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Volume 30 Issue 5
Oct.  2019
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Chemical Compositions and Distribution Characteristics of Cements in Longmaxi Formation Shales, Southwest China

  • Shale gas resources have been regarded as a viable energy source, and it is of great significance to characterize the shale composition of different cements, such as quartz and dolomite. In this research, chemical analysis and the multifractal method have been used to study the mineral compositions and petrophysical structures of cements in shale samples from the Longmaxi Formation, China. X-ray diffraction, electron microprobe, field emission scanning electron microscopy, cathodoluminescence microscopy and C-O isotope analyses confirmed that cements in the Longmaxi Formation shales are mainly composed of Fe-bearing dolomite and quartz. Fe-bearing dolomite cements concentrate around dolomite as annuli, filling micron-sized inorganic primary pores. Quartz cements in the form of nanoparicles fill primary inter-crystalline pores among clay minerals. Theoretical calculation shows that the Fe-bearing dolomite cements formed slightly earlier than the quartz cements, but both were related to diagenetic illitization of smectite. Moreover, multifractal analysis reveals that the quartz cements are more irregularly distributed in pores than the Fe-bearing dolomite cements. These results suggest that the plugging effect of the quartz cements on the primary inoraganic pore structures is the dominant factor resulting in low interconnected porosity of shales, which are unfavorable for the enrichment of shale gas.
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Chemical Compositions and Distribution Characteristics of Cements in Longmaxi Formation Shales, Southwest China

    Corresponding author: Zhengyu Bao, tinaxie@cug.edu.cn
  • 1. State Key Laboratory of Geological Processes and Mineral Resources (GPMR), School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
  • 2. Faculty of Chemistry and Material Sciences, China University of Geosciences, Wuhan 430074, China
  • 3. Geological Sciences, School of Agricultural, Earth and Environmental Sciences, University of KwaZulu-Natal, Westville 3629, South Africa
  • 4. Economic Geology Research Centre (EGRU), James Cook University, Townsville QLD 4811, Australia

Abstract: Shale gas resources have been regarded as a viable energy source, and it is of great significance to characterize the shale composition of different cements, such as quartz and dolomite. In this research, chemical analysis and the multifractal method have been used to study the mineral compositions and petrophysical structures of cements in shale samples from the Longmaxi Formation, China. X-ray diffraction, electron microprobe, field emission scanning electron microscopy, cathodoluminescence microscopy and C-O isotope analyses confirmed that cements in the Longmaxi Formation shales are mainly composed of Fe-bearing dolomite and quartz. Fe-bearing dolomite cements concentrate around dolomite as annuli, filling micron-sized inorganic primary pores. Quartz cements in the form of nanoparicles fill primary inter-crystalline pores among clay minerals. Theoretical calculation shows that the Fe-bearing dolomite cements formed slightly earlier than the quartz cements, but both were related to diagenetic illitization of smectite. Moreover, multifractal analysis reveals that the quartz cements are more irregularly distributed in pores than the Fe-bearing dolomite cements. These results suggest that the plugging effect of the quartz cements on the primary inoraganic pore structures is the dominant factor resulting in low interconnected porosity of shales, which are unfavorable for the enrichment of shale gas.

0.   INTRODUCTION
  • Cements play an important role in controlling the inorganic pore structures of shales. With increasing cement contents, primary pores in shales are filled and, consequently, their porosity and permeability decrease (Baig et al., 2016; Walderhaug et al., 2012; Ramm et al., 1997). Studying the types and distribution characteristics of cements in shales is, therefore, of great significance for understanding the development of shale inorganic pore structures (Dowey and Taylor, 2017; Zhou et al., 2017; Li et al., 2016; Zhou et al., 2016).

    The study of cements in conventional reservoirs is well established. Cements in sedimentary rocks are commonly divided into calcareous, siliceous, clay and iron (Sliaupa et al., 2008; Towe, 1962). Various types of cements have different effects on the inorganic pore structure of sedimentary rocks. For example, the presence of chlorite cements may increase the anti-compaction capacity of the rock and restrains the secondary enlargement of quartz, promotes dissolution, and forms favorable pore structures for oil reservoirs (Ajdukiewicz and Larese, 2012; Liu et al., 2009). Siliceous cements, on the other hand, fill the primary pores of reservoirs and decrease the permeability of a sedimentary formation (Luo et al., 2015).

    In general, the particle sizes of shale cements are close to the nano-meter levels (Chen et al., 2016; Weinberg et al., 2011), making it difficult to test characteristics of cements in shale. Traditionally, X-ray diffractometry (XRD), micron computerized tomography (micron-CT), isotope and micro-element analysis, nuclear magnetic resonance (NMR) and other advanced analytical methods had been used to investigate the type, source and formation time of shale cements, temperatures of diagenetic processes, and other information about shales (Ukar et al., 2017; Ge et al., 2015; Porten et al., 2015; Walderhaug et al., 2009; Midtbø et al., 2000). Other analytical techniques such as field emission scanning electron microscopy (FE-SEM) and infrared spectroscopy also have been used to show that quartz cements in Late Cretaceous shales from the northern North Sea are nano-particles in size and are distributed among clay minerals, filling nano-pore spaces (Thyberg et al., 2010; Peltonen et al., 2009; Worden et al., 2005). Multifractal analysis has been wildly applied to quantify pore structures and element distribution patterns in different media (Torre et al., 2018; Liu and Ostadhassan, 2017; Vega and Jouini, 2015; Xie et al., 2010a; Mandelbrot, 1977). In this paper, multifractal will be used to quantitatively study the distribution characteristics and formation time of cements in shales.

    The Longmaxi Formation is an important shale gas reservoir in Southwest China and has attracted a lot of attention among researchers in recent years (Zhou et al., 2018; Chen et al., 2017; Liang et al., 2017; Ye et al., 2017). Recent studies suggested that cements in the Longmaxi Formation shales are mainly composed of quartz derived from illitization of smectite (Zhao et al., 2017; Kong et al., 2016). Also, the mineral assemblages of the Longmaxi Formation shales are broadly similar to those of other shale gas reservoirs (e.g., Wufeng Formation) in China (Yang et al., 2017). However, the distribution characteristics and formation time of cements in the Longmaxi Formation shales and other Chinese shale gas reservoirs remain poorly understood.

    Accordingly, we initiated a detailed investigation on cements in shales from the Longmaxi Formation using a large number of analytical techniques from XRD to electron probe micro-analysis (EPMA), C-O isotope analysis, FE-SEM, energy dispersive spectrometry (EDS), and cathodoluminescence (CL). Results reported herein are intended to further evaluate the mineralogy of the cements in the Longmaxi shales, including the discovery of a new cement type (i.e., Fe-bearing dolomite). Multifractal analysis has been used to quantitatively determine the spatial distribution characteristics of the two distinct types of shale cements. In addition, the effects of the Fe-bearing dolomite and quartz cements on the primary inorganic pore structures of shale have been evaluated as well.

1.   SAMPLES
  • Representative samples of the Longmaxi Formation shales were collected from the Lucheng Village profile located in the Xishui County, Guizhou Province, southeastern Sichuan Basin (Fig. 1). Based on lithologies and depositional environment, the Longmaxi Formation has been divided into two parts––the upper SQ2 and the lower SQ1––which are distinguished by the existence and absence of carbonate minerals in the former and the latter, respectively (Wu et al., 2016; Wang et al., 2015; Li et al., 2012). Based on lithofacies variations of the Longmaxi Formation (Liang et al., 2016), which indicate that the sedimentary depositional environment of this formation is relatively stable and the types of cements in its shale units are simple, 10 samples of Longmaxi Formation shales were collected for this study from the Lucheng profile (Fig. 1). Five samples (L1 to L5) were taken in sequence at roughly equal intervals from SQ1 (which is 35 m thick) with sample L1 at the bottom of SQ1, whereas five samples (L6 to L10) were collected in sequence at roughly equal intervals from SQ2 (which is 45 m thick) with sample L10 at the top of SQ2. Field observations of the materials in SQ1 and SQ2 are relatively homogeneous, and so the 10 samples collected from the 80-m thick Lucheng profile are considered representative for analysis.

    Figure 1.  Map of the Sichuan Basin (after Dai et al., 2014) showing the profile, where shale samples were collected, in the Lucheng Village, Xishui, Guizhou Province, China.

    Samples for FE-SEM, EPMA, EDS and CL analyses were polished by argon ion, which will not cause mechanical damage to the samples (Stevens et al., 2011).

2.   METHODS
  • The XRD analysis, which was applied in this study to determine the mineralogical composition of the shale samples, was carried out at the Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan. XRD patterns of the samples were recorded by a D8-FOCUS X-ray Diffractometer (Bruker AXS, Germany), equipped with a Lynx-Eye Detector with Co Kα radiation at 35 kV and 40 mA. Specifically, the XRD analysis was divided into two parts: whole-rock analysis and clay mineral analysis. The separation of clay minerals from the shale samples was based on the Stokes law in water (Jiang et al., 2017; Bettison-Varga et al., 1991).

    Identification of minerals was made using the Evaluation (EVA) phase analysis software (Bruker AXS, Germany) by comparison with reference mineral patterns archived in the Powder Diffraction Files of the International Centre for Diffraction Data and other available databases. Quantitative analysis was carried out using TOPAS (Bruker AXS, Germany), a PC-based program for Rietveld refinements of the XRD spectra (Puphaiboon et al., 2013).

  • Two samples (L3 and L7) were selected for detailed EMPA analyses on a JXA-8100 electron probe micro-analyzer (JEOL, Japan). Specifically, sample L7, which has the highest dolomite content of 39.79%, was selected for investigating the dolomite and quartz cements in the SQ2 part of the Longmaxi Formation. Sample L3 from the middle part of SQ1 was selected for studying the distribution of the quartz cements. Resolution of the EMPA analysis is about 1 μm in diameter. Depending on the content of the element of all area, values of element content from the highest to the lowest are assigned to the corresponding color averagely. White corresponds to the highest element content, and black corresponds to the lowest. The analytical temperature was 20 ℃ and the analytical conditions/procedures were similar to those described in Lavrent'Ev et al. (2015) and Korolyuk (2008).

  • It is difficult to separate dolomite in shales. Therefore, the C-O isotope analysis of dolomite was made by using the powders of the whole rocks. To ensure reliable data, samples L7 and L9 were chosen for C-O isotope analysis because they had the highest dolomite contents. The C-O isotope analysis was achieved using the Finnigan MAT 253 Gas Isotope Mass Spectrometer (ThermoFisher, Germany) at the Key Lab of Carbonate Reservoirs, CNPC. The results of the analysis were relative to the PDB standard and were reported as δ(‰) with assumed δ13C and δ18O values of 0.04‰ and 0.08‰, respectively (Li et al., 2013; Bao and Thiemens, 2000). The GBW-04405 was taken as the standard sample.

  • Two representative samples (L3 and L7) were selected for FE-SEM and EDS analysis. FE-SEM imaging was made on a SU8010 instrument (HITACHI, Japan), with the working distances from 7 to 17 mm and acceleration voltages of 5 to 15 kV (Liu et al., 2017; Tan et al., 2015). The analytical mode was back-scattered diffraction (BSD). The resolution is about 400 nm. Minerals in the sample L3 were observed by FE-SEM after ultrasonic mineral separation (Moore and Reynolds, 1989; Gipson, 1963). However, the as-is sample L7 was used for FE-SEM analysis to determine the form and elemental composition of cements.

    EDS was carried out using ESCA+ (Oxford Instruments, Germany) and the resolution ratio was 0.4%. The beam size of the electron beam gun was ~75 nm in diameter. The energy resolution was 0.5 eV. The proportions of oxygen were obtained by stoichiometry (Rusk and Reed, 2002).

  • CL images were captured by a Mono CL4 system (Gatan, USA) on the SU8010 instrument under the working condition of 13.5 mm distance, 10 kV voltage and 120 μm aperture (Lupan et al., 2008; Jacobs et al., 2007).

  • Based on the element composition of the cements and the accuracy of the EPMA, we selected the appropriate mass ratio of calcium and silicon from the EPMA results as the proxies for analyzing the distribution patterns of the cements. The theoretical concentrations of calcium in Fe-bearing dolomite are ~20.6%–21.7%. Considering the accuracy of the EPMA, the color gamut corresponding to the calcium mass ratios between 17% and 25% were considered as the distribution areas of the Fe-bearing dolomite cements. The mass ratios of silicon in illite are about 23.2% to 32.1%, and the mass ratios of silicon between 27% and 30% were considered as the distribution areas of the quartz cements in the sample. The color gamut where the cement is located is extracted by the color gamut selection function of Coreldraw. We then used the IMAGEJ software to digitize the processed EPMA images into gray scale (Abramoff et al., 2004).

  • The multifractality of the shale cements in the EPMA images was investigated. Similar to pore structure analysis for soil pore systems (Bird et al., 2006) and carbonate pore systems (Xie et al., 2010b), the multifractal measures associated with cements in the digital images of shales were defined herein. Firstly, superimpose a square grid box with size δ on the parts with cements in the digital images. Then calculate the pixel number mi in each grid box with cements, where mi ranges from 1 to δ×δ. The multifractal measure, μi(δ), of the ith box covering the space of cements, is defined as mi/M, where M is the total number of cement pixels in the image. Thus, a partition function, χq(δ), with the moment q of μi(δ) can be constructed by using the method of moments to measure the multifractal properties (Bird et al., 2006; Halsey et al., 1986)

    where n(δ) is the total number of grid boxes covering the cement pixels, and Nj is the number of grid boxes containing j pixels. In this way, for a multifractal measure, a power-law relationship can be found between the partition function χq(δ) and the box size δ

    where τ(q) is defined as the mass exponent of order q, which can be obtained by plotting the data of χq(δ) and δ on log-log diagrams as the limit when δ→0. The generalized multifractal dimensions, D(q), is related to τ(q) as

    The generalized multifractal spectrum function, f(α), measure of strength of each moment, can be calculated from the mass distribution of pixels through Legendre transform (Evertsz and Mandelbrot, 1992)

    where the singularity exponent, α(q), can be effectively deduced by $ {\rm{}}\alpha \left(q \right) = \partial \tau \left(q \right)/\partial q$. For multifractal distribution patterns, the spectra of f(α) are typically humped with α-values falling into a wide range, whereas graphs between α(q) and f(α) of data from mono-fractal patterns coverage on certain values and, α(q) would vary a little bit for all grid boxes of the same sizes covering cements. Several parameters are deduced from Eq. (4), such as, $ \Delta \alpha = {\alpha _{{\rm{max}}}} - {\alpha _{{\rm{min}}}}$, $ \Delta {{f}} = {{{f}}_{{\rm{max}}}} - {{{f}}_{{\rm{min}}}}$, $ \Delta {\alpha _L} = {\alpha _0} - {\alpha _{{\rm{min}}}}$, $ \Delta {\alpha _R} = {\alpha _{{\rm{max}}}} - {\alpha _0}$, and asymmetry index $ R = \frac{{\Delta {\alpha _L} - \Delta {\alpha _R}}}{{\Delta \alpha }}$.

  • The 10 samples were mechanically crushed to particle sizes of 60–80 meshes for nitrogen adsorption measurements (Zhu et al., 2015), which were carried out using the ASAP2020 instrument (Micro Corporation, American) at the Key Lab of Carbonate Reservoirs, CNPC. The analytical gas was N2; the test model was BJH; the temperature was -196 ℃; the calculated curve was the adsorption curve (Chen and Xiao, 2014; Steins et al., 2014). The analysis method of the adsorption curve was BJH (Barrett-Joyner-Halenda), which is based on the Kelvin Equation (Li et al., 2004).

3.   RESULTS
  • The XRD results show that the brittle minerals in the Longmaxi Formation shales are mainly quartz and albite. Specifically, the brittle minerals in SQ1 account for ~76%–86% of the total rock, but only ~32%–45% in SQ2. In particular, the quartz content of SQ2 is significantly lower than that in SQ1 (Table 1). Also, the SQ1 generally lacks any carbonate minerals, whereas the SQ2 contains abundant carbonate minerals (calcite+ dolomite). Clay minerals in the Longmaxi Formation shales are mainly illite, illite-smectite mixed layer and small amounts of chlorite. Illite and illite-smectite mixed layer were products of illitization of smectite (Hu et al., 2017; Geng et al., 2016; Kong et al., 2016; Zhang et al., 2015).

    Sample Quartz Albite K-feldspar Calcite Dolomite Illite Illite-smectite mixed layer Chlorite Kaolinite Pyrite
    L10 SQ2 22.07 14.62 3.51 2.3 1.62 20.37 28.07 6.60 / 0.84
    L9 25.15 15.97 / 1.76 12.02 18.12 21.21 4.86 / 0.91
    L8 25.93 17.58 2.2 4.94 0.66 17.43 24.78 2.75 0.92 2.81
    L7 27.2 5.59 / 4 39.79 7.39 11.91 0.82 0.41 1.99
    L6 53.01 14.21 3.22 0.32 0.15 12.58 13.98 1.340 / 0.71
    L5 SQ1 72.89 5.93 / / / 8.90 12.07 0.21 / /
    L4 78.55 4.32 0.74 / / 6.06 10.32 / / /
    L3 57.64 10.85 17.47 / / 5.56 7.55 0.13 / 0.80
    L2 72.68 10.44 / / / 7.60 9.12 0.17 / /
    L1 70.84 5.6 / / / 8.95 12.72 0.71 1.18 /
    /. means not detected.

    Table 1.  Mineral composition (wt.%) of the Longmaxi Formation shales determined by XRD

    The measured peak intensities of (015) and (110) in the XRD spectrum of the sample L7 (Goldsmith and Graf, 1958) yield the degree of cation ordering of Fe-bearing dolomite at 1, suggesting that the Fe-bearing dolomite cements in this sample are stable and conform to the characteristics of buried dolomite (Samtani et al., 2001).

  • The EPMA results of samples L3 and L7 show that silicon is distributed as a continuous band at micron-scale in the concentrated area of clay minerals. No significant calcium concentration was found in sample L3 (Fig. 2a). However, calcium in sample L7 is totally concentrated in the carbonate minerals, and the calcium content in the core region of dolomite is higher than that in the annuli (Fig. 2d). The EPMA results (Figs. 2b, 2e) show that silicon and brittle minerals comprise a dense mesh, nearly blocking all inter-connected pores. The EPMA results also show the area (number) of quartz cements in sample L3 is much bigger than that in sample L7 (Figs. 2b, 2e).

    Figure 2.  Distributions of calcium and silicon in the Longmaxi Formation shales. (a) Distribution of calcium in sample L3; (b) distribution of silicon in sample L3; (c) the test area of sample L3; (d) distribution of calcium in sample L7; (e) distribution of silicon in sample L7; (f) the test area of sample L7.

  • The δ18O and δ13C compositions of sample L7 are -8.37 ‰ and -4.91‰, respectively. The δ18O and δ13C compositions of sample L9 are -8.94‰ and -4.29‰, respectively.

  • The FE-SEM results show that dolomite in the sample L7 occurs as euhedral crystals with particle sizes of ~40 μm and exhibits cloudy centers and clear rims (CCCR) (Fig. 3a). There are stylolite and anamorphic clay minerals between dolomite crystals (Fig. 3a). Closer observation shows that the cores of individual dolomite crystals are close to the ideal composition CaMg(CO3)2, whereas the rims are Fe-bearing dolomite (low-gray area) with a formula of CaMg0.69Fe0.31(CO3)2 (Fig. 4). Some Fe-bearing dolomite cements combine with each other (Fig. 3b). Part of Fe-bearing dolomite is limited by brittle minerals, leading to incomplete crystals (Fig. 3c).

    Figure 3.  Fe-bearing dolomite cements in sample L7. (a) Stylolites and anamorphic clay minerals between dolomite crystals; (b) combination of Fe-bearing dolomite cements; (c) Fe-bearing dolomite cements between minerals.

    Figure 4.  EDS results of Fe-bearing dolomite cements in sample L7.

    By observing the brittle minerals (feldspar, quartz) separated by ultrasonic crushing (Moore and Reynolds, 1989; Gipson, 1963), it is obvious that clay minerals aggregate together with the quartz cements (Figs. 5a5b).

    Figure 5.  Quartz cements in sample L3. (a) (b) Clay minerals aggregation; (c) (d) the surface of brittle minerals is neat and there is nearly no quartz cement.

  • The CL intensity of dolomite varies significantly in individual grains and between different grains. Pure dolomite with high-gray value gleams weakly, whereas Fe-bearing dolomite with low-gray value does not gleam (Fig. 6).

    Figure 6.  The CL result of dolomite in sample L7.

  • After processing the EPMA results for samples L3 and L7, the distributions of quartz and Fe-bearing dolomite cements were investigated (Figs. 79). Based on the Fraclac plug-in analysis of the processed figures, the multifractal parameters of each sample were obtained (Fig. 10, Tables 24) to characterize the spatial distribution patterns of the cements in Figs. 79. By setting q ranging from -10 to 10, the multifractal parameters were calculated. Figure 10 shows the multifractal spectrum curves for cements at micron-scales in the samples. The multifractal spectrum curves are continuous and fluctuate in a relatively broad range, indicating a heterogeneous distribution of the cements.

    Figure 7.  Distribution of quartz cements in sample L3.

    Figure 8.  Distribution of quartz cements in sample L7.

    Figure 9.  Distribution of Fe-bearing dolomite cements in sample L7.

    Figure 10.  Multifractal spectra describing the spatial distribution patterns of cements. (a) Multifractal spectra of quartz cements in sample L3, based on Fig. 7; (b) multifractal spectra of quartz cements in sample L7, based on Fig. 8; (c) multifractal spectra of Fe-bearing dolomite cements in sample L7, based on Fig. 9.

    Figure D ΔαL ΔαR Δα R Δf(α)
    Fig. 7a 1.72 0.053 0.591 0.644 -0.836 1.190
    Fig. 7b 1.74 0.168 0.591 0.759 -0.557 0.752
    Fig. 7c 1.73 0.099 0.581 0.680 -0.709 0.925
    Fig. 7d 1.75 0.048 0.584 0.632 -0.847 1.103

    Table 2.  Results of multifractal analysis of quartz cements in sample L3

    Figure D ΔαL ΔαR Δα R Δf(α)
    Fig. 8a 1.43 0.531 0.613 1.144 -0.073 0.195
    Fig. 8b 1.48 0.292 0.660 0.952 -0.389 1 0.471
    Fig. 8c 1.47 0.284 0.798 1.082 -0.482 3 0.659
    Fig. 8d 1.44 0.312 0.582 0.894 -0.306 0.196

    Table 3.  Results of multifractal analysis of quartz cements in sample L7

    Figure D ΔαL ΔαR Δα R Δf(α)
    Fig. 9a 1.47 0.198 0.575 0.773 -0.487 0.707
    Fig. 9b 1.44 0.272 0.714 0.986 -0.448 0.638
    Fig. 9c 1.45 0.189 0.593 0.782 -0.515 0.613 4
    Fig. 9d 1.47 0.167 0.652 0.819 -0.592 0.796 3

    Table 4.  Results of multifractal analysis of Fe-bearing dolomite cements in sample L7

    The multifractal parameters shown in Tables 24 reveal the spatial invariance of the cements. The multifractal parameter ΔαL reflects large cements distributed in shales, and ΔαR reflects small cements dispersed in shales. All the asymmetry index R values are smaller than 0. Also, ΔαL values are smaller than ΔαR, suggesting that the small cements are more dispersedly distributed in the Longmaxi Formation shales.

    The box-counting dimension (D) reflects the degree of regularity of morphology. The larger the box-counting dimension is, the more irregular the distribution of cements is. The value of D also reflects the complexity of the distribution of cements. The higher the value of D, the more chaotic is the distribution of cements. The D values of quartz cements in sample L3 (1.72–1.75) were significantly higher than those in sample L7 (1.43–1.48).

  • The results of nitrogen adsorption analysis show that pores in 10 samples are mainly nano-scale pores with pore diameters of less than 60 nm, and those with 2–4 nm in diameter predominate. The pore volume decreases gradually with corresponding increase in pore sizes (Figs. 11 and 12). These results can be divided into two categories: a peak of pore volume at pore size of ~350 nm (samples L1 to L5, which have no dolomite in shales) and no peak at ~300 nm (samples L6 to L10, which have dolomite in shales) (Figs. 11 and 12). In samples L6 to L10, the pore sizes are more concentrated below 50 nm and micro-pores (> 0.3 μm) are not developed (Fig. 12).

    Figure 11.  BJH results of dV/dD distribution of pore sizes of shale samples (L1–L5) from SQ1 of the Longmaxi Formation.

    Figure 12.  BJH results of dV/dD distribution of pore sizes of shale samples (L6–L10) from SQ2 of the Longmaxi Formation.

4.   DISCUSSION
  • Dolomite in the SQ2 part of the Longmaxi Formation shales generally shows cloudy centers and clear rims (CCCR), and their outer annuli are Fe-bearing dolomite (Figs. 3 and 4). The SQ2 part of the Longmaxi Formation belongs to the hemipelagic facies and carbonates are saturated in the sea (Wang et al., 2015). According to the euhedral crystal forms, corroded border of dolomite nuclei (Fig. 3) and the sedimentary facies of the Longmaxi Formation shales, the pure dolomite nuclei is interpreted to be authigenic formed during syndiagenesis but be corroded during the following diagenesis. According to the XRD results (Table 1) and previous research of mineral characteristics of the Longmaxi Formation shales (Zhao et al., 2017; Kong et al., 2016), dolomite only exists in the SQ2 part of the Longmaxi Formation shales. The combination with surrounding minerals and deformed clay minerals between Fe-bearing dolomite crystals (Fig. 3) show that Fe-bearing dolomite is not detrital in origin. What is more, the Ca/Mg ratios of the nuclei and annule of dolomite are significantly different (Figs. 3c and 4), suggesting two kinds of dolomite formed in different chemical environments. The results of CL imaging (Fig. 6) show that the Fe-bearing dolomite with Fe and Mn contents is consistent with hot-water dolomite (Gasparrini et al., 2006). The degree of cation ordering of dolomite in sample L7 is 1, which is consistent with burial dolomite (Jones et al., 2001). The C-O isotope compositions of samples L7 and L9 also match those of burial dolomite as well (Ai-Aasm and Packard, 2000; Mountjoy et al., 1999; Machel, 1997). These data collectively demonstrate that the Fe-bearing dolomite cements in the shale samples are mainly burial dolomite.

  • Quartz cements exist in both the SQ1 and SQ2 parts of the Longmaxi Formation shales. FE-SEM observations show that nano-sized quartz cements occur on the surfaces of clay minerals (Figs. 5a, 5b). However, the surfaces of brittle minerals are generally clean without significant amounts of quartz cements (Figs. 5c, 5d). Comparing the EPMA and FE-SEM results, it can be observed that the quartz cements are mainly distributed in the clay minerals accumulation area (Figs. 7, 8). Clay minerals bond with each other to form a stable mineral aggregation, with the help of quartz cements (Figs. 5a and 5b). Considering material balance of SiO2 during diagenesis and the spatial association of quartz cements and clay minerals (Fig. 2), the quartz cements were probably formed by precipitation of SiO2 released from illitization of smectite during the early phase of the middle diagenetic stage (Zhao et al., 2017; Kong et al., 2016; Bjorkum et al., 1993). According to the EPMA results (Figs. 2, 7 and 8), the contents of quartz cements in sample L3 is much higher than those in sample L7.

  • The relative timing for the formation of the two distinct types of cements in the Longmaxi Formation shales was determined by calculating the fluid volume required for the formation of Fe-bearing dolomite using the method of Land (1985). At least 443.8 unit volumes of formation water (assumed as seawater composition) are required per unit volume of CaMg0.69Fe0.31(CO3)2. This result suggests that the formation of Fe-bearing dolomite cement in the stratum of sample L7 requires an enormous amount of formation water from the adjacent strata. However, the gray levels of Fe-bearing dolomite cements are consistent under the BSD condition of FE-SEM, indicating the homogeneous element composition of crystals, which are formed with stable and continuous supplement of the source element. This reflects the high permeability of the shale primary inorganic pore structure during the formation of Fe-bearing dolomite cements. In contrast, considering the permeability of the SQ1 part of the Longmaxi Formation shales, which have only the quartz cements (Wang et al., 2013), it would be difficult for such large amount of formation water to flow through. However, no significant micron-scale pores were found in sample L7 by FE-SEM, indicating that the inter- connected pores were not blocked by the quartz cements during the formation of Fe-bearing dolomite cements; allowing the existence of some micron-scale pores. Therefore, quartz cements were formed after Fe-bearing dolomite cements. Because the Mg2+ cations in formation water from deposition are low (assumed to be normal seawater composition), the Mg2+ required in the formation of Fe-bearing dolomite cements would have come mainly from illitization of smectite (Boles and Franks, 1979) and the SiO2 required for quartz cements formation would also have come from this process. Thus, the formation of Fe-bearing dolomite cements in the Longmaxi Formation shales was only slightly earlier than that of the quartz cements, both occurred during diagenesis.

  • The two distinct types of cements in the Longmaxi Formation shales have different distribution patterns. The results of FE-SEM and EPMA show that the Fe-bearing dolomite cements exist in the form of micron-sized crystal annuli. In contrast, the quartz cements are distributed densely among clay minerals as nano-sized crystallites (Figs. 5a, 5b) and distributed as continuous banding at micron-scale (Figs. 78).

    Multifractal results help us differentiate the difference of distribution of the cements. Usually, the width of the multifractal spectrum curve, represented by Δα, reflects the degrees of multifractality. The stronger the multifractality is, the larger the Δα value will be, and vice versa (Xie and Bao, 2004). The value of Δf(α) can show the distribution patterns of relatively continuously or dispersedly distributed cements (Ouyang et al., 2015). Generally speaking, the smaller the Δf(α) value is, the larger the proportion of continuously distributed cements is. In order to discern the relationship between the parameters, a scatterplot of Δα versus Δf(α) is shown in Fig. 13. It is clear in Fig. 11 that the Δα values are smaller and the Δf(α) values reach the maximum for quartz cements in sample L3, quartz cements in sample L7 with medium Δα and Δf(α) values, whereas Fe-bearing dolomite cements in sample L7 have larger Δα and smaller Δf(α) values. In this aspect, with the weakest multifractality, the Fe-bearing dolomite cements in sample L7 distribute relatively homogeneously, but the quartz cements in samples L7 and L3 are more irregularly distributed in micro-space. The homogeneous distribution of Fe-bearing dolomite cements, which occupy micron-size primary pore spaces, affected the illitization of smectite and inhibited the release of quartz, leading to the irregular distributions of quartz cements in the SQ1 and SQ2 parts of the Longmaxi Formation shales.

    Figure 13.  Scatterplot of Δα versus Δf(α).

  • EPMA analyses (Fig. 2) show that calcium in sample L7 is highly concentrated in the micron-size carbonate minerals. The nitrogen adsorption analyses yield no peak at 300 nm of pore sizes in samples L6 to L10 (Fig. 12), but a clear peak at 300 nm of pore sizes in samples L1 to L5 (Fig. 11). These results show that, due to the presence of Fe-bearing dolomite cements, micro- pores in samples L6 to L10 are substantially filled. From the FE-SEM results, most Fe-bearing dolomite crystals are euhedral in morphology, suggesting that Fe-bearing dolomite cements tend to fill the shale primary micron-size pores (Fig. 3).

  • The results of EPMA and FE-SEM show that the quartz cements in samples are distributed mainly among clay minerals. Inter-granular pores among clay minerals are an important part of shale primary inter-connected inorganic pore structure, accounting for the vast majority of inter-connected pores (Curtis et al., 2010). Therefore, the quartz cements have blocked the inter-connected pores between clay minerals and have played a major role in the destruction of shale primary pores. According to the EPMA results (Figs. 7 and 8) and nitrogen adsorption analysis results (Figs. 11 and 12), the volume of nano-pore in shales decreases with the increasing number of the quartz cements. This further confirms that the formation of the quartz cements played the decisive role in plugging connected pores in shales. On the other hand, with the increase of the quartz cements, loose clay minerals are closely adhered together, and made the shales harder. Consequently, there are more interconnected pore spaces in the SQ2 part of the Longmaxi Formation of shales, hence having greater potential for storage and migration of gas than SQ1.

5.   CONCLUSIONS
  • (1) The distribution patterns of quartz and Fe-bearing dolomite cements in the Longmaxi Formation shales can be divided into three types directly, based on the scatterplot of multifractal parameters between Δα and Δf(α). Fe-bearing dolomite cements tend to fill micron-scale primary pores of shales. In contrast, quartz cements in the form of nano-scale crystallites tend to fill nano-scale primary pores and bond clay minerals to form stable micro-scale mineral aggregations.

    (2) The formation of Fe-bearing dolomite cements in the Longmaxi Formation shales was slightly earlier than that of quartz cements.

    (3) To a certain degree, quartz cements were more critical to the shale permeability than the Fe-bearing dolomite cements. Plugging effect of quartz cements on shale primary inorganic pore structures would significantly result in low interconnected porosity.

ACKNOWLEDGMENTS
  • This work was financially funded by the National Key R & D Program of China (No. 2016YFC0600501), the Natural Science Foundation of China (Nos. 41572315, 41872250), and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUG170104). The authors would like to thank Dr. Tianfu Zhang and Ying Wang in the Key Lab of Carbonate Reservoirs, CNPC, Hangzhou. Also the authors want to express their sincere appreciations for the English polishing and constructive suggestions of the anonymous reviewers during the peer review. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1013-7.

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