| Citation: |
Yong Ma, Cheng-yu Yang, Da-hua Li, Hong-wei Zhao, Zhe-jun Pan, Yong-shui Zhou, Dai-duo Zhu, Ning-ning Zhong, 2025. Genetic relationship between shell fossils and shale oil: A case study of Jurassic shale reservoir in the northeast Sichuan Basin, China Geology, 8, 360-372. doi: 10.31035/cg2024221
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Genetic relationship between shell fossils and shale oil: A case study of Jurassic shale reservoir in the northeast Sichuan Basin
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Abstract
Benthic bivalves, the most widely distributed mollusks since the Mesozoic era, often inhabited environments where their fossilized remains are found adjacent to or intermingled with organic-rich shale. Recent Jurassic shale oil exploration in the Sichuan Basin has revealed that bioclastic layers, composed of abundant fossil bivalves and closely associated with shales and, exhibit significant hydrocarbon potentials. However, the microscopic structures of these bivalve fossils and their role in hydrocarbon storage and migration remain poorly understood. In this study, we characterized the microporosity of bivalve shells within the Middle-Lower Jurassic bioclastic shale in the northeastern Sichuan Basin using a combination of 2D imaging (thin section, SEM), 3D reconstruction (FIB-SEM), and permeability simulation. The micropores within the shell fossils range from 100 to 1000 nm in radius and are uniformly distributed in a grid-like pattern within the shell interior, where they host liquid hydrocarbons. The bioclastic carbonate layers exhibit an overall porosity of approximately 0.8%. Comparative analysis with extant bivalve shells suggests that these micropores represent residual pores from the nacreous brick wall structure. Due to the regular orientation of the shells and their microporous nacres, permeability coefficients along the long bivalve fossil axes are three to five times higher than those along the short axes. These residual micropores within the bioclastic fossil shells have a positive influence on both the storage and migration of shale oil and gas, making bioclastic fossil-bearing shalespromising sweet spots for shale oil and gas exploration in similar sedimentary environments.
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Keywords:
- Micro-porosity /
- Bioclastic carbonates /
- Bivalve shells /
- Shale oil /
- Sichuan Basin
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References
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Yong Ma, Cheng-yu Yang, Da-hua Li, Hong-wei Zhao, Zhe-jun Pan, Yong-shui Zhou, Dai-duo Zhu, Ning-ning Zhong, 2025. Genetic relationship between shell fossils and shale oil: A case study of Jurassic shale reservoir in the northeast Sichuan Basin, China Geology, 8, 360-372. doi: 10.31035/cg2024221
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Figure 1.
Sedimentary stratigraphy and sedimentary evolution characteristics of the Jurassic Strata in the Northeastern Sichuan Basin (Modified from He WY et al., 2022). (a) Sedimentary stratigraphy of the Lower and Middle Jurassic in NE Sichuan. (b) Map of sedimentary facies of Sichuan Basin in middle Jurassic. (c) Map of sedimentary facies of Sichuan Basin in early Jurassic.
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Figure 2.
Characteristics of shell fossils in bioclastic shale. (a) Pure bioclastic shale; PL8, 2711 m. (b) bioclastic shale-micrite shale; PL7, 2968.3 m. (c) Shell-rich mudstone; Outcrop, Liangping. (d) Three-dimensional reconstruction of a bioclastic shale using CT images; D1, 3162.8 m. (e) Three-dimensional reconstruction of shells within a bioclastic shale; D1, 3162.8 m.
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Figure 3.
Thin section images showing the characteristics of shell fossils in bioclastic layers of the Da'anzhai shale, Sichuan Basin. (a) Large shell embedded in shale with inner and outer layer indicated; plane polarized light, D1-3162.8 m. (b) Layer composed almost totally of large shells; plane polarized light, DY-3363.7 m. (c) Large and small shells embedded in shale; plane polarized light, D1-3364.9 m. (d) Layer composed of both large and small shells; plane polarized light,DY-3364.9 m. (e) Large shell, plane polarized light, DY-3162.8 m. (f) Large shell containing relatively pure calcite crystals, plane polarized light; D1-3368.2 m. (g) Small shell embedded in shale; plane polarized light, D1-3162.8 m. (h) Large shell; cross polarized light, D1-3162.8 m. (i) Large shells containing relatively pure calcite crystals; cross polarized light, D1-3368.2 m.
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Figure 4.
SEM images showing the micropores within a bivalve shell. (a) Oriented micropores along the shell, PL8, 2570.3 m. (b) Micropores in the pore-rich layer within the shell, D1-3044.5 m. (c) Pseudomorphic crystals filling the pores within a shell, D1- 3044.5 m. (d) Fossil shell in the bioclastic layer, D1-3028.3 m. (e) Boundary of pore-rich layer in a shell, D1-3028.3 m. (f) The microfracture in a shell, D1-3028.3 m.
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Figure 5.
Three-dimensional reconstruction of the fossil shell layer (PL8, 2570.3 m) from 1500 sequential BSE images by FIB-SEM. (a) Reconstruction of the fossil shell layser from BSE images. (b) Pores within the fossil shell layer. (c) Separated pores shown in different colors within the shell fossil. (d) Pore-size distribution of the pores within the fossil shell layer.
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Figure 6.
Simulated 3-dimensional permeability distributions of fossil shell layers, PL8, 2570.3 m. (a) Permeability simulations were performed for the green box in the pore network of the shell layer. (b) Results of permeability simulation of the fossil shell layer in x-direction. (c) y-direction and (d) z-direction.
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Figure 7.
Images showing the large shells and the liquid hydrocarbons inside under plane-polarized light (left) and reflected fluorescence (right). (a) Deformed large shell, D1, 3363.7 m. (b) Hydrocarbons appear in white-blue fluorescence under ultraviolet light within the deformed shell, D1, 3363.7 m. (c) Individual large shell, D1, 3364.9 m. (d) Hydrocarbons within the micropores exhibiting bright blue fluorescence under ultraviolet light along the long axis of the shell, D1-3364.9 m. (e) Large shell, D1, 3364.9 m. (f) Hydrocarbon filling arranged in ribbons along the short axis of the shell, showing bright blue fluorescence under ultraviolet light, D1, 3364.9 m.
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Figure 8.
Typical shell structure of bivalvia and SEM image of the section of inner nacreous shell layer of a modern sample (Lutz RA, 1976; Zhang GS and Xie XD, 2000).
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Figure 9.
Comparison of growth lines of Jurassic and modern shells. (a) Large shell with growth line, plane polarized light, DY1-3162.8 m. (b) Modern specimen sampled 29 November 1972 from Damariscotta River shore population (Lutz RA, 1976). (c) Large shell with growth line, cross polarized light, DY1-3162.8 m. (d) Acetate peel of umbonal region of (b) (Lutz RA, 1976).
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Figure 10.
Schematic diagram showing the concept of the filling of shell fossils in the Jurassic bioclastic layers in the northeastern Sichuan Basin. The evolution of shell fossils modified after Lutz RA (1976).