Shale gas refers to thermogenic or biogenic gas stored as free gas, absorbed gas and/or dissolved gas within organic shales [1, 2, 3]. Shales commonly have complex porous networks with different pore types and wide pore size distribution [4, 5, 6]. Investigation of pore structure is significant to understand methane storage and flow mechanisms in gas shales. Pore structure can be effectively characterized by microscopic observation, radiation detection and fluid intrusion techniques [7, 8, 9, 10, 11]. In addition, fractal theory provides a novel method for quantitatively describing the heterogeneity of porous rocks [12, 13]. Based on low pressure N2 physisorption (LPNP) isotherms, fractal dimensions of pores in coals and shales can be characterized by the Frenkel-Halsey-Hill (FHH) model [10, 14, 15, 16, 17]. The fractal dimension D varies from 2 to 3. The lowest value 2 corresponds to a perfectly smooth surface or homogenous pore structure, while the upper value 3 refers to an irregular surface or heterogeneous pore structure [8, 14]. Recently, fractal characteristics of pores in marine shales around the world were extensively studied [10, 17, 18, 19, 20, 21], however, few investigations about the nanopore fractal dimensions of lacustrine shale were reported.
The black shales and mudstones in the Chang-7 and Chang-9 members of the Triassic Yanchang formation deposited in deep lacustrine sedimentary facies are the primary targets for lacustrine shale gas exploration [22, 23]. To understand the pore structure and fractal characteristics of lacustrine shales, a series of experiments were conducted including Total organic matter contents (TOC), vitrinite reflectance (Ro), X-ray diffraction (XRD) and experiments of field emission-scanning electron microscopy (FE-SEM) and LPNP.
The objectives of the present work are to: (1) investigate the fractal dimensions of pores in lacustrine shales using FHH method based on LPNP data; (2) compare the fractal dimensions of pores in the Chang-7 shale and the Chang-9 shale; (3) discuss the controls of matrix compositions and pore structure parameters on the fractal dimensions of the lacustrine Yanchang shales.
2 Samples and Methods
In total, 25 lacustrine shale samples were collected from wells YC-7 and YC-8 in the study region, which is in southern Ordos Basin (Figure 1). These core samples were from a depth range of 1343.77-1613.35m. Shales in the Chang-7 and Chang-9 members in the Yanchang Formation are deposited in a semi-deep lacustrine environment, with relatively large thickness (Figure 1c). The organic matter types are primarily sapropelic [22, 23].
Total organic matter contents (TOC), vitrinite reflectance (Ro), X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM) and LPNP experiments were conducted following the Chinese national standard for shale gas evaluation (GB/T 31483-2015). Briefly, TOC contents were tested on a LECO CS230 carbon/sulfur analyzer using 100 mesh samples after carbonates removed. Ro values were measured on an MPV-SP micro-photometer. Bulk mineral compositions were analyzed using a D8 Discover XRD apparatus. FE-SEM observations were conducted using a Zeiss SUPRA 55 Sapphire SEM equipped with secondary electron (SE), backscattered electron (BSE) detectors. LPNP experiments were conducted with a Quantachrome SI surface area analyzer with temperature at 77K and relative pressure from 0.001 to 0.998. The BET (Brunauer-Emmett-Teller) and BJH (Barrette-Joyner-Halenda) models were applied to calculate pore structure parameters. The FHH model, for calculating fractal dimensions, can be described as :(1)
Where, P is the equilibrium pressure, MPa; Po is the gas saturation pressure, MPa; V is the volume corresponding to the equilibrium pressure P, cm3/g; D is the fractal dimension, and C is a constant.
3.1 Mineral compositions
Bulk mineral compositions of the lacustrine shale samples are listed in Table 1. The samples are clay rich, with average clay mineral contents of 50.23 wt%(40 wt%-58 wt%) in Chang-7 shales and 54.2 wt% (43 wt%-58 wt%) in Chang-9 shales, respectively (Table 1, Figure 2a). In addition, a mixed-layer of illite/smectite (I/S) contributes the greatest proportion in clay contents, with a mean value of 60.59 wt% (45 wt%-79 wt%) in Chang-7 shales and 65.38 wt% (44 wt%-88 wt%) in Chang-9 shales (Table 1, Figure 2b). The quartz content varies from 22 wt% to 37 wt% with an average of 29.05 wt% in Chang-7 shales, and varies from 34 wt% to 44 wt% with an average of 39.12 wt% in Chang-9 shales. Trace abundance of carbonates, dolomite and pyrite were identified in both members (Table 1).
The Chang-7 shales are richer in organic matter than the Chang-9 shales. Average TOC contents of Chang-7 shales and Chang-9 shales are 2.61 wt% (0.49 wt%-6.86 wt%) and 4.52 wt% (2.14 wt%-6.65 wt%). The Ro values of Chang-7 shales ranges from 0.848% to 0.976%, while Ro of Chang-9 shale varies from 0.93%to 1.094%, indicating that both the Chang-7 and Chang-9 member are in the mature stage, and shale gas and residual oil may coexist in the shale pores (Table 1, Figure 2c).
3.3 Pore structure characteristics
3.3.1 FE-SEM imaging
Following the classification of Loucks et al (2012), organic matter (OM) pores, intergranular pores and microfractures were observed in FE-SEM images (Figure 3). The width of typical pores displays a wide range of 6.25-433.2nm, indicating heterogeneous pore systems in the lacustrine shale samples. OM pores show heterogeneous distribution with abundant organic matter grains have very few isolated OM pores (Figure 3a-c). The intergranular pores are the dominate types in the selected lacustrine shales. Such pores are primarily hosted in the framework of clay interlayers, quartz grains and pyrite framboids (Figure 3d-g). Inter-crystallite pores are identified within pyrite framboids and quartz grains (Figure 3g, 3h). A few microfractures, with widths of 33-200 nm and lengths of 5 μm, were also observed along grain rims (Figure 3i).
3.3.2 Pore structure parameters obtained from LPNP data
Pore structure parameters, including BET specific surface area (SSA), BJH pore volume (PV) and average pore diameter (PD), were listed in Table 2. SSAs of the Chang-7 shales are slightly lower than that of the Chang-9 shales, with average SSAs of 7.37 m2/g and 12.66 m2/g of Chang-7 shales and Chang-9 shales, respectively. PVs of Chang-9 member are greater than that of Chang-7 member. The PVs in Chang-7 shales vary from 0.022 cm3/g to 0.047 cm3/g, while the PVs in Chang-9 shales range from 0.026 cm3/g to 0.049 cm3/g. The average pore width (PD) of the Chang-7 member is bigger than that of the Chang-9 member (Table 2). The plot of dV/d(logd) versus pore size is shown in Figure 4 to illustrate the pore size distribution of the selected lacustrine shales in this paper. Bimodal features are observed, indicating the shales are dominated by mesopores with different pore width. Pore size distributions of the Chang-7 and Chang-9 shales are in the range of 2-16 nm(mesopores).
3.4 Fractal dimensions
Two linear segments occur in the ln(V)-ln(ln(P0/P)) plots at relative pressures of 0-0.45 and 0.45-1 (Figure 5), indicating different nitrogen adsorption mechanisms [10, 14]. The fractal dimension D1 at a lower relative pressure of 0-0.45 represents the effects of Van der Waals forces and indicates the surface fractal dimension. While, the fractal dimension D2 at a higher relative pressure of 0.45-1 corresponds to the results of capillary condensation and represents the pore structure fractal dimension .
The fitting equations, correlation coefficients and fractal dimensions are listed in Table 2. For the Chang-7 shales, the D1 values are ranging from 2.17 to 2.36 and the D2 values are ranging from 2.63 to 2.46. For Chang-9 shales, the D1 values are in the range of 2.40 to 2.23 and the D 2 values are in the range of 2.64 to 2.46. The generally larger D2 values indicate that larger pores in the selected samples have more heterogeneous pore volumes compared with pores of smaller width. These results indicate that the lacustrine shales in the present study have complex pore structure.
4.1 Relationships between fractal dimensions and shale compositions
As shown in Figure 6, fractal dimensions D1 and D2 have negative correlations with TOC content of the lacustrine shales in thiswork. This result is inconsistent with the fractal dimensions of the over-mature marine shales [10, 17, 18]. Shales may contain OM pores due to thermal maturation [3, 4, 24, 25], and OM pores with smaller pore width may result in more complex pore networks, consequently, increasing the fractal dimensions D1 and D2. In FE-SEM images, very few OM pores were observed and inorganic pores especially intergranular pores, intercrystalline pores and micro fracture are mostly developed in the selected lacustrine shales (Figure 3), which may explain the opposite correlations of fractal dimensions with TOC content.
Figure 7 presented the relationships between fractal dimensions (D1, D2) and bulk mineral compositions including clay minerals, quartz, feldspar and I/S. With the increase of clays and I/S, both D1 and D2 increase. Clay minerals are the main constituent in lacustrine shales with an average content of 50% (Table 1). In addition, illite/smectite (I/S) is the main component in the clay minerals. As the clay minerals host abundant complicated inorganic pores, enhancing the heterogeneity of pore structures. In the Figures 7e-f, the relationship between fractal dimensions and quartz content displays slightly positive correlations. This is because the development of few intergranular pores and fractures are associated with quartz detrital (Figure 3), and the irregular shaped may result in more complex pore system and increasing in fractal dimensions .
4.2 Relationships between fractal dimensions and pore structure parameters
To reveal the impacts of pore structure on fractal dimensions of lacustrine shales, correlations of fractal dimensions with pore volume and average pore diameter were discussed. Both D1 and D2 have undefined correlations with PV (Figure 8a, b), suggesting pore volume has little effect on fractal dimensions. It may be because complex pore systems have larger pore volumes, resulting in more complicated fractal dimensions [8, 17, 26]. The difference between D1 and D2 indicate that pore volume has some different influences on fractal dimensions D1 and D2.
Both fractal dimensions D1 and D2 increase with decreasing average pore diameters (Figure 8). Additionally, the correlation of D2 with the average pore diameter is better than that of D1 with the average pore diameter, indicating that D2 may best represent the pore structure fractal dimension and thus, it is more sensitive to the average pore diameter than D1. These findings are consistent with the findings of coal , marine gas shale  and lacustrine shale , which indicating the micropores and mesopores are more complicated than macropores, probably because the small pore diameters have lager fractal dimensions. Shale samples with smaller average pore diameters may also contain more throats and micropores [8, 14], which lead to more heterogonous and complicated pore structure and higher fractal dimension D2 values.
4.3 Comparison of fractal characteristics between Chang-7 and Chang-9 shales
Both fractal dimensions D1 and D2 in Chang-9 shales are higher than those in Chang-7 shales (Figure 9, Table 2), suggesting more irregular and nonhomogeneous pores in Chang-9 shales. The maturity of Chang-9 shale is generally larger than that of Chang-7 shale due to the burial depth, resulting in the fractal dimensions (D1 and D2) of the Chang-9 shale being higher than that of Chang-7 shale (Figure 9, Table 2). With the increase of burial depth and thermal maturity, the generation of more hydrocarbons break brittle minerals to form micro fractures which make pores more irregular. In addition, more smectite converted into I/S mixed layer, leading to a higher SSA and PV [10, 26]. In addition, the fractal dimensions keep steady,maybe because the generated oil occupying the macro-nanopores is balancing the generated organic pores [26, 27]. For pore structure parameters of the Yanchang shale samples, SSA of Chang-7 member is lower than that of in the Chang-9 member. Chang-9 member has a much larger PV than that in the Chang-7 member. For the average PD, the Chang-7 shale is bigger than that of the Chang-9 shale. Further study should be focused on the difference in shale reservoir characteristics, especially pore structure features between two target layers Chang-7 member and Chang-9 member, which has not been further quantitatively revealed and compared.
Based on our study, the following conclusions can be drawn.
Two fractal dimensions, D1 and D2,were obtained at relative pressures of 0-0.45 and 0.45-1 using the FHH method. The Chang-7 shales have D1 and D2 range of 2.17-2.36 and 2.46-2.63, while the Chang-9 shales have D1 values of 2.23-2.40 and D2 values of 2.46-2.64. The fractal dimensions (D1, D2) of Chang-7 shale are generally lower than that of Cheng-9 shale, which results in Chang 9 shale developing more complex pore structure due to its higher maturity.
Fractal dimensions of the selected lacustrine shales are affected by shale mineral compositions and pore structure parameters. Positive correlations of D1 and D2 with Clay minerals and quartz contents and negative correlations of D1 and D2 with TOC contents were presented in the present study. Observations of a few organic matter pores and abundant inorganic pores hosted in the Yanchang shales may contribute to these correlations.
Comparisons of matrix composition and pore characteristics between the Chang-7 shales and the Chang-9 shales suggest the latter may have a more irregular and heterogeneous pore structure.
This study was supported by the by the National Natural Science Foundation of China (No. 19641502123 and No. U1562215) and the National Major Project of China (No. 2017ZX05035-197 002 and No. 201605034-001).
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Published Online: 2019-05-17