Xin Tang

Lacustrine shale oil has become an important target for increasing unconventional oil and gas reserves and production in China. However, compared with shallow to medium-depth shale oil systems, ultra-deep shale oil systems have experienced more complex burial evolution, diagenetic alteration, and fluid activities, and their large-scale enrichment mechanisms remain poorly understood. This study focuses on the ultra-deep shale oil system of the fourth member of the Paleogene Funing Formation (E₁f₄) in the Gaoyou Sag, Subei Basin. Core observations, mineralogical analyses, organic geochemical tests, scanning electron microscopy (SEM), pore structure characterization, and laser confocal microscopy were integrated to investigate shale lithofacies, organic matter characteristics, and shale oil occurrence mechanisms, and to identify the key factors controlling shale oil enrichment. The results show that five major lithofacies are developed in the E₁f₄, including laminated calcareous–dolomitic shale and laminated clay-rich mixed shale. Among them, carbonate-rich laminated lithofacies exhibit the highest organic matter abundance and the best reservoir quality. Laser confocal microscopy further reveals that heavy hydrocarbons are mainly retained within organic-rich clay laminae, whereas light hydrocarbons preferentially migrate into and accumulate within adjacent carbonate-rich and felsic laminae, forming an efficient source–reservoir association at the microscale. The integrated results indicate that abundant algal organic matter and thick high-quality source rocks provided sufficient hydrocarbon-generation potential. Differential source–reservoir associations controlled by laminated structures, together with multi-scale pore–fracture networks, promoted short-distance hydrocarbon migration and effective accumulation. In addition, thick mudstone roof and floor seals, a weak faulting background, and an overpressure system ensured long-term hydrocarbon preservation. This study reveals the enrichment mechanism of ultra-deep mixed shale oil in faulted lacustrine basins of eastern China, improves the understanding of lacustrine shale oil accumulation, and provides a theoretical basis for sweet spot evaluation and favorable area selection in similar basins.

Summary: The pore structure in shale oil reservoirs exerts a crucial control over hydrocarbon accumulation and migration. Taking medium-low maturity shale (R0 = 0.71–0.90%) from the Beibuwan Basin as the research object, we study thoroughly investigate the dynamic evolution of multiscale pore structures under high-temperature conditions (0–1,000°C) and their optimization mechanism for fluid percolation and transport efficiency. A comprehensive approach, combining gas adsorption experimental [low-temperature nitrogen (N2) adsorption and carbon dioxide (CO2) adsorption), scanning electron microscopy (SEM), X-ray diffraction (XRD), and high-pressure mercury injection experiments, along with quantitative indicators such as Shannon entropy and Lempel-Ziv complexity (LZC), was utilized to reveal the pore network reconstruction process from the perspectives of full pore size range and spatial heterogeneity. The results indicate that the evolution of shale pore structure exhibits two critical temperature thresholds: The first threshold (400–500°C) is where organic matter pyrolysis leads to a significant increase in pore volume and specific surface area, accompanied by an increase in Shannon entropy. However, the LZC value fluctuates minimally, and pores are mostly isolated and dispersed, resulting in limited improvement in connectivity. The second threshold (500–600°C) is where high-pressure mercury injection data show a sharp increase in permeability, and the LZC value reaches its peak, indicating that the pore-fracture network achieves efficient connectivity, forming a highly complex, interwoven multiscale percolation pathway. At higher temperatures (>600°C), mineral phase transformation further induces stress fractures, which synergistically interact with organic matter pyrolysis fractures (microcracks formed by the pyrolysis of organic matter and mineral phase transformation) to construct a pervasive permeable network within the matrix. This study reveals that the evolution of the shale pore network follows a dynamic process from “pore increment but limited connectivity” to “interconnected pore-fracture and efficient transport,” with the synergistic response between 500°C and 600°C being key to significant optimization of pore connectivity. This structural optimization enhances the pore-throat matching and effective fluid pathway development, providing a crucial micromechanistic basis for evaluating the fluid transport potential and enhanced hydrocarbon recovery in high-temperature shale reservoirs.

To address the challenges of conflicting macroscopic mechanical tests and the inability to reveal complex nanoscale mechanisms during deep shale thermal modification, this study comprehensively investigates the microstructural and nanomechanical evolution of Longmaxi shale under heat treatment. This involves the innovative combination of gas adsorption, atomic force microscope high-resolution mapping, and multi-level spatial statistical analysis to systematically elucidate the spatial evolution of the pore structure of shale, surface morphology, and nanomechanical properties. The findings reveal a unique hardening-softening-rehardening three-stage pattern: From 25-400 °C, the average reduced modulus increases due to dehydration; from 400-600°C, organic matter pyrolysis significantly decreases the modulus, with intense atomic force microscope topographic uplift; from 600-900 °C, the modulus slightly increases again due to structural collapse and pore network regeneration. Local indicators of spatial association analysis shows that this macro evolution stems from synergistic microscopic phase space reshaping. Crucially, the mesopore volume increases most significantly in the 400-500°C range, and it exhibits its most notable increase. Considering energy efficiency and feasibility, this study demonstrates for the first time that 400-500 °C is an ideal temperature window for effective organic matter pyrolysis and nanopore optimization from a nanoscale spatial distribution and geometric stability perspective. This work provides crucial micromechanical mechanism support and precise temperature guidance for deep shale thermal modification, significantly outperforming the temperature ranges from traditional macroscopic experiments and filling the gap in macro-nanoscale mechanical discrepancies and spatial feature analysis. Document Type: Original article Cited as: Chen, Q., Tang, X., Shi, Y., Zhang, R., Shang, F. Nanomechanics and pore structure evolution in organic-rich shale reservoirs during high-temperature treatment: A multi-scale analysis of microscopic stability. Advances in Geo-Energy Research, 2025, 17(3): 241-255. https://doi.org/10.46690/ager.2025.09.06

This study employs transformative nanoscale-spatial methodology, which reveals the complete thermal damage evolution mechanism of sandstone from 25 to 700 ◦C, fundamentally challenging the conventional linear degradation paradigm. Spatial autocorrelation analysis is innovatively applied to atomic force microscopy data, establishing that thermal damage follows a non-monotonic pathway governed by spatial heterogeneity evolution rather than simple progressive weakening. Four critical temperature thresholds are identified: 200 ◦C marks the initiation of localized damage through expansion of weak zones with 61.4% stiffness reduction; 400 ◦C features unexpected transient hardening due to clay mineral dehydroxylation and pore restructuring, reaching the peak adhesion force; 500 ◦C represents not only increased damage but also the thermal decomposition of the 400 ◦C hardened structure, evidenced by maximum spatial heterogeneity and negative adhesion; and the 700 ◦C following quartz phase transition establishes a completely reconfigured structure with a novel "harder-in-lower-areas" pattern. Crucially, this research demonstrates that the spatial clustering of mechanical properties evolves from balanced high-low zones at 200 ◦C to a percolating low-stiffness network at 500 ◦C, establishing spatial reorganization as the governing principle of thermal damage. The findings provide a quantitative framework that can accurately predict rock behavior in geothermal systems and underground energy storage applications. Document Type: Original article Cited as: Yan, H., Zhou, T., Zhou, X., Liu, X., Tang, X. Non-monotonic evolution and spatial reorganization mechanism of thermally induced micro-damage in sandstone. Advances in Geo-Energy Research, 2025, 17(2): 135-148. https://doi.org/10.46690/ager.2025.08.05

Image observation method is a key method for shale reservoir evaluation. At the micro scale, scanning electron microscope images can be used to accurately understand the structural characteristics of shale. Most of the current research is to artificially identify the microstructure of shale. This approach has subjective limitations and makes it difficult to process images in batches on a large scale. We take the scanning electron microscope image as the research object, and the shale deep learning theory as the research method to realize the intelligent identification of microscopic substances in the shale scanning electron microscope image. The results show that the improved deep learning model performs better than other deep learning models. The maximum values of Precision, Recall, mAP50 and mAP50-95 reached 0.94442, 0.91695, 0.9579 and 0.71547, respectively. The functions of the optimized Yolov8 model were integrated with SEM technology. In engineering practice, it can assist researchers to quickly locate object substances and obtain high-quality SEM images, effectively improving the efficiency and accuracy of reservoir evaluation. In addition, this technology has great potential for development, and it is expected to play an important role in expanding to a variety of fields such as medicine and materials science by changing the test object.

Pore structure is the main controlling factor for oil and gas reservoir storage and migration. Recent years, in-situ high-temperature modification of pore structure has become a common method to improve permeability and enhance shale oil and gas recovery efficiency. This study aims to investigate the expansion and variation patterns of micro-pores in medium-low maturity shale under different temperatures. The medium-low maturity shale samples from Heitianchi, Liangping District, Chongqing were selected for scanning electron microscopy (SEM) experiments and gas adsorption-desorption experiments to explore the optimal reservoir modification temperature and its effect. The changes in micro-pore structure and morphology of shale under different temperatures were qualitatively and quantitatively described. The results demonstrate that an optimal temperature threshold for shale pores between 250 and 300 °C exists, where the surface area and volume of shale pores undergo significant improvement. Furthermore, under high-temperature conditions, pore morphology becomes more irregular, greatly increasing pore opening and cracks, indicating that direct heating of shale reservoirs within the range of 250–300 °C can enhance fluidity of shale oil and connectivity within the reservoir space. It not only effectively improves reservoir permeability but also reduces costs while minimizing environmental pollution, providing some fundamental guidance for large-scale in-situ extraction of shale oil.

Summary: In the field of geological engineering, heat treatment techniques are crucial for optimizing the mechanical properties of rocks, particularly in soft formations and those with special geological conditions. These techniques significantly enhance the pore volume (PV) and optimize the overall performance of rocks. In the study, atomic force microscopy (AFM) and fluid injection experiments were used to analyze the PV, surface properties, and micromechanical behaviors of Niutitang shale under different temperature treatments. The results showed that the PV of shale significantly improved under temperature treatment at 400°C, especially the volume of pores 2–20 nm in size, and the projection area of a pore also expanded compared with that under ambient temperature. In addition, the geometric instability of the pore cross section is enhanced, and the reduced modulus reaches its minimum in the 400°C treated shale. The study reveals the threshold effect of temperature on the microstructure of shale provides a scientific basis for temperature control in the heat treatment process and offers theoretical guidance for optimizing the performance of rocks in engineering applications.