence assemblages. Blue-fluorescing inclusions are characterized by fluorescence λ_max below 485 nm and CIE chromaticity coordinates (X, Y) lower than 0.3, indicating a relatively high level of thermal maturity. Homogenization temperatures of hydrocarbon inclusions and their coexisting aqueous inclusions show good correspondence with the diagenetic stages of host minerals, reflecting multiple episodes of fluid trapping. Trapping ages of hydrocarbon inclusions inferred from burial–thermal history reconstruction are broadly consistent with in situ U–Pb ages of calcite veins (65.3 ± 4.8 Ma and 58.2 ± 5.2 Ma), indicating that vein formation and hydrocarbon-bearing fluid activity were essentially synchronous at the study area scale. Image-based analysis of the hydrocarbon inclusion abundance index (IHI-AI) shows that blue-fluorescing inclusions are relatively enriched in the central Gulong Sag, and their spatial distribution closely matches the occurrence of high-maturity, light shale oil. Integrated interpretation of fluorescence spectral parameters, equivalent hydrocarbon inclusion reflectance derived from the image-based fluorescence CIE chromaticity (IF–CIE) method (EqHIR_o), homogenization temperature data, diagenetic evolution, and in situ U–Pb dating indicates that, after the Qingshankou Formation reached a high level of thermal maturity near the end of the Cretaceous, shale oil retained within the source rock continued to undergo intrasource microscale migration and redistribution from the middle to late Paleogene through the Neogene. Distinct fluorescence–chromaticity signatures of hydrocarbon inclusions indicate that shale oil accumulation was closely associated with continuous hydrocarbon generation and sustained intrasource micromigration. This study provides an integrated framework for constraining the timing and evolutionary pathways of shale oil accumulation in lacustrine shale systems.

Eastern Sichuan Basin is a critical region for oil and gas production in China, and the thermal effect of the Emeishan Mantle Plume (EMP) on the basin remains unclear, which limits the study of the Paleozoic hydrocarbon accumulation process. Thus, clarifying the thermal history is crucial for oil and gas evaluation and exploration. This study combines zircon fission track (ZFT), zircon (U-Th)/He (ZHe), and vitrinite reflectance analyses to reconstruct the thermal history since the Paleozoic and investigate the maturity evolution of Paleozoic source rocks in the eastern Sichuan Basin. Additionally, the contributing factors of the Permian abnormal heat flow and the thermal effect range of the EMP were analyzed. Thermal history results show a stable low heat flow phase (45–55 mW/m²) from the Cambrian to Permian, a rapid increase to peak values (62–70 mW/m²) during the Middle Permian, and a gradual decline to current stable values (47–62 mW/m²) from the Triassic onward. Thermal evolution revealed that Paleozoic source rocks exhibited maturation stagnation due to tectonic events, while the Permian peak heat flow and the Jurassic-Late Cretaceous rapid burial accelerated maturation. The Cambrian Qiongzhusi Formation and Silurian Longmaxi Formation source rocks began generating hydrocarbons (R_equ = 0.5%) in the Late Cambrian–Late Ordovician and Late Silurian–Early Devonian, respectively, while it was Late Permian–Early Triassic for Permian source rocks. Peak maturity (R_equ > 2%) occurred in the Late Cretaceous, with natural gas as the dominant product. The EMP significantly influenced the thermal regime of the western and central Sichuan Basin, with a thermal effect radius of ~600–650 km. In contrast, the Permian thermal anomaly in the eastern Sichuan Basin is attributed to the lithospheric extension associated with the Middle Permian Kaijiang-Liangping trough. These findings provide critical insights for deep and ultra-deep gas exploration and enhance the understanding of the EMP's regional thermal impacts.

The absence of humic vitrinite complicates the determination of thermal maturity in pre-Devonian sediments. Lower Paleozoic shales contain structured zooclasts, solid bitumen, and vitrinite-like materials, and their reflectances have been found to be valid maturity proxies. Yet there remains much controversy as to the degree of evolution of reflectances. The current work aims to investigate the reflectance of dispersed organic matter (DOM), as well as their transformation degree and organic pore development. To this end, this study evaluated an Upper Ordovician zooclasts-bearing shale and a Middle Jurassic coal sample via anhydrous-pyrolysis to compare differences in the maturation pathways between marine-derived DOM and terrigenous vitrinite. These two original samples were cut into small blocks and placed in an identical vacuum stainless vessel simultaneously. The pyrolysis was carried out at isothermal temperatures ranging from 250 to 550 °C for 48 h. The morphologic and reflectance changes of DOM in pyrolysis residues were studied and compared with previous pyrolysis results.

Adopting the combined use of optical reflectance and scanning electron microscope (SEM) techniques, the study shows that DOM in the shale components consists of solid bitumen (SB), bituminite, chitinozoans, a few graptolites, vitrinite-like particles (VLP), alginate and liptodetrinite. The reflectance sequence, from high to low, is zooclasts, VLP and SB. Their reflectances gradually increases as the pyrolysis temperature rises, although the rate of growth is slower than that of co-heating coal vitrinite. Notably, zooclasts yield significantly higher reflectance values than those of VLP and SB at each pyrolysis temperature. However, the VLP undergoes two distinct phases in reflectance development, despite gradual morphology changes similar to vitrinite. Based on evidence of the optical texture and organic pore evolution, we argue that a fair amount of VLP in studied Upper Ordovician shale does not consist of graptolite-type fragments. Consequently, the VLPR_o serves as a reliable indicator for assessing organic matter maturity in Ordovician shales with a reflectance value of less than 2.0%. This investigation enhances scholarly understandings of marine-derived DOM evolution issues, providing clearer correlations among reflectances of different DOM and reducing uncertainties in thermal maturity determination.