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Pore Structure and Fractal Characteristics in Unconventional Oil and Gas...
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Pore Structure and Fractal Characteristics in Unconventional Oil and Gas Reservoirs, 2nd Edition

A special issue of Fractal and Fractional (ISSN 2504-3110). This special issue belongs to the section "Engineering".

Deadline for manuscript submissions: closed (15 June 2026) | Viewed by 13690

Editors

Dr. Zine El Abiddine Fellah

E-Mail Website
Guest Editor
Laboratory of Mechanics and Acoustics, French National Centre for Scientific Research LMA, CNRS, UMR 7031, Centrale Marseille, Aix-Marseille University, CEDEX 20, F-13402 Marseille, France
Interests: acoustic propagation in porous materials; fractional calculus; time-domain modelling; direct and inverse problems; Biot model–equivalent fluid model; causality and Kramers–Kronig relations; experimental characterization (transmission, reflection, inverse problems; non-destructive testing; ultrasound 50–500 kHz; low-frequency measurements in pipe 20 Hz–3 kHz); ultrasonic propagation in human cancellous bone (direct and inverse scattering problems); inhomogeneous porous materials (wave-splitting approach); non-local effects (fractional derivatives in space domain); acoustic waves in fractal materials (non-integer dimensional space model, fractal dimension); acoustic waves in micropolar media (gyration/rotational waves); radiation function
Special Issues, Collections and Topics in MDPI journals
Dr. Jizhen Zhang

E-Mail Website
Guest Editor
Key Laboratory of Exploration Technologies for Oil and Gas Resources, Ministry of Education, Yangtze University, Wuhan 430100, China Laboratory of Reservoir Microstructure Evolution and Digital Characterization, Yangtze University, Wuhan 430100, China
Interests: pore structure; pore heterogeneity; complexity; fractal characteristics
Special Issues, Collections and Topics in MDPI journals
Dr. Quanzhong Guan

E-Mail Website
Guest Editor
Energy College, Chengdu University of Technology, Chengdu 610059, China
Interests: shale gas/oil; pore structure; pore heterogeneity; fractal characteristics; reservoir characteristics
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Fractals possess self-similar patterns repeated at different scales and spatially. Many applications have been found for fractals, not only because of their beauty, which has itself received attention, but also because their governing rules simplify complex features in nature. The length and volume of a fractal are related to its characteristic size. Researchers have used fractals to model transport properties of porous media. There has been a growing interest in using fractals for understanding the transport properties of tight formations. Researchers have employed fractals to capture the heterogeneity of stimulated volume, complex geometries of fractures, and change in apparent and relative permeabilities. Fractals have found applications in analyzing the topology of the pore space. Researchers have used fractals to model transport properties of porous media. There has been a growing interest in using fractals for understanding the transport properties of tight formations. The pore structure and their fractal characteristics can have a significant effect on the spatial distributions of the wetting and nonwetting phases, occurrence, enrichment, and flow migration of unconventional oil and gas, which play a significant role in the theoretical research and exploration and development deployment of unconventional oil and gas resources.

In this Special Issue, “Pore Structure and Fractal Characteristics in Unconventional Oil and Gas Reservoirs, 2nd Edition", we would like to solicit your innovative ideas and work regarding the investigation and application of fractal dimensions in geological and geophysical science in the form of original articles. In addition, your study could focus on any aspect of geological and geophysical science, such as geological and geophysical material properties, numerical analysis, experimental and theoretical verifications, etc. The purpose of this Special Issue is to promote the deeper and wider investigation and application of fractal theory in fields of geological and geophysical science. Please note that manuscripts are required to utilize fractal theory, and should include fractal analysis or fractal applications. The submitted manuscripts will be peer reviewed, and those accepted will be published in the open access journal Fractal and Fractional. The topics to be considered in this Special Issue include, but are not limited to, the following:

  • Earth science;
  • Microstructures of shale, tight sandstone and coal;
  • Geotechnical engineering;
  • Engineering geology;
  • Granular aggregate properties;
  • Modelling of cracking behavior;
  • The impact of fractal characteristics on reservoirs;
  • Fractal characteristics of fractures;
  • Experimental and theoretical study.

Dr. Zine El Abiddine Fellah
Dr. Jizhen Zhang
Dr. Quanzhong Guan
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 250 words) can be sent to the Editorial Office for assessment.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-anonymized peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Fractal and Fractional is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • pore structure
  • pore heterogeneity
  • complexity
  • fractal characteristics
  • microstructures
  • fractal cracks

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Published Papers (15 papers)

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Research

29 pages, 35292 KB  
Article
Multiscale Fractal Characterization of Pore Structure and Reservoir Quality Based on Deep-Learning-Assisted Pore Extraction in the Majiagou Tight Dolomite Gas Reservoir, Central Ordos Basin, China
by Xiaohong Deng, Congjun Feng, Xiaoping Gao, Jing Li, Bin Guan, Xinglei Song and Mengsi Sun
Fractal Fract. 2026, 10(8), 502; https://doi.org/10.3390/fractalfract10080502 - 23 Jul 2026
Viewed by 230
Abstract
Tight dolomite gas reservoirs are promising exploration targets, yet their evaluation is complicated by multiscale pore-throat heterogeneity and poor seepage connectivity. Here, high-pressure mercury intrusion (HPMI), nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), and deep-learning-assisted pore extraction were integrated to characterize the [...] Read more.
Tight dolomite gas reservoirs are promising exploration targets, yet their evaluation is complicated by multiscale pore-throat heterogeneity and poor seepage connectivity. Here, high-pressure mercury intrusion (HPMI), nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), and deep-learning-assisted pore extraction were integrated to characterize the pore-throat structure and fractal features of the Middle Ordovician Majiagou Formation in the Ordos Basin. The reservoir is dominated by diagenetic-origin pores, mainly intercrystalline and intragranular dissolution pores, together with microfractures, and can be classified into three types with progressively poorer connectivity and flow capacity. Type I reservoirs contain more regular pores, larger pore-throat systems, and better storage and seepage capacity; Type II reservoirs are intermediate, whereas Type III reservoirs exhibit complex pore morphology, isolated pore networks, poor petrophysical properties, and limited gas-flow potential. The corresponding fractal dimensions are weakly correlated but complementary: DSEM captures pore-boundary complexity, DHPMI reflects pore-throat architecture and capillary-pressure-controlled seepage pathways, and DNMR reflects multiscale movable-fluid distribution. Clay minerals, especially illite-rich mixed layers, further intensify pore-throat heterogeneity. Increasing fractal dimension is generally associated with higher displacement and median pressures, but poorer connectivity, porosity, permeability, movable-fluid content, and gas deliverability. These results provide a basis for the quantitative evaluation of multiscale pore systems and reservoir quality in tight dolomite gas reservoirs. Full article
(This article belongs to the Special Issue Pore Structure and Fractal Characteristics in Unconventional Oil and Gas Reservoirs, 2nd Edition)
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26 pages, 12931 KB  
Article
Calibrated Multi-Method Fractal Characterization of Full-Scale Pore Structure and Geological Controls in Deep Anthracite: Case Study from Daning–Jixian Block, Ordos Basin
by Bin Zhang, Ya Meng, Song Yang, Xiangting Wang, Dejie Zhou and Kun Zhao
Fractal Fract. 2026, 10(7), 443; https://doi.org/10.3390/fractalfract10070443 - 29 Jun 2026
Viewed by 268
Abstract
Deep coal reservoirs commonly exhibit strong multiscale heterogeneity, which directly affects coalbed methane (CBM) storage, diffusion, and flow. In this study, deep No. 8 coal samples from the Daning–Jixian block, Ordos Basin, were comprehensively and quantitatively characterized using low-pressure CO2 adsorption, low-temperature [...] Read more.
Deep coal reservoirs commonly exhibit strong multiscale heterogeneity, which directly affects coalbed methane (CBM) storage, diffusion, and flow. In this study, deep No. 8 coal samples from the Daning–Jixian block, Ordos Basin, were comprehensively and quantitatively characterized using low-pressure CO2 adsorption, low-temperature N2 adsorption, mercury intrusion porosimetry (MIP), and nuclear magnetic resonance (NMR). A method-constrained calibration framework was developed to assign reliable fractal dimensions to different pore-size intervals and to calculate a volume-weighted comprehensive fractal index. The scale-dependent pore structure was evaluated, and its relationships with coal quality, maceral composition, and maximum vitrinite reflectance (Ro,max) were analyzed. The results show that deep anthracite has a trimodal pore-size distribution, with micropores dominating both specific surface area and pore volume. Fractal behavior is strongly scale-dependent, and calibrated full-pore-size fractal dimensions provide a more reliable measure of reservoir heterogeneity than single-method interpretations. Pore development and heterogeneity are closely associated with coalification degree, coal quality, and maceral composition. Ash tends to inhibit pore development, whereas fixed carbon and vitrinite promote micropore development; inertinite mainly contributes to macropores and fractures. These findings provide a quantitative basis for evaluating pore-structure heterogeneity and optimizing deep CBM reservoir development. Full article
(This article belongs to the Special Issue Pore Structure and Fractal Characteristics in Unconventional Oil and Gas Reservoirs, 2nd Edition)
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26 pages, 19008 KB  
Article
Multifractal Characterization of Pore Structure in Different Members Tight Sandstones of the Triassic Yanchang Formation, Ordos Basin, China
by Yong Wang, Yan Zhu, Hengquan Li, Fangkai Liu, Hongzhou Chen, Zhikai Liang and Xixin Wang
Fractal Fract. 2026, 10(7), 425; https://doi.org/10.3390/fractalfract10070425 - 23 Jun 2026
Viewed by 271
Abstract
Tight oil reservoir quality and development effectiveness are highly dependent on microscopic pore structure characteristics and spatial heterogeneity. In this study, tight sandstones from the Chang 3, Chang 6, Chang 7, and Chang 8 members of the Triassic Yanchang Formation in the Xunyi [...] Read more.
Tight oil reservoir quality and development effectiveness are highly dependent on microscopic pore structure characteristics and spatial heterogeneity. In this study, tight sandstones from the Chang 3, Chang 6, Chang 7, and Chang 8 members of the Triassic Yanchang Formation in the Xunyi exploration area, southern Ordos Basin, were selected as research objects. By integrating X-ray diffraction (XRD), cast thin sections, scanning electron microscopy (SEM), high-pressure mercury injection (HPMI) experiments, and multifractal theory, the multi-scale heterogeneity characteristics of pore structures in different layers were quantitatively characterized. The response relationships between multifractal parameters, macroscopic physical properties, and pore size distributions were discussed, and the geological control mechanisms of sedimentation and diagenesis on heterogeneity were revealed. The results indicate that the sedimentary environment plays a fundamental role in controlling reservoir physical properties. The Chang 3 and Chang 8 members, deposited in underwater distributary channels, are dominated by primary and dissolution pores, with physical properties significantly superior to the gravity flow-deposited Chang 6 and Chang 7 members. Multifractal analysis shows that the Chang 3 member has the largest singularity spectrum width (Δα =1.943 ± 0.56) and heterogeneity index (Rd = 1.782 ± 0.99), reflecting its broadest pore size distribution, strongest heterogeneity, and significant intra-layer differences; while the pore structures from Chang 6 to Chang 8 are relatively stable, with the Chang 8 member exhibiting high spatial connectivity. This study demonstrates that the quantitative evaluation method based on multifractal theory can effectively identify microscopic structural differences in tight sandstones, providing a critical supporting basis for reservoir classification characterization and favorable layer selection in the Yanchang Formation of the Ordos Basin. Full article
(This article belongs to the Special Issue Pore Structure and Fractal Characteristics in Unconventional Oil and Gas Reservoirs, 2nd Edition)
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21 pages, 18316 KB  
Article
NMR and Multifractal Characterization of Pore Heterogeneity in Transitional-Marine Shales: A Case Study from the Longtan Formation, Sichuan Basin
by Longyi Wang, Xizhe Li, Ya’na Chen, Yuce Wang, Zan Hang, Nijun Qi, Wenxuan Yu, Sijie He, Liangji Jiang and Yuchuan Chen
Fractal Fract. 2026, 10(6), 417; https://doi.org/10.3390/fractalfract10060417 - 18 Jun 2026
Viewed by 339
Abstract
Transitional marine–continental shale reservoirs are typified by intricate pore architectures and pronounced heterogeneity; accurate characterization of their pore network and fluid mobility underpins reservoir appraisal and sweet-spot forecasting. Focusing on the Longtan Formation transitional shales in the Sichuan Basin, this study integrates NMR [...] Read more.
Transitional marine–continental shale reservoirs are typified by intricate pore architectures and pronounced heterogeneity; accurate characterization of their pore network and fluid mobility underpins reservoir appraisal and sweet-spot forecasting. Focusing on the Longtan Formation transitional shales in the Sichuan Basin, this study integrates NMR T2 spectrometry, geochemical–mineralogical assays and multifractal analysis to elucidate multi-scale heterogeneity of the pore framework and its governing mechanisms. Results reveal that the investigated shales are characterized by low porosity (0.46–7.43%) and high bound fluid saturation (66.77–97.28%). Multifractal spectral width (Δα) and degree of multifractality (ΔD) serve as robust metrics of pore heterogeneity, correlating closely with rock composition (e.g., TOC and clay content). By combining multifractal indices, mineralogical assemblage and fluid movability, the samples are classified into three reservoir archetypes, with Type I (weakly heterogeneous—high quality) identified as the prospective developmental sweet spot. This work provides a theoretical and methodological backbone for quality assessment and play-ranking of transitional marine–continental shale reservoirs. Full article
(This article belongs to the Special Issue Pore Structure and Fractal Characteristics in Unconventional Oil and Gas Reservoirs, 2nd Edition)
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23 pages, 5786 KB  
Article
Fractal Characteristics and Heterogeneity Evaluation of Shale Reservoirs Based on MIP and Gas Adsorption: A Case Study of Marine Shale in the Sichuan Basin
by Meng Wang, Shu Liu, Yuxi Wang, Xinan Yu, Jun Lang, Yulin Cheng, Xingming Duan and Jingjing Guo
Fractal Fract. 2026, 10(5), 349; https://doi.org/10.3390/fractalfract10050349 - 21 May 2026
Cited by 1 | Viewed by 553
Abstract
The deep marine shale of the Wufeng–Longmaxi (WF–LMX) Formation in the Sichuan Basin is characterized by laterally continuous thickness, high porosity, and significant gas content, making it a representative shale reservoir with considerable resource potential. This study investigates the heterogeneity of pore structures [...] Read more.
The deep marine shale of the Wufeng–Longmaxi (WF–LMX) Formation in the Sichuan Basin is characterized by laterally continuous thickness, high porosity, and significant gas content, making it a representative shale reservoir with considerable resource potential. This study investigates the heterogeneity of pore structures and their controlling factors using shale samples from three representative wells, based on low-temperature nitrogen adsorption and mercury intrusion data. The reservoir can be classified into three main lithofacies: mixed siliceous shale (MSS), clay-rich siliceous shale (CSS), and siliceous clay mixed shale (SMS). The results show that siliceous shales (MSS and CSS) exhibit higher total organic carbon and quartz contents, with more developed pore systems. Among them, the CSS exhibits the highest specific surface area and the largest mesopore and macropore volumes, indicating a greater development of larger pores and superior reservoir quality. All three shale facies exhibit clear single and multifractal characteristics. The average D1 and D2 values (fractal dimensions from nitrogen adsorption at P/P0 < 0.45 and >0.45, respectively) are higher than DHg, (fractal dimension from mercury intrusion), indicating greater pore-surface roughness than internal pore structure complexity and stronger heterogeneity in larger pores. The D(q)–q spectrum shows a left-wide/right-narrow pattern, whereas the α–f(α) spectrum exhibits the opposite trend. The branch-width ratios Skd and Ska (indices of pore-size distribution complexity and heterogeneity) are both <0.1, suggesting that heterogeneity is more pronounced in low-probability regions. Fractal and multifractal analyses reveal significant pore structure heterogeneity across different lithofacies, with CSS showing relatively more homogeneous pore structures, whereas MSS exhibits stronger heterogeneity and poorer connectivity. The heterogeneity of shale reservoirs is primarily controlled by pore development, especially micropores and mesopores, and is strongly influenced by total organic carbon and quartz content. Full article
(This article belongs to the Special Issue Pore Structure and Fractal Characteristics in Unconventional Oil and Gas Reservoirs, 2nd Edition)
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22 pages, 5570 KB  
Article
Macroscopic Characterization and Microscopic Pore Structure of Permian Shale Reservoirs in Hunan–Guizhou–Guangxi Basin: Insights from NMRC, Fractal and Image-J Methods
by Yue Sun, Yuqiang Jiang, Miao Li, Xiangfeng Wei, Jingyu Hao and Yifan Gu
Fractal Fract. 2026, 10(5), 279; https://doi.org/10.3390/fractalfract10050279 - 23 Apr 2026
Viewed by 536
Abstract
Permian shale is the largest and most promising shale gas exploration target in southern China after Silurian shale. The fine evaluation of shale reservoirs is a prerequisite for large-scale exploration and development. Based on the fractal method, this study, through the combined technology [...] Read more.
Permian shale is the largest and most promising shale gas exploration target in southern China after Silurian shale. The fine evaluation of shale reservoirs is a prerequisite for large-scale exploration and development. Based on the fractal method, this study, through the combined technology of nuclear magnetic resonance cryoporometry (NMRC) and Image recognition software (Image-J), clarifies the pore size distribution of Permian shale in the HGG Basin. The purpose of this study is to characterize the macroscopic parameters of Permian shale and reveal the level of reservoir space development in Permian shale. The controlling factors of porosity and pore structure are demonstrated. It is suggested that Permian shales in the HGG Basin have organic carbon contents similar to marine shales. In the favorable interval of the Dalong Formation, the average organic carbon content is comparable to that of the LMX pay zone. The lower Longtan shales have the highest organic carbon and the greatest gas generation potential, followed by the Dalong shales. TOC is the primary control on porosity in the lower Longtan and Dalong formations, whereas clay minerals dominate the control in the upper Longtan. Abundant pores between grains and between layers within clay minerals account for most of the porosity in Upper Longtan shale. In the lower Longtan and Dalong formations, organic pores are pervasive, explaining the difference in the dominant controls on porosity between these intervals. Clay minerals are a key control on the development of Permian shale reservoirs. The fractal dimension of adsorption pores (DA) has no clear relationship with the total clay content, is negatively correlated with the illite content, and shows no clear relationship with the chlorite content. In contrast, the fractal dimension of flow pores (DS) shows a weak positive correlation with the total clay content, a clear positive correlation with the illite content, and a negative correlation with the chlorite content. When illite interacts with water, it tends to break down and plug pores, an effect that is especially pronounced in the smallest pores hosted by organic matter; this accounts for the negative correlation between DA and the illite content. In larger, flow-bearing pores, disintegrated illite roughens otherwise smooth walls between and within grains, increasing structural complexity and raising DS. By contrast, reactions between chlorite and pore fluids tend to smooth the walls of flow pores, reducing structural complexity and lowering DS. Full article
(This article belongs to the Special Issue Pore Structure and Fractal Characteristics in Unconventional Oil and Gas Reservoirs, 2nd Edition)
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39 pages, 11584 KB  
Article
Pore Structure Characterization and Fractal Analysis of Lacustrine Shales: Integrating N2 Adsorption, Mercury Intrusion, and Deep Learning-Assisted FIB–SEM 3D Pore Surface Point Cloud Reconstruction
by Guanlin Li, Bixiao Xin and Zongmin Li
Fractal Fract. 2026, 10(3), 179; https://doi.org/10.3390/fractalfract10030179 - 11 Mar 2026
Cited by 3 | Viewed by 1212
Abstract
Lacustrine shales are key targets for shale oil exploration, yet the quantitative characterization of their complex and heterogeneous pore systems remains a significant challenge, constraining effective reservoir evaluation and development. This study investigates lacustrine shales from the Second Member of the Kongdian Formation [...] Read more.
Lacustrine shales are key targets for shale oil exploration, yet the quantitative characterization of their complex and heterogeneous pore systems remains a significant challenge, constraining effective reservoir evaluation and development. This study investigates lacustrine shales from the Second Member of the Kongdian Formation by integrating N2 adsorption, mercury intrusion porosimetry, and focused ion beam scanning electron microscopy with fractal analysis. A Mamba-based deep learning model was applied to improve two-dimensional (2D) pore segmentation, and three-dimensional (3D) pore surface point clouds were reconstructed for 3D surface fractal characterization to reduce artifacts associated with conventional 3D reconstruction. The results indicate that the pore system is dominated by inorganic pores, mainly irregular interparticle pores and dissolution pores, while organic pores are scarce. Pore sizes are predominantly concentrated in the range of 5 to 200 nm. Adsorption-derived fractal dimensions exhibit systematic lithofacies differences, with D1 and D2 averaging around 2.47 and 2.56, respectively. These trends are consistent with the 3D pore surface fractal dimension derived from pore surface point clouds (mean 2.48), which supplements the bulk statistical results with direct geometric quantification of surface roughness. The heterogeneity of the pore system is influenced by the coupled effects of mineral composition, organic matter content, and diagenesis. Specifically, the enrichment of clay minerals and dolomite increases the irregularity of pore morphology and results in higher fractal dimensions. In contrast, samples enriched in feldspars and calcite are supported by a rigid granular framework that corresponds to lower 3D surface complexity. Ultimately, these fractal dimensions effectively quantify pore network complexity and reservoir heterogeneity in the Kong 2 shales, offering quantitative support for reservoir characterization and lacustrine shale oil exploration. Full article
(This article belongs to the Special Issue Pore Structure and Fractal Characteristics in Unconventional Oil and Gas Reservoirs, 2nd Edition)
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24 pages, 5671 KB  
Article
Temperature-Dependent Pore Size Redistribution and Fractal Complexity in Low-Maturity Shale: Implications for In Situ Conversion
by Qiansong Guo, Xianda Sun, Yuchen Wang, Chengwu Xu, Wei Li and Changxin He
Fractal Fract. 2026, 10(2), 132; https://doi.org/10.3390/fractalfract10020132 - 22 Feb 2026
Cited by 1 | Viewed by 695
Abstract
Low-maturity shale is a prime target for in situ conversion (ICP), yet heating window selection remains largely empirical because pore evolution and hydrocarbon generation are rarely quantified in tandem. Nenjiang Formation shale from the Songliao Basin (TOC = 8.91%; Ro,max = 0.54%) [...] Read more.
Low-maturity shale is a prime target for in situ conversion (ICP), yet heating window selection remains largely empirical because pore evolution and hydrocarbon generation are rarely quantified in tandem. Nenjiang Formation shale from the Songliao Basin (TOC = 8.91%; Ro,max = 0.54%) was subjected to closed-system pyrolysis at 300–500 °C (20 °C h−1; 72 h per step). Released oil and gas and residual chloroform-extractable bitumen (“A”) were quantified, and pore evolution was characterized using 2D low-field NMR, SEM, micro-CT, and low-pressure N2 adsorption. Fractal dimensions (Ds and Dp) were derived from Frenkel–Halsey–Hill (FHH) fitting. Oil yield and bitumen “A” increased sharply above 350 °C and peaked at 375 °C, whereas gas generation accelerated above 400 °C and continued to increase to 500 °C. NMR indicates a temperature-dependent shift in retained hydrocarbons toward weaker confinement and higher mobility, with enhanced expulsion/mobility signals near 375 °C. At 375 °C, BJH pore volume and average pore diameter reached maxima (0.0675 cm3 g−1 and 15.36 nm), while Ds and Dp reached minima (2.343 and 2.444). The coincidence of peak oil expulsion with minimum fractal complexity suggests that FHH-based fractal indices provide a quantitative metric for comparing ICP heating windows in low-maturity shale. Full article
(This article belongs to the Special Issue Pore Structure and Fractal Characteristics in Unconventional Oil and Gas Reservoirs, 2nd Edition)
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18 pages, 6208 KB  
Article
Fractal Characteristics of Pore Structure in Lacustrine Shale Oil Reservoirs and Controlling Factors of Oil Occurrence State: A Case Study of Da’anzhai Member, Sichuan Basin
by Miao Li, Xueying Yan, Yuqiang Jiang, Hongzhan Zhuang and Zhanlei Wang
Fractal Fract. 2026, 10(2), 111; https://doi.org/10.3390/fractalfract10020111 - 5 Feb 2026
Viewed by 545
Abstract
The Jurassic lacustrine oil shale in southwest China has become a primary production layer due to its high yield and substantial reserves. However, influenced by the lacustrine environment, the vertical profile of the lacustrine shale reservoir shows alternating deposits of shale and carbonate [...] Read more.
The Jurassic lacustrine oil shale in southwest China has become a primary production layer due to its high yield and substantial reserves. However, influenced by the lacustrine environment, the vertical profile of the lacustrine shale reservoir shows alternating deposits of shale and carbonate rock. This complex lithological combination results in significant heterogeneity in reservoir types, reservoir distribution, and internal structure. Currently, research on micro-pore structure and hydrocarbon storage mechanisms in lacustrine shales is insufficient, necessitating the elucidation of their micro-characteristics to support future exploration and development. This research focuses on the Da’anzhai Member of Jurassic Ziliujing Formation. Various techniques—including organic geochemical analysis, X-ray diffraction, physical property testing, gradient centrifugation, and gradient drying NMR monitoring—were employed to investigate the micro-pore structure and fluid storage mechanisms of the lacustrine shale reservoir. The following insights were gained from this research. The organic matter pores (OMP) and inorganic pores (IP) developed within the Da’anzhai lacustrine shale reservoir together create the storage space for shale oil, while micro-fractures further enhance the reservoir’s storage capacity and flow performance. Lacustrine shale oil exists in three storage states: mobile oil, bound oil, and adsorbed oil. Mobile oil is primarily located within the micro-fractures and large pores (greater than 350 nm) of the shale reservoir and is the main target for industrial extraction. Bound oil is mainly found in the meso-pores, micropores, and narrow pore structures between rock grains (30 nm to 350 nm), and, theoretically, could potentially be developed through engineering methods such as hydraulic fracturing. Adsorbed oil, due to its close binding with organic matter and clay mineral surfaces, is difficult to release effectively using conventional techniques. The OM abundance, the mineral composition of lacustrine shale, and the pore structure all influence the storage states of shale oil. While a high TOC value increases the amount of mobile oil, the strong adsorption properties of kerogen and organic matter lead to the accumulation of adsorbed oil, which inhibits oil flow. Clay minerals further restrict oil flow by enhancing adsorption, while brittle minerals facilitate the movement of mobile oil by expanding pore space. Based on fractal geometry theory and multi-scale testing results, the large pores in the Da’anzhai lacustrine shale have a high fractal dimension and exhibit complex shapes. However, as pore complexity increases, the amount of adsorbed oil rises significantly, which in turn reduces the proportion of movable oil. Full article
(This article belongs to the Special Issue Pore Structure and Fractal Characteristics in Unconventional Oil and Gas Reservoirs, 2nd Edition)
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27 pages, 6279 KB  
Article
Sedimentary Paleo-Environment and Reservoir Heterogeneity of Shale Revealed by Fractal Analysis in the Inter-Platform Basin: A Case Study of Permian Shale from Outcrop of Nanpanjiang Basin
by Meng Wang, Xinan Yu, Shu Liu, Yulin Cheng, Jingjing Guo, Zhanlei Wang and Xingming Duan
Fractal Fract. 2025, 9(12), 795; https://doi.org/10.3390/fractalfract9120795 - 4 Dec 2025
Cited by 13 | Viewed by 933
Abstract
The Upper Permian marine shale of the inter-platform basin in the Nanpanjiang Basin are rich in organic matter, widely distributed, and relatively thick, indicating abundant resource potential for hydrocarbon exploration. To clarify the sedimentary condition and the variability of reservoir properties, the paleo-environment [...] Read more.
The Upper Permian marine shale of the inter-platform basin in the Nanpanjiang Basin are rich in organic matter, widely distributed, and relatively thick, indicating abundant resource potential for hydrocarbon exploration. To clarify the sedimentary condition and the variability of reservoir properties, the paleo-environment was reconstructed by using geochemical, mineralogical, rock-property, and pore-structure data. Building on a lithofacies classification, the development patterns of different shale lithofacies were revealed. Reservoir characteristics among lithofacies were compared using scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), and low-temperature Nuclear Magnetic Resonance Cryoporometry (NMRC) experiments. A fractal analysis was performed based on NMR and NMRC data to quantify pore-scale heterogeneity, calculate fractal dimensions (D1, D2, and Dc), and evaluate the complexity of pore systems across lithofacies. Correlation analysis and redundancy analysis were applied to further explore the controlling factors of reservoir heterogeneity. The results showed that organic-rich shale in the Permian Linghao Formation occurred mainly in the 1st Member, with average total organic carbon (TOC) content of 2.57%, and the lower part of the 3rd Member (average TOC content 2.88%). In the 1st Member, high-carbon shale was deposited under humid conditions with intense weathering, abundant fine-grained clastic input from basin margins, strongly reducing (anoxic) bottom waters, vigorous phosphorus recycling, and moderate to low primary productivity. Using TOC and mineral composition, seven shale lithofacies were identified in the Linghao Formation, and their development patterns were established based on depositional paleo-environment characteristics and evolution. In the 1st Member, organic-rich shale was dominated by mixed lithofacies with moderate to high TOC. The paleo-environment exerted a primary control on reservoir properties, gas content, pore structure, and heterogeneity. The high-carbon lithofacies had the most favorable rock properties—higher porosity, greater pore volume, and higher gas content—and contained a larger proportion of well-developed organic pores. Fractal analysis revealed that seepage pores exhibited greater structural complexity than adsorption-related pores, with the high-carbon lithofacies showing the highest overall fractal dimensions and thus the strongest heterogeneity. Across the formation, higher clay content and TOC were the primary drivers of increased pore-scale heterogeneity, whereas greater feldspar and quartz contents tended to diminish it. Carbonates exerted a minor effect. Heterogeneity in adsorption pores exerted the strongest influence on differences among lithofacies. These results highlighted the utility of fractal analysis in quantitatively linking shale mineralogy and organic content to multiscale heterogeneity in inter-platform basin settings. Full article
(This article belongs to the Special Issue Pore Structure and Fractal Characteristics in Unconventional Oil and Gas Reservoirs, 2nd Edition)
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25 pages, 5662 KB  
Article
From Compaction to Porosity Reconstruction: Fractal Evolution and Heterogeneity of the Qingshankou Shale Reservoir in the Songliao Basin
by Qi Yao, Chengwu Xu and Hongyu Li
Fractal Fract. 2025, 9(12), 777; https://doi.org/10.3390/fractalfract9120777 - 28 Nov 2025
Cited by 1 | Viewed by 924
Abstract
The Qingshankou Formation shale in the Changling Sag of the Songliao Basin represents a typical lacustrine pure-shale reservoir, characterized by high organic matter abundance, high maturity, high clay mineral content, and strong heterogeneity. To elucidate the pore structure and heterogeneity of this shale, [...] Read more.
The Qingshankou Formation shale in the Changling Sag of the Songliao Basin represents a typical lacustrine pure-shale reservoir, characterized by high organic matter abundance, high maturity, high clay mineral content, and strong heterogeneity. To elucidate the pore structure and heterogeneity of this shale, a comprehensive suite of analytical techniques—including X-ray diffraction (XRD), scanning electron microscopy (SEM), high-pressure mercury intrusion porosimetry (MICP), and low-temperature nitrogen adsorption—was employed to investigate its pore types and fractal characteristics systematically. On this basis, lithofacies classification and FHH fractal modeling were conducted to quantitatively assess the complexity of pore–throat structures and their influence on reservoir properties. The results indicate that shale-dominated lithofacies (Types A–C) exhibit higher surface fractal dimensions (D1 = 2.51–2.58) and structural fractal dimensions (D2 = 2.73–2.81), corresponding to low porosity, low permeability, and high displacement pressure. In contrast, carbonate- and clastic-dominated lithofacies (Types D–G) display lower fractal dimensions, suggesting more regular pore–throat structures and better connectivity. Overall, both D1 and D2 show negative correlations with porosity and permeability but positive correlations with displacement pressure, and are negatively correlated with TOC content, reflecting the intrinsic coupling among pore–throat complexity, reservoir capacity, and organic matter abundance. These findings reveal that the Qingshankou shale reservoir has undergone a geometric evolutionary pathway of “shale compaction → siltstone transition → carbonate porosity reconstruction.” The fractal dimensions effectively characterize the reservoir heterogeneity and pore–throat connectivity, providing a new theoretical basis for the quantitative characterization, classification, and potential prediction of continental shale oil reservoirs. Full article
(This article belongs to the Special Issue Pore Structure and Fractal Characteristics in Unconventional Oil and Gas Reservoirs, 2nd Edition)
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23 pages, 18709 KB  
Article
Fractal Characteristics and Controlling Factors of Pore-Throat Structure in Tight Sandstone Reservoirs: A Case Study of the 2nd Member of the Kongdian Formation in the Nanpi Slope, Cangdong Sag, Bohai Bay Basin
by Yong Zhou, Guomeng Han, Yanxin Liu, Liangang Mou, Ke Wang, Peng Yang and Kexin Yan
Fractal Fract. 2025, 9(9), 608; https://doi.org/10.3390/fractalfract9090608 - 20 Sep 2025
Cited by 5 | Viewed by 1208
Abstract
Tight sandstone reservoirs generally exhibit poor physical properties and characterization of microscopic pore structure is crucial for evaluating reservoir quality and fluid flow behavior. Fractal dimension provides an effective means to quantify the complexity and heterogeneity of pore structures in such reservoirs. This [...] Read more.
Tight sandstone reservoirs generally exhibit poor physical properties and characterization of microscopic pore structure is crucial for evaluating reservoir quality and fluid flow behavior. Fractal dimension provides an effective means to quantify the complexity and heterogeneity of pore structures in such reservoirs. This study investigates tight sandstone reservoirs of the Kongdian Formation in the Nanpi Slope, Cangdong Sag, using cast thin sections, scanning electron microscopy (SEM), high-pressure mercury injection (HPMI), and constant-rate mercury injection (CRMI) experiments. We establish a full-range fractal model to characterize pore-throat distributions and elucidate the correlation between fractal dimensions and reservoir properties, alongside factors influencing pore-structure heterogeneity. Key findings include that (1) pore types are predominantly residual intergranular pores, intergranular dissolution pores, and clay mineral intercrystalline pores, with throats primarily consisting of sheet-like and curved sheet-like types, exhibiting strong pore-structure heterogeneity; (2) full-range fractal dimensions D1, D2 and D4 effectively characterize the heterogeneity of pore structure, where higher D1 and D2 values correlate with increased macro–mega pore and micro-fine throat abundance, respectively, indicating enhanced pore connectivity and superior flow capacity, while elevated D4 reflects greater nano throat complexity, degrading reservoir properties and impeding hydrocarbon flow; (3) compared to conventional methods splicing HPMI and CRMI data at 0.12 μm, the fractal-derived integration point more accurately resolves full-range pore-throat distributions, revealing significant disparities in pore-throat size populations; (4) the fractal dimensions D1, D2, and D4 are collectively governed by clay mineral content, average throat radius, displacement pressure, and tortuosity. Full article
(This article belongs to the Special Issue Pore Structure and Fractal Characteristics in Unconventional Oil and Gas Reservoirs, 2nd Edition)
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27 pages, 16665 KB  
Article
Microscopic Pore Structure Heterogeneity on the Breakthrough Pressure and Sealing Capacity of Carbonate Rocks: Insight from Monofractal and Multifractal Investigation
by Siqi Ouyang, Yiqian Qu, Yuting Cheng, Yupeng Wu and Xiuxiang Lü
Fractal Fract. 2025, 9(9), 589; https://doi.org/10.3390/fractalfract9090589 - 8 Sep 2025
Cited by 4 | Viewed by 1541
Abstract
Reservoirs and caprocks overlap with each other in heterogeneous carbonate rocks. The sealing capacity of caprocks and their controlling factors are not clear, which restricts the prediction, exploration, and development of carbonate hydrocarbon reservoirs. We selected core samples from the Ordovician reservoirs and [...] Read more.
Reservoirs and caprocks overlap with each other in heterogeneous carbonate rocks. The sealing capacity of caprocks and their controlling factors are not clear, which restricts the prediction, exploration, and development of carbonate hydrocarbon reservoirs. We selected core samples from the Ordovician reservoirs and caprocks in the Tarim Basin, China, for scanning electron microscopy, thin section, breakthrough pressure (BP), high-pressure mercury intrusion porosimetry (HMIP), and nitrogen adsorption method (N2GA). The experimental results show that the reservoir and caprock can be distinguished by BP. The BP of the reservoir is less than 3.0 MPa, and the BP of the caprock is less than 3.0 Mpa. We analyzed the heterogeneity characteristics and differences in reservoirs and caprocks with different lithologies from the perspectives of monofractal and multifractal. The results indicate that the differences in pore structure of grainstone, dolomite, and micrite/argillaceous limestone result in significant heterogeneity differences between samples. The correlation analysis between the fractal parameters and BP indicates that the characteristics of reservoir microporous structures have a decisive impact on BP (correlation coefficient > 0.7). The pore structure of the carbonate reservoir–caprock system exhibits self-similarity. The heterogeneity of the caprock has no significant control effect on BP (correlation coefficient < 0.3), while the higher the heterogeneity of the reservoir, the greater the BP. The sealing capacity of the caprock depends on the heterogeneity differences in pore types and pore structures between the reservoirs and caprocks. When both the reservoir and the caprock are grainstone, the micropores in the reservoirs and caprocks are dispersed but evenly distributed, and little heterogeneous differences can achieve sealing. When the lithology of reservoirs and caprocks is different, the enhancement of heterogeneity differences in micropores will improve the sealing capacity of the caprock. In summary, fractal dimension is an effective method for studying the heterogeneous structure and sealing capacity of pore–throat in carbonate caprocks. This study proposes a new perspective that the difference between the heterogeneity of micropore structures of reservoirs and caprocks affects the sealing capacity of carbonate rocks, and provides a new explanation and model for the sealing mode of carbonate rock caprocks. Full article
(This article belongs to the Special Issue Pore Structure and Fractal Characteristics in Unconventional Oil and Gas Reservoirs, 2nd Edition)
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30 pages, 11585 KB  
Article
Multifractal Characterization of Marine Shale Pore Structure Alteration Induced by Supercritical CO2–Water–Rock Interaction
by Haonan Wei, Yi Du, Changqing Fu, Gaoqiang Fu, Yingfang Zhou, Jinfeng Ma, Zhenliang Wang, Zhejun Pan and Wei Gao
Fractal Fract. 2025, 9(9), 582; https://doi.org/10.3390/fractalfract9090582 - 2 Sep 2025
Cited by 5 | Viewed by 1744
Abstract
Supercritical CO2 (ScCO2) injection has emerged as a promising method to enhance shale gas recovery while simultaneously achieving CO2 sequestration. This research investigates how ScCO2 interacts with water and shale rock, altering the pore structure characteristics of shale [...] Read more.
Supercritical CO2 (ScCO2) injection has emerged as a promising method to enhance shale gas recovery while simultaneously achieving CO2 sequestration. This research investigates how ScCO2 interacts with water and shale rock, altering the pore structure characteristics of shale reservoirs. The study examines shale samples from three marine shale formations in southern China under varying thermal and pressure regimes simulating burial conditions at 1000 m (45 °C and 10 MPa) and 2000 m (80 °C and 20 MPa). The research employs multiple analytical techniques including XRD for mineral composition analysis, MICP, N2GA, and CO2GA for comprehensive pore characterization, FE–SEM for visual observation of mineral and pore changes, and multifractal theory to analyze pore structure heterogeneity and connectivity. Key findings indicate that ScCO2–water–shale interactions lead to dissolution of minerals such as kaolinite, calcite, dolomite, and chlorite, and as the reaction proceeds, substantial secondary mineral precipitation occurs, with these changes being more pronounced under 2000 m simulation conditions. Mineral dissolution and precipitation cause changes in pore structure parameters of different pore sizes, with macropores showing increased PV and decreased SSA, mesopores showing decreased PV and SSA, and micropores showing insignificant changes. Moreover, mineral precipitation effects are stronger than dissolution effects. These changes in pore structure parameters lead to alterations in multifractal parameters, with mineral precipitation reducing pore connectivity and consequently enhancing pore heterogeneity. Correlation analysis further revealed that H and D−10–D10 exhibit a significant negative correlation, confirming that reduced connectivity corresponds to stronger heterogeneity, while mineral composition strongly controls the multifractal responses of macropores and mesopores, with micropores mainly undergoing morphological changes. However, these changes in micropores are mainly manifested as modifications of internal space. Siliceous shale samples exhibit stronger structural stability compared to argillaceous shale, which is attributed to the mechanical strength of the quartz framework. By integrating multifractal theory with multi–scale pore characterization, this study achieves a unified quantification of shale pore heterogeneity and connectivity under ScCO2–water interactions at reservoir–representative pressure–temperature conditions. This novelty not only advances the methodological framework but also provides critical support for understanding CO2–enhanced shale gas recovery mechanisms and CO2 geological sequestration in depleted shale gas reservoirs, highlighting the complex coupling between geochemical reactions and pore structure evolution. Full article
(This article belongs to the Special Issue Pore Structure and Fractal Characteristics in Unconventional Oil and Gas Reservoirs, 2nd Edition)
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20 pages, 11478 KB  
Article
Pore Evolution and Fractal Characteristics of Marine Shale: A Case Study of the Silurian Longmaxi Formation Shale in the Sichuan Basin
by Hongzhan Zhuang, Yuqiang Jiang, Quanzhong Guan, Xingping Yin and Yifan Gu
Fractal Fract. 2025, 9(8), 492; https://doi.org/10.3390/fractalfract9080492 - 28 Jul 2025
Cited by 7 | Viewed by 1323
Abstract
The Silurian marine shale in the Sichuan Basin is currently the main reservoir for shale gas reserves and production in China. This study investigates the reservoir evolution of the Silurian marine shale based on fractal dimension, quantifying the complexity and heterogeneity of the [...] Read more.
The Silurian marine shale in the Sichuan Basin is currently the main reservoir for shale gas reserves and production in China. This study investigates the reservoir evolution of the Silurian marine shale based on fractal dimension, quantifying the complexity and heterogeneity of the shale’s pore structure. Physical simulation experiments were conducted on field-collected shale samples, revealing the evolution of total organic carbon, mineral composition, porosity, and micro-fractures. The fractal dimension of shale pore was characterized using the Frenkel–Halsey–Hill and capillary bundle models. The relationships among shale components, porosity, and fractal dimensions were investigated through a correlation analysis and a principal component analysis. A comprehensive evolution model for porosity and micro-fractures was established. The evolution of mineral composition indicates a gradual increase in quartz content, accompanied by a decline in clay, feldspar, and carbonate minerals. The thermal evolution of organic matter is characterized by the formation of organic pores and shrinkage fractures on the surface of kerogen. Retained hydrocarbons undergo cracking in the late stages of thermal evolution, resulting in the formation of numerous nanometer-scale organic pores. The evolution of inorganic minerals is represented by compaction, dissolution, and the transformation of clay minerals. Throughout the simulation, porosity evolution exhibited distinct stages of rapid decline, notable increase, and relative stabilization. Both pore volume and specific surface area exhibit a trend of decreasing initially and then increasing during thermal evolution. However, pore volume slowly decreases after reaching its peak in the late overmature stage. Fractal dimensions derived from the Frenkel–Halsey–Hill model indicate that the surface roughness of pores (D1) in organic-rich shale is generally lower than the complexity of their internal structures (D2) across different maturity levels. Additionally, the average fractal dimension calculated based on the capillary bundle model is higher, suggesting that larger pores exhibit more complex structures. The correlation matrix indicates a co-evolution relationship between shale components and pore structure. Principal component analysis results show a close relationship between the porosity of inorganic pores, microfractures, and fractal dimension D2. The porosity of organic pores, the pore volume and specific surface area of the main pore size are closely related to fractal dimension D1. D1 serves as an indicator of pore development extent and characterizes the changes in components that are “consumed” or “generated” during the evolution process. Based on mineral composition, fractal dimensions, and pore structure evolution, a comprehensive model describing the evolution of pores and fractal dimensions in organic-rich shale was established. Full article
(This article belongs to the Special Issue Pore Structure and Fractal Characteristics in Unconventional Oil and Gas Reservoirs, 2nd Edition)
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