Structure and Fractal Characteristics of Organic Matter Pores in Wufeng–Lower Longmaxi Formations in Southern Sichuan Basin, China
Abstract
1. Introduction
2. Geologic Setting and Well Description
3. Samples and Analytical Methods
3.1. Samples
3.2. TOC Content and Mineralogy
3.3. Field Emission-Scanning Electron Microscopy (FE-SEM) for Organic Matter Pore Analysis
3.4. Low-Pressure N2 Gas Adsorption for OM Extraction
3.5. Fractal Theory
4. Results
4.1. TOC Content and Mineral Composition
4.2. FE-SEM Observation and Analyses
4.3. Pore Size Distribution, Pore Volume, and Special Surface Area (SSA)
4.4. Fractal Analysis
5. Discussion
5.1. Effect of Shale Compositions on Organic Matter Pore Structure
5.2. Effect of Diagenesis on Organic Matter Pore Structure
5.3. Effect of Tectonism on Organic Matter Pore Structure
5.4. Relationship Between Fractal Characteristics of Organic Matter Pores and Shale Gas Occurrence
6. Conclusions
Author Contributions
Funding
Data Availability Statement
Conflicts of Interest
References
- Zou, C.; Dong, D.; Wang, Y.; Li, X.; Huang, J.; Wang, S.; Guan, Q.; Zhang, C.; Wang, H.; Liu, H.; et al. Shale Gas in China: Characteristics, Challenges and Prospects (II). Shiyou Kantan Yu Kaifa/Pet. Explor. Dev. 2016, 43, 166–178. [Google Scholar] [CrossRef]
- Dong, D.; Liang, F.; Guan, Q.; Jiang, Y.; Zhou, S.; Yu, R.; Gu, Y.; Zhang, S.; Qi, L.; Liu, Y. Development Model and Identification of Evaluation Technology for Wufeng Formation–Longmaxi Formation Quality Shale Gas Reservoirs in the Sichuan Basin. Nat. Gas Ind. B 2023, 10, 165–182. [Google Scholar] [CrossRef]
- WANG jian; GUO Qiulin; ZHAO Chenlei; WANG Yuman; YU Jingdu; LIU Zhuangxiaoxue; CHEN Ningsheng Potentials and Prospects of Shale Oil-Gas Resources in Major Basins of China. Acta Pet. Sin. 2023, 44, 2033–2044.
- Zhang, J.; Li, Z.; Wang, D.; Xu, L.; Li, Z.; Niu, J.; Chen, L.; Sun, Y.; Li, Q.; Yang, Z.; et al. Shale Gas Accumulation Patterns in China. Nat. Gas Ind. B 2023, 10, 14–31. [Google Scholar] [CrossRef]
- Loucks, R.G.; Reed, R.M.; Ruppel, S.C.; Hammes, U. Spectrum of Pore Types and Networks in Mudrocks and a Descriptive Classification for Matrix-Related Mudrock Pores. Am. Assoc. Pet. Geol. Bull. 2012, 96, 1071–1098. [Google Scholar] [CrossRef]
- Curtis, J.B. Fractured Shale-Gas Systems. Am. Assoc. Pet. Geol. Bull. 2002, 86, 1921–1938. [Google Scholar] [CrossRef]
- Sun, L.; Fu, D.; Wu, Y.; Wang, Z. The Current Study and Development of Organic Matter Pores in Organic-Rich Shale. Arab. J. Geosci. 2022, 15, 62. [Google Scholar] [CrossRef]
- Bai, B.; Wang, G.; Shang, F.; Zhou, H.; Liu, Y.; Shi, Z. Effects of SEM Image Resolution on Organic Pore Structure Estimation of Shale Reservoirs. Gas Sci. Eng. 2024, 121, 205170. [Google Scholar] [CrossRef]
- Chen, Z.; Song, Y.; Jiang, Z.; Liu, S.; Li, Z.; Shi, D.; Yang, W.; Yang, Y.; Song, J.; Gao, F.; et al. Identification of Organic Matter Components and Organic Pore Characteristics of Marine Shale: A Case Study of Wufeng-Longmaxi Shale in Southern Sichuan Basin, China. Mar. Pet. Geol. 2019, 109, 56–69. [Google Scholar] [CrossRef]
- Guan, Q.; Lü, X.; Dong, D.; Cai, X. Origin and Significance of Organic-Matter Pores in Upper Ordovician Wufeng-Lower Silurian Longmaxi Mudstones, Sichuan Basin. J. Pet. Sci. Eng. 2019, 176, 554–561. [Google Scholar] [CrossRef]
- Katz, B.J.; Arango, I. Organic Porosity: A Geochemist’ s View of the Current State of Understanding. Org. Geochem. 2018, 123, 1–16. [Google Scholar] [CrossRef]
- Wang, G. Deformation of Organic Matter and Its Effect on Pores in Mud Rocks. Am. Assoc. Pet. Geol. Bull. 2020, 104, 21–36. [Google Scholar] [CrossRef]
- Teng, J.; Liu, B.; Mastalerz, M.; Schieber, J. Origin of Organic Matter and Organic Pores in the Overmature Ordovician-Silurian Wufeng-Longmaxi Shale of the Sichuan Basin, China. Int. J. Coal Geol. 2022, 253, 103970. [Google Scholar] [CrossRef]
- Liu, B.; Mastalerz, M.; Schieber, J. SEM Petrography of Dispersed Organic Matter in Black Shales: A Review. Earth-Sci. Rev. 2022, 224, 103874. [Google Scholar] [CrossRef]
- Nie, H.; Jin, Z.; Zhang, J. Characteristics of Three Organic Matter Pore Types in the Wufeng-Longmaxi Shale of the Sichuan Basin, Southwest China. Sci. Rep. 2018, 8, 7014. [Google Scholar] [CrossRef]
- Hu, G.; Pang, Q.; Jiao, K.; Hu, C.; Liao, Z. Development of Organic Pores in the Longmaxi Formation Overmature Shales: Combined Effects of Thermal Maturity and Organic Matter Composition. Mar. Pet. Geol. 2020, 116, 104314. [Google Scholar] [CrossRef]
- Pang, Q.; Hu, G.; Hu, C.; Zhang, C. Variation of Organic Pore Structure With Maceral Types in the Longmaxi Shale, Sichuan Basin. Front. Earth Sci. 2021, 9, 715278. [Google Scholar] [CrossRef]
- Borjigin, T.; Lu, L.; Yu, L.; Zhang, W.; Pan, A.; Shen, B.; Wang, Y.; Yang, Y.; Gao, Z. Formation, Preservation and Connectivity Control of Organic Pores in Shale. Shiyou Kantan Yu Kaifa/Pet. Explor. Dev. 2021, 48, 687–699. [Google Scholar] [CrossRef]
- Chang, J.; Fan, X.; Jiang, Z.; Wang, X.; Chen, L.; Li, J.; Zhu, L.; Wan, C.; Chen, Z. Differential Impact of Clay Minerals and Organic Matter on Pore Structure and Its Fractal Characteristics of Marine and Continental Shales in China. Appl. Clay Sci. 2022, 216, 106334. [Google Scholar] [CrossRef]
- Ma, Z.; Tang, X.; Deng, E.; Liu, Y.; Wang, Y.; Zhang, J.; Liang, G.; Zhang, J.; Luo, H. Lithofacies and Its Controls on the Organic Matter-Hosted Pores in the Cambrian Gas-Rich Niutitang Shale in Upper Yangtze Plate, China. J. Pet. Sci. Eng. 2022, 218, 111052. [Google Scholar] [CrossRef]
- Milliken, K.L.; Ergene, S.M.; Ozkan, A. Quartz Types, Authigenic and Detrital, in the Upper Cretaceous Eagle Ford Formation, South Texas, USA. Sediment. Geol. 2016, 339, 273–288. [Google Scholar] [CrossRef]
- Guan, Q.; Dong, D.; Zhang, H.; Sun, S.; Zhang, S.; Guo, W. Types of Biogenic Quartz and Its Coupling Storage Mechanism in Organic-Rich Shales: A Case Study of the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation in the Sichuan Basin, SW China. Shiyou Kantan Yu Kaifa/Pet. Explor. Dev. 2021, 48, 700–709. [Google Scholar] [CrossRef]
- Zhang, K.; Song, Y.; Jia, C.; Jiang, Z.; Han, F.; Wang, P.; Yuan, X.; Yang, Y.; Zeng, Y.; Li, Y.; et al. Formation Mechanism of the Sealing Capacity of the Roof and Floor Strata of Marine Organic-Rich Shale and Shale Itself, and Its Influence on the Characteristics of Shale Gas and Organic Matter Pore Development. Mar. Pet. Geol. 2022, 140, 105647. [Google Scholar] [CrossRef]
- Yu, Y.; Wang, Z.; Zhang, K.; Feng, X.; Cheng, M. Effects of Shear on Development Characteristics of Organic Matter Pores in Shale: A Case Study of Shale in the Niutitang Formation of the Well XAD1. J. Pet. Sci. Eng. 2022, 211, 110166. [Google Scholar] [CrossRef]
- Guan, Q.; Dong, D.; Zhang, H.; Zhang, S.; Lyu, X.; Wang, Y. Characterizing the Characteristics of Natural Fractures in Shale Based on the Modified Petrophysical Model. Nat. Gas Ind. B 2021, 8, 325–333. [Google Scholar] [CrossRef]
- Zhu, B.; Meng, J.; Pan, R.; Song, C.; Zhu, Z.; Jin, J.; Zhang, C.; Liu, L.; Xu, L.; Zhou, X. Multi-Scale Characterization of Organic Matter Pore Space in Deep Marine Shale Combined with Mathematical Morphology. Geoenergy Sci. Eng. 2023, 223, 211539. [Google Scholar] [CrossRef]
- Zhang, W.; Hu, W.; Borjigin, T.; Zhu, F. Pore Characteristics of Different Organic Matter in Black Shale: A Case Study of the Wufeng-Longmaxi Formation in the Southeast Sichuan Basin, China. Mar. Pet. Geol. 2020, 111, 33–43. [Google Scholar] [CrossRef]
- Pommer, M.; Milliken, K. Pore Types and Pore-Size Distributions across Thermal Maturity, Eagle Ford Formation, Southern Texas. Am. Assoc. Pet. Geol. Bull. 2015, 99, 1713–1744. [Google Scholar] [CrossRef]
- Wu, J.; Luo, C.; Zhong, K.; Li, Y.; Li, G.; Du, Z.; Yang, J. Innovative Characterization of Organic Nanopores in Marine Shale by the Integration of HIM and SEM. Energy 2023, 282, 128390. [Google Scholar] [CrossRef]
- Guan, Q.; Dong, D.; Sun, S.; Hu, L.; Qi, L.; Li, C.; Chen, C.; Chen, X. Fractal Characteristics of Organic-Rich Shale Pore Structure and Its Geological Implications: A Case Study of the Lower Silurian Longmaxi Formation in the Weiyuan Block, Sichuan Basin. Nat. Gas Ind. 2024, 44, 108–118. [Google Scholar] [CrossRef]
- Peng, N.; He, S.; Hu, Q.; Zhang, B.; He, X.; Zhai, G.; He, C.; Yang, R. Organic Nanopore Structure and Fractal Characteristics of Wufeng and Lower Member of Longmaxi Shales in Southeastern Sichuan, China. Mar. Pet. Geol. 2019, 103, 456–472. [Google Scholar] [CrossRef]
- Li, X.; Wang, P.; Wang, S.; Jin, C.; Zhang, C.; Zhao, R.; Jiang, Z. Influence of the Duration of Tectonic Evolution on Organic Matter Pore Structure and Gas Enrichment in Marine Shale: A Case Study of the Lower Silurian Longmaxi Shale in Southern China. Front. Earth Sci. 2022, 9, 787697. [Google Scholar] [CrossRef]
- Ma, Y.; Cai, X.; Zhao, P. China’s Shale Gas Exploration and Development: Understanding and Practice. Shiyou Kantan Yu Kaifa/Pet. Explor. Dev. 2018, 45, 561–574. [Google Scholar] [CrossRef]
- Wang, S.; Dong, D.; Wang, Y.; Li, X.; Huang, J.; Guan, Q. Sedimentary Geochemical Proxies for Paleoenvironment Interpretation of Organic-Rich Shale: A Case Study of the Lower Silurian Longmaxi Formation, Southern Sichuan Basin, China. J. Nat. Gas Sci. Eng. 2016, 28, 691–699. [Google Scholar] [CrossRef]
- Dong, D.; Gao, S.; Huang, J.; Guan, Q.; Wang, S.; Wang, Y. Discussion on the Exploration & Development Prospect of Shale Gas in the Sichuan Basin. Nat. Gas Ind. B 2015, 2, 9–23. [Google Scholar] [CrossRef]
- Wei, Z.; Chu, R.; Xie, J.; Bao, F.; Zeng, S.; Sheng, M.; Zeng, Q. Crustal Structure in the Weiyuan Shale Gas Field, China, and Its Tectonic Implications. Tectonophysics 2022, 837, 229449. [Google Scholar] [CrossRef]
- Guo, X.; Hu, D.; Li, Y.; Wei, Z.; Wei, X.; Liu, Z. Geological Factors Controlling Shale Gas Enrichment and High Production in Fuling Shale Gas Field. Pet. Explor. Dev. 2017, 44, 513–523. [Google Scholar] [CrossRef]
- Feng, Z.; Dong, D.; Tian, J.; Qiu, Z.; Wu, W.; Zhang, C. Geochemical Characteristics of Longmaxi Formation Shale Gas in the Weiyuan Area, Sichuan Basin, China. J. Pet. Sci. Eng. 2018, 167, 538–548. [Google Scholar] [CrossRef]
- Guan, Q.; Dong, D.; Wang, S.; Huang, J.; Wang, Y.; Lu, H.; Zhang, C. Preliminary Study on Shale Gas Microreservoir Characteristics of the Lower Silurian Longmaxi Formation in the Southern Sichuan Basin, China. J. Nat. Gas Sci. Eng. 2016, 31, 382–395. [Google Scholar] [CrossRef]
- Wu, J.; Yuan, Y.; Niu, S.; Wei, X.; Yang, J. Multiscale Characterization of Pore Structure and Connectivity of Wufeng-Longmaxi Shale in Sichuan Basin, China. Mar. Pet. Geol. 2020, 120, 104514. [Google Scholar] [CrossRef]
- Mandelbrot, B. How Long Is the Coast of Britain? Statistical Self-Similarity and Fractional Dimension. Science 1967, 156, 636–638. [Google Scholar] [CrossRef] [PubMed]
- Sing, K.S.W. Reporting Physisorption Data for Gas/Solid Systems with Special Reference to the Determination of Surface Area and Porosity (Recommendations 1984). Pure Appl. Chem. 1985, 57, 603–619. [Google Scholar] [CrossRef]
- Milliken, K.L.; Olson, T. Silica Diagenesis, Porosity Evolution, and Mechanical Behavior In Siliceous Mudstones, Mowry Shale (Cretaceous), Rocky Mountains, USA. J. Sediment. Res. 2017, 87, 366–387. [Google Scholar] [CrossRef]
- Ko, L.T.; Ruppel, S.C.; Loucks, R.G.; Hackley, P.C.; Zhang, T.; Shao, D. Pore-Types and Pore-Network Evolution in Upper Devonian-Lower Mississippian Woodford and Mississippian Barnett Mudstones: Insights from Laboratory Thermal Maturation and Organic Petrology. Int. J. Coal Geol. 2018, 190, 3–28. [Google Scholar] [CrossRef]
- WU, J.; Zhou, W.; Sun, S.; Zhou, S.; Shi, Z. Graptolite-Derived Organic Matter and Pore Characteristics in the Wufeng-Longmaxi Black Shale of the Sichuan Basin and Its Periphery. Acta Geol. Sin. 2019, 93, 982–995. [Google Scholar] [CrossRef]
- İnan, S.; Al Badairy, H.; İnan, T.; Al Zahrani, A. Formation and Occurrence of Organic Matter-Hosted Porosity in Shales. Int. J. Coal Geol. 2018, 199, 39–51. [Google Scholar] [CrossRef]
- Ko, L.T.; Loucks, R.G.; Zhang, T.; Ruppel, S.C.; Shao, D. Pore and Pore Network Evolution of Upper Cretaceous Boquillas (Eagle Ford-Equivalent) Mudrocks: Results from Gold Tube Pyrolysis Experiments. Am. Assoc. Pet. Geol. Bull. 2016, 100, 1693–1722. [Google Scholar] [CrossRef]
- Eliyahu, M.; Emmanuel, S.; Day-Stirrat, R.; Macaulay, C.I. Mechanical Properties of Organic Matter in Shales Mapped at the Nanometer Scale. Mar. Pet. Geol. 2014, 59, 294–304. [Google Scholar] [CrossRef]
- Guo, T. Key Geological Issues and Main Controls on Accumulation and Enrichment of Chinese Shale Gas. Pet. Explor. Dev. 2016, 43, 349–359. [Google Scholar] [CrossRef]
- Zhu, H.; Ju, Y.; Qi, Y.; Huang, C.; Zhang, L. Impact of Tectonism on Pore Type and Pore Structure Evolution in Organic-Rich Shale: Implications for Gas Storage and Migration Pathways in Naturally Deformed Rocks. Fuel 2018, 228, 272–289. [Google Scholar] [CrossRef]
Sample No. | TOC % | Clay % | Quartz % | Feldspar % | Calcite % | Dolomite % | Pyrite % | Anhydrite % | Siderite % |
---|---|---|---|---|---|---|---|---|---|
2-5 | 2.54 | 22 | 21 | 9 | 28 | 17 | 1 | 2 | 0 |
2-11 | 2.48 | 14 | 68 | 6 | 7 | 3 | 2 | 0 | 0 |
2-13 | 6.06 | 34 | 41 | 8 | 3 | 7 | 2 | 2 | 3 |
2-14 | 2.29 | 7 | 25 | 2 | 35 | 21 | 10 | 0 | 0 |
5-1 | 1.05 | 31 | 39 | 17 | 5 | 2 | 2 | 3 | 1 |
5-5 | 3.00 | 28 | 39 | 9 | 13 | 7 | 2 | 2 | 0 |
5-8 | 2.84 | 27 | 33 | 9 | 21 | 6 | 2 | 2 | 0 |
Sample No. | Pores | Area/nm2 | Perimeter/nm | Length/nm | Width/nm | Equivalent Circular Diameter/nm | Elongation |
---|---|---|---|---|---|---|---|
2-5 | 1798 | 277.00–153,739.61 (2580.92) | 33.62–5595.64 (184.25) | 5.26–1185.76 (65.74) | 4.08–488.47 (35.16) | 14.56–560.52 (45.01) | 0.09–1.00 (0.59) |
2-11 | 1709 | 650.36–2,087,905.77 (33,272.35) | 66.43–14,565.63 (744.00) | 8.06–3393.23 (253.60) | 4.49–1695.35 (132.44) | 28.78–1630.46 (167.10) | 0.05–1.00 (0.58) |
2-13 | 1042 | 226.76–238,962.07 (5347.66) | 39.22–4761.47 (242.49) | 7.41–1143.86 (86.03) | 4.76–467.86 (46.90) | 16.99–551.59 (59.66) | 0.12–1.00 (0.57) |
2-14 | 247 | 8910.67–45,500,000.00 (228,316.75) | 245.88–62,729.13 (1083.17) | 29.85–30,418.80 (449.83) | 26.93–1599.36 (159.12) | 106.51–7608.62 (239.24) | 0.02–1.00 (0.53) |
5-1 | 436 | 142.40–32,566.75 (1886.11) | 31.43–2724.89 (215.87) | 10.67–634.58 (63.18) | 4.98–256.18 (34.26) | 13.47–203.63 (38.71) | 0.20–1.00 (0.59) |
5-5 | 405 | 49.38–27,577.32 (1376.73) | 18.30–2520.00 (174.39) | 6.67–397.78 (52.41) | 3.65–237.62 (29.48) | 7.93–187.83 (33.19) | 0.20–0.96 (0.63) |
5-8 | 1307 | 326.53–18,971.43 (1518.06) | 47.07–1386.75 (169.13) | 5.71–424.35 (57.52) | 5.71–165.77 (32.37) | 20.39–155.42 (39.99) | 0.16–1.00 (0.59) |
Sample No. | SSA (m2/g) | Pore Volume (mL/g) | Mean Radius (nm) | Mode Radius (nm) |
---|---|---|---|---|
2-5 | 96.4838 | 0.295448 | 9.14 | 2.89 |
2-11 | 65.783 | 0.257775 | 10.24 | 2.97 |
2-13 | 163.0092 | 0.282815 | 5.32 | 2.88 |
2-14 | 121.4183 | 0.272646 | 6.72 | 3.12 |
5-1 | 60.1268 | 0.215191 | 8.55 | 2.9 |
5-5 | 96.734 | 0.397203 | 11.09 | 2.95 |
5-8 | 105.4049 | 0.368997 | 9.82 | 2.85 |
Sample No. | Slope>50 | R2>50 | D>50 | Slope2–50 | R22–50 | D2–50 | Slope<2 | R2<2 | D<2 |
---|---|---|---|---|---|---|---|---|---|
2-5 | −0.39 | 0.991 | 2.61 | −0.38 | 0.996 | 2.52 | −0.99 | 0.949 | 2.01 |
2-11 | −0.45 | 0.993 | 2.55 | −0.44 | 0.996 | 2.56 | −0.97 | 0.943 | 2.03 |
2-13 | −0.24 | 0.962 | 2.76 | −0.32 | 0.992 | 2.68 | −0.98 | 0.961 | 2.02 |
2-14 | −0.27 | 0.989 | 2.73 | −0.35 | 0.999 | 2.65 | −0.98 | 0.946 | 2.02 |
5-1 | −0.37 | 0.973 | 2.63 | −0.42 | 0.999 | 2.58 | −0.97 | 0.948 | 2.03 |
5-5 | −0.53 | 0.992 | 2.47 | −0.41 | 0.996 | 2.59 | −0.99 | 0.936 | 2.01 |
5-8 | −0.49 | 0.988 | 2.51 | −0.37 | 0.998 | 2.63 | −0.99 | 0.948 | 2.01 |
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Guan, Q.; Dong, D.; Deng, B.; Chen, C.; Li, C.; Jiao, K.; Ye, Y.; Liang, H.; Yue, H. Structure and Fractal Characteristics of Organic Matter Pores in Wufeng–Lower Longmaxi Formations in Southern Sichuan Basin, China. Fractal Fract. 2025, 9, 410. https://doi.org/10.3390/fractalfract9070410
Guan Q, Dong D, Deng B, Chen C, Li C, Jiao K, Ye Y, Liang H, Yue H. Structure and Fractal Characteristics of Organic Matter Pores in Wufeng–Lower Longmaxi Formations in Southern Sichuan Basin, China. Fractal and Fractional. 2025; 9(7):410. https://doi.org/10.3390/fractalfract9070410
Chicago/Turabian StyleGuan, Quanzhong, Dazhong Dong, Bin Deng, Cheng Chen, Chongda Li, Kun Jiao, Yuehao Ye, Haoran Liang, and Huiwen Yue. 2025. "Structure and Fractal Characteristics of Organic Matter Pores in Wufeng–Lower Longmaxi Formations in Southern Sichuan Basin, China" Fractal and Fractional 9, no. 7: 410. https://doi.org/10.3390/fractalfract9070410
APA StyleGuan, Q., Dong, D., Deng, B., Chen, C., Li, C., Jiao, K., Ye, Y., Liang, H., & Yue, H. (2025). Structure and Fractal Characteristics of Organic Matter Pores in Wufeng–Lower Longmaxi Formations in Southern Sichuan Basin, China. Fractal and Fractional, 9(7), 410. https://doi.org/10.3390/fractalfract9070410