Journal of Earth Science, Vol. 30, No. 5, p. 879–892, October 2019 ISSN 1674-487X Printed in China https://doi.org/10.1007/s12583-019-1013-7

Chemical Compositions and Distribution Characteristics of Cements in Longmaxi Formation Shales, Southwest China

Wenda Zhou 1, Shuyun Xie *1, Zhengyu Bao2, Emmanuel John M. Carranza3, 4, Lei Lei1, Zhenzhen Ma2 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 Cooko University, Townsville QLD 4811, Australia Wenda Zhou: https://orcid.org/0000-0001-8109-7710; Shuyun Xie: https://orcid.org/0000-0002-7443-6486

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 quartzz 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. KEEY WORDS: cement, pore structure, multifractal, shale gas reservoir, petroleum geology.

0 INTRODUCTION anti-compaction capacity of the rock and restrains the second- Cements play an important role in controlling the inor- ary enlargement of quartz, promotes dissolution, and forms ganic pore structures of shales. With increasing cement con- favorable pore structures for oil reservoirs (Ajdukiewicz and tents, primary pores in shales are filled and, consequently, their Larese, 2012; Liu et al., 2009). Siliceous cements, on the other porosity and permeability decrease (Baig et al., 2016; Walder- hand, fill the primary pores of reservoirs and decrease the per- haug et al., 2012; Ramm et al., 1997). Studying the types and meability of a sedimentary formation (Luo et al., 2015). distribution characteristics of cements in shales is, therefore, of In general, the particle sizes of shale cements are close to great significance for understanding the development of shale the nano-meter levels (Chen et al., 2016; Weinberg et al., 2011), inorganic pore structures (Dowey and Taylor, 2017; Zhou et al., making it difficult to test characteristics of cements in shale. 2017; Li et al., 2016; Zhou et al., 2016). Traditionally, X-ray diffractometry (XRD), micron compute- The study of cements in conventional reservoirs is well rized tomography (micron-CT), isotope and micro-element established. Cements in sedimentary rocks are commonly di- analysis, nuclear magnetic resonance (NMR) and other ad- vided into calcareous, siliceous, clay and iron (Sliaupa et al., vanced analytical methods had been used to investigate the type, 2008; Towe, 1962). Various types of cements have different source and formation time of shale cements, temperatures of effects on the inorganic pore structure of sedimentary rocks. diagenetic processes, and other information about shales (Ukar For example, the presence of chlorite cements may increase the et al., 2017; Ge et al., 2015; Porten et al., 2015; Walderhaug et al., 2009; Midtbø et al., 2000). Other analytical techniques such *Corresponding author: [email protected] as field emission scanning electron microscopy (FE-SEM) and © China University of Geosciences (Wuhan) and Springer-Verlag infrared spectroscopy also have been used to show that quartz GmbH Germany, Part of Springer Nature 2019 cements in Late Cretaceous shales from the northern North Sea are nano-particles in size and are distributed among clay min- Manuscript received November 3, 2018. erals, filling nano-pore spaces (Thyberg et al., 2010; Peltonen Manuscript accepted February 24, 2019. et al., 2009; Worden et al., 2005). Multifractal analysis has

Zhou, W. D., Xie, S. Y., Bao, Z. Y., et al., 2019. Chemical Compositions and Distribution Characteristics of Cements in Longmaxi Formation Shales, Southwest China. Journal of Earth Science, 30(5): 879–892. https://doi.orgg/10.1007/s12583-019-1013-7. http://en.earth-science.net 880 Wenda Zhou, Shuyun Xie, Zhengyu Bao, Emmanuel John M. Carranza, Lei Lei and Zhenzhen Ma been wildly applied to quantify pore structures and element Xishui County, Guizhou Province, southeastern Sichuan Basin distribution patterns in different media (Torre et al., 2018; Liu (Fig. 1). Based on lithologies and depositional environment, the and Ostadhassan, 2017; Vega and Jouini, 2015; Xie et al., Longmaxi Formation has been divided into two parts––the

2010a; Mandelbrot, 1977). In this paper, multifractal will be upper SQ2 and the lower SQ1––which are distinguished by the used to quantitatively study the distribution characteristics and existence and absence of carbonate minerals in the former and formation time of cements in shales. the latter, respectively (Wu et al., 2016; Wang et al., 2015; Li et The Longmaxi Formation is an important shale gas reser- al., 2012). Based on lithofacies variations of the Longmaxi voir in Southwest China and has attracted a lot of attention Formation (Liang et al., 2016), which indicate that the sedi- among researchers in recent years (Zhou et al., 2018; Chen et mentary depositional environment of this formation is relative- al., 2017; Liang et al., 2017; Ye et al., 2017). Recent studies ly stable and the types of cements in its shale units are simple, suggested that cements in the Longmaxi Formation shales are 10 samples of Longmaxi Formation shales were collected for mainly composed of quartz derived from illitization of smectite this study from the Lucheng profile (Fig. 1). Five samples (L1 (Zhao et al., 2017; Kong et al., 2016). Also, the mineral assem- to L5) were taken in sequence at roughly equal intervals from blages of the Longmaxi Formation shales are broadly similar to SQ1 (which is 35 m thick) with sample L1 at the bottom of SQ1, those of other shale gas reservoirs (e.g., Wufeng Formation) in whereas five samples (L6 to L10) were collected in sequence at

China (Yang et al., 2017). However, the distribution characte- roughly equal intervals from SQ2 (which is 45 m thick) with ristics and formation time of cements in the Longmaxi Forma- sample L10 at the top of SQ2. Field observations of the mate- tion shales and other Chinese shale gas reservoirs remain poor- rials in SQ1 and SQ2 are relatively homogeneous, and so the 10 ly understood. samples collected from the 80-m thick Lucheng profile are Accordingly, we initiated a detailed investigation on ce- considered representative for analysis. ments in shales from the Longmaxi Formation using a large Samples for FE-SEM, EPMA, EDS and CL analyses were number of analytical techniques from XRD to electron probe polished by argon ion, which will not cause mechanical dam- micro-analysis (EPMA), C-O isotope analysis, FE-SEM, ener- age to the samples (Stevens et al., 2011). gy dispersive spectrometry (EDS), and cathodoluminescence (CL). Results reported herein are intended to further evaluate 2 METHODS the mineralogy of the cements in the Longmaxi shales, includ- 2.1 XRD ing the discovery of a new cement type (i.e., Fe-bearing dolo- The XRD analysis, which was applied in this study to de- mite). Multifractal analysis has been used to quantitatively termine the mineralogical composition of the shale samples, determine the spatial distribution characteristics of the two was carried out at the Faculty of Materials Science and Chemi- distinct types of shale cements. In addition, the effects of the stry, China University of Geosciences, Wuhan. XRD patterns Fe-bearing dolomite and quartz cements on the primary inor- of the samples were recorded by a D8-FOCUS X-ray Diffrac- ganic pore structures of shale have been evaluated as well. tometer (Bruker AXS, Germany), equipped with a Lynx-Eye Detector with Co Kα radiation at 35 kV and 40 mA. Specifi- 1 SAMPLES cally, the XRD analysis was divided into two parts: whole-rock Representative samples of the Longmaxi Formation shales analysis and clay mineral analysis. The separation of clay min- were collected from the Lucheng Village profile located in the erals from the shale samples was based on the Stokes law in

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.

Chemical Compositions and Distribution Characteristics of Cements in Longmaxi Formation Shales, Southwest China 881 water (Jiang et al., 2017; Bettison-Varga et al., 1991). 2.5 CL Identification of minerals was made using the Evaluation CL images were captured by a Mono CL4 system (Gatan, (EVA) phase analysis software (Bruker AXS, Germany) by USA) on the SU8010 instrument under the working condition comparison with reference mineral patterns archived in the of 13.5 mm distance, 10 kV voltage and 120 μm aperture (Lu- Powder Diffraction Files of the International Centre for Dif- pan et al., 2008; Jacobs et al., 2007). fraction Data and other available databases. Quantitative analy- sis was carried out using TOPAS (Bruker AXS, Germany), a 2.6 Multifractal Analysis PC-based program for Rietveld refinements of the XRD spectra 2.6.1 Image processing (Puphaiboon et al., 2013). Based on the element composition of the cements and the accuracy of the EPMA, we selected the appropriate mass ratio 2.2 EPMA of calcium and silicon from the EPMA results as the proxies for Two samples (L3 and L7) were selected for detailed analyzing the distribution patterns of the cements. The theoret- EMPA analyses on a JXA-8100 electron probe micro-analyzer ical concentrations of calcium in Fe-bearing dolomite are (JEOL, Japan). Specifically, sample L7, which has the highest ~20.6%–21.7%. Considering the accuracy of the EPMA, the dolomite content of 39.79%, was selected for investigating the color gamut corresponding to the calcium mass ratios between dolomite and quartz cements in the SQ2 part of the Longmaxi 17% and 25% were considered as the distribution areas of the Formation. Sample L3 from the middle part of SQ1 was se- Fe-bearing dolomite cements. The mass ratios of silicon in illite lected for studying the distribution of the quartz cements. Res- are about 23.2% to 32.1%, and the mass ratios of silicon be- olution of the EMPA analysis is about 1 μm in diameter. De- tween 27% and 30% were considered as the distribution areas pending on the content of the element of all area, values of of the quartz cements in the sample. The color gamut where the element content from the highest to the lowest are assigned to cement is located is extracted by the color gamut selection the corresponding color averagely. White corresponds to the function of Coreldraw. We then used the IMAGEJ software to highest element content, and black corresponds to the lowest. digitize the processed EPMA images into gray scale (Abramoff The analytical temperature was 20 ºC and the analytical et al., 2004). conditions/procedures were similar to those described in Lavrent’Ev et al. (2015) and Korolyuk (2008). 2.6.2 Multifractal analysis The multifractality of the shale cements in the EPMA im- 2.3 C-O Isotope Analysis ages was investigated. Similar to pore structure analysis for soil It is difficult to separate dolomite in shales. Therefore, the pore systems (Bird et al., 2006) and carbonate pore systems C-O isotope analysis of dolomite was made by using the (Xie et al., 2010b), the multifractal measures associated with powders of the whole rocks. To ensure reliable data, samples cements in the digital images of shales were defined herein. L7 and L9 were chosen for C-O isotope analysis because they Firstly, superimpose a square grid box with size δ on the parts had the highest dolomite contents. The C-O isotope analysis with cements in the digital images. Then calculate the pixel was achieved using the Finnigan MAT 253 Gas Isotope Mass number mi in each grid box with cements, where mi ranges Spectrometer (ThermoFisher, Germany) at the Key Lab of from 1 to δ×δ. The multifractal measure, µi(δ), of the ith box Carbonate Reservoirs, CNPC. The results of the analysis were covering the space of cements, is defined as mi/M, where M is relative to the PDB standard and were reported as δ(‰) with the total number of cement pixels in the image. Thus, a parti- 13 18 assumed δ C and δ O values of 0.04‰ and 0.08‰, respec- tion function, χq(δ), with the moment q of µi(δ) can be con- tively (Li et al., 2013; Bao and Thiemens, 2000). The structed by using the method of moments to measure the multi- GBW-04405 was taken as the standard sample. fractal properties (Bird et al., 2006; Halsey et al., 1986)

χ () = ∑()( ) = ∑()( ) = ∑× ( ) (1) 2.4 FE-SEM and EDS Two representative samples (L3 and L7) were selected for where n(δ) is the total number of grid boxes covering the ce- FE-SEM and EDS analysis. FE-SEM imaging was made on a ment pixels, and Nj is the number of grid boxes containing j SU8010 instrument (HITACHI, Japan), with the working dis- pixels. In this way, for a multifractal measure, a power-law tances from 7 to 17 mm and acceleration voltages of 5 to 15 kV relationship can be found between the partition function χq(δ) (Liu et al., 2017; Tan et al., 2015). The analytical mode was and the box size δ back-scattered diffraction (BSD). The resolution is about 400 () nm. Minerals in the sample L3 were observed by FE-SEM after () ∝ (2) ultrasonic mineral separation (Moore and Reynolds, 1989; where τ(q) is defined as the mass exponent of order q, which Gipson, 1963). However, the as-is sample L7 was used for can be obtained by plotting the data of χ (δ) and δ on log-log FE-SEM analysis to determine the form and elemental compo- q diagrams as the limit when δ→0. The generalized multifractal sition of cements. dimensions, D(q), is related to τ(q) as EDS was carried out using ESCA+ (Oxford Instruments, () Germany) and the resolution ratio was 0.4%. The beam size of () = ( ≠ 1) (3) the electron beam gun was ~75 nm in diameter. The energy resolution was 0.5 eV. The proportions of oxygen were ob- The generalized multifractal spectrum function, f(α), tained by stoichiometry (Rusk and Reed, 2002). measure of strength of each moment, can be calculated from

882 Wenda Zhou, Shuyun Xie, Zhengyu Bao, Emmanuel John M. Carranza, Lei Lei and Zhenzhen Ma the mass distribution of pixels through Legendre transform sizes of 60–80 meshes for nitrogen adsorption measurements (Evertsz and Mandelbrot, 1992) (Zhu et al., 2015), which were carried out using the ASAP2020 instrument (Micro Corporation, American) at the Key Lab of ()=()−() (4) Carbonate Reservoirs, CNPC. The analytical gas was N2; the where the singularity exponent, α(q), can be effectively de- test model was BJH; the temperature was -196 ºC; the calcu- duced by () =()/. For multifractal distribution pat- lated curve was the adsorption curve (Chen and Xiao, 2014; terns, the spectra of f(α) are typically humped with α-values Steins et al., 2014). The analysis method of the adsorption falling into a wide range, whereas graphs between α(q) and f(α) curve was BJH (Barrett-Joyner-Halenda), which is based on the of data from mono-fractal patterns coverage on certain values Kelvin Equation (Li et al., 2004). and, α(q) would vary a little bit for all grid boxes of the same sizes covering cements. Several parameters are deduced from 3 RESULTS 3.1 Mineral Composition Eq. (4), such as, ∆ = − , ∆ƒ = ƒ − ƒ , The XRD results show that the brittle minerals in the ∆ = −, ∆ = −, and asymmetry index Longmaxi Formation shales are mainly quartz and albite. Spe- ∆ ∆ cifically, the brittle minerals in SQ account for ~76%–86% of = . 1 ∆ the total rock, but only ~32%–45% in SQ2. In particular, the

quartz content of SQ2 is significantly lower than that in SQ1 2.7 Nitrogen Adsorption (Table 1). Also, the SQ1 generally lacks any carbonate minerals, The 10 samples were mechanically crushed to particle

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

Sample Quartz Albite K-feldspar Calcite Dolomite Illite Illite-smectite Chlorite Kaolinite Pyrite mixed layer

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.

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.

Chemical Compositions and Distribution Characteristics of Cements in Longmaxi Formation Shales, Southwest China 883

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.

whereas the SQ2 contains abundant carbonate minerals (calcite+ son, 1963), it is obvious that clay minerals aggregate together dolomite). Clay minerals in the Longmaxi Formation shales are with the quartz cements (Figs. 5a–5b). mainly illite, illite-smectite mixed layer and small amounts of chlorite. Illite and illite-smectite mixed layer were products of 3.5 CL Results illitization of smectite (Hu et al., 2017; Geng et al., 2016; Kong The CL intensity of dolomite varies significantly in indi- et al., 2016; Zhang et al., 2015). vidual grains and between different grains. Pure dolomite with The measured peak intensities of (015) and (110) in the high-gray value gleams weakly, whereas Fe-bearing dolomite XRD spectrum of the sample L7 (Goldsmith and Graf, 1958) with low-gray value does not gleam (Fig. 6). yield the degree of cation ordering of Fe-bearing dolomite at 1, suggesting that the Fe-bearing dolomite cements in this sample 3.6 Results of Multifractal Analysis are stable and conform to the characteristics of buried dolomite After processing the EPMA results for samples L3 and L7, (Samtani et al., 2001). the distributions of quartz and Fe-bearing dolomite cements

3.2 Elemental Composition The EPMA results of samples L3 and L7 show that silicon is distributed as a continuous band at micron-scale in the con- centrated area of clay minerals. No significant calcium concen- tration 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 sam- ple L3 is much bigger than that in sample L7 (Figs. 2b, 2e).

3.3 C-O Isotope Composition 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.

3.4 FE-SEM and EDS Results 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 min- erals, leading to incomplete crystals (Fig. 3c). By observing the brittle minerals (feldspar, quartz) sepa- rated by ultrasonic crushing (Moore and Reynolds, 1989; Gip- Figure 4. EDS results of Fe-bearing dolomite cements in sample L7.

884 Wenda Zhou, Shuyun Xie, Zhengyu Bao, Emmanuel John M. Carranza, Lei Lei and Zhenzhen Ma

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.

Figure 6. The CL result of dolomite in sample L7. were investigated (Figs. 7–9). Based on the Fraclac plug-in The box-counting dimension (D) reflects the degree of analysis of the processed figures, the multifractal parameters of regularity of morphology. The larger the box-counting dimen- each sample were obtained (Fig. 10, Tables 2–4) to characterize sion is, the more irregular the distribution of cements is. The the spatial distribution patterns of the cements in Figs. 7–9. By value of D also reflects the complexity of the distribution of setting q ranging from -10 to 10, the multifractal parameters cements. The higher the value of D, the more chaotic is the were calculated. Figure 10 shows the multifractal spectrum distribution of cements. The D values of quartz cements in curves for cements at micron-scales in the samples. The multi- sample L3 (1.72–1.75) were significantly higher than those in fractal spectrum curves are continuous and fluctuate in a rela- sample L7 (1.43–1.48). tively broad range, indicating a heterogeneous distribution of the cements. 3.7 Results of Nitrogen Adsorption Analysis The multifractal parameters shown in Tables 2–4 reveal The results of nitrogen adsorption analysis show that pores the spatial invariance of the cements. The multifractal parame- in 10 samples are mainly nano-scale pores with pore diameters ter ΔαL reflects large cements distributed in shales, and ΔαR of less than 60 nm, and those with 2–4 nm in diameter predo- reflects small cements dispersed in shales. All the asymmetry minate. The pore volume decreases gradually with correspond- index R values are smaller than 0. Also, ΔαL values are smaller ing increase in pore sizes (Figs. 11 and 12). These results can than ΔαR, suggesting that the small cements are more disper- be divided into two categories: a peak of pore volume at pore sedly distributed in the Longmaxi Formation shales. size of ~350 nm (samples L1 to L5, which have no dolomite in

Chemical Compositions and Distribution Characteristics of Cements in Longmaxi Formation Shales, Southwest China 885

Figure 7. Distribution of quartz cements in sample L3.

Figure 8. Distribution of quartz cements in sample L7.

886 Wenda Zhou, Shuyun Xie, Zhengyu Bao, Emmanuel John M. Carranza, Lei Lei and Zhenzhen Ma

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

Table 2 Results of multifractal analysis of quartz cements in sample L3 shales) and no peak at ~300 nm (samples L6 to L10, which have dolomite in shales) (Figs. 11 and 12). In samples L6 to Figure D Δα Δα Δα R Δf(α) L R L10, the pore sizes are more concentrated below 50 nm and Fig. 7a 1.72 0.053 0.591 0.644 -0.836 1.190 micro-pores (＞0.3 μm) are not developed (Fig. 12). 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 4 DISCUSSION Fig. 7d 1.75 0.048 0.584 0.632 -0.847 1.103 4.1 Cement Types in the Longmaxi Formation Shales 4.1.1 Fe-bearing dolomite cements Table 3 Results of multifractal analysis of quartz cements in sample L7 Dolomite in the SQ2 part of the Longmaxi Formation shales generally shows cloudy centers and clear rims (CCCR), Figure D ΔαL ΔαR Δα R Δf(α) Fig. 8a 1.43 0.531 0.613 1.144 -0.073 0.195 and their outer annuli are Fe-bearing dolomite (Figs. 3 and 4). Fig. 8b 1.48 0.292 0.660 0.952 -0.389 1 0.471 The SQ2 part of the Longmaxi Formation belongs to the hemi- Fig. 8c 1.47 0.284 0.798 1.082 -0.482 3 0.659 pelagic facies and carbonates are saturated in the sea (Wang et Fig. 8d 1.44 0.312 0.582 0.894 -0.306 0.196 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 Table 4 Results of multifractal analysis of Fe-bearing dolomite interpreted to be authigenic formed during syndiagenesis but be cements in sample L7 corroded during the following diagenesis. According to the

Figure D ΔαL ΔαR Δα R Δf(α) XRD results (Table 1) and previous research of mineral cha- Fig. 9a 1.47 0.198 0.575 0.773 -0.487 0.707 racteristics of the Longmaxi Formation shales (Zhao et al.,

Fig. 9b 1.44 0.272 0.714 0.986 -0.448 0.638 2017; Kong et al., 2016), dolomite only exists in the SQ2 part Fig. 9c 1.45 0.189 0.593 0.782 -0.515 0.613 4 of the Longmaxi Formation shales. The combination with sur- Fig. 9d 1.47 0.167 0.652 0.819 -0.592 0.796 3 rounding minerals and deformed clay minerals between Fe-bearing dolomite crystals (Fig. 3) show that Fe-bearing

Chemical Compositions and Distribution Characteristics of Cements in Longmaxi Formation Shales, Southwest China 887

dolomite is not detrital in origin. What is more, the Ca/Mg CaMg0.69Fe0.31(CO3)2. This result suggests that the formation of ratios of the nuclei and annule of dolomite are significantly Fe-bearing dolomite cement in the stratum of sample L7 re- different (Figs. 3c and 4), suggesting two kinds of dolomite quires an enormous amount of formation water from the adja- formed in different chemical environments. The results of CL cent strata. However, the gray levels of Fe-bearing dolomite imaging (Fig. 6) show that the Fe-bearing dolomite with Fe and cements are consistent under the BSD condition of FE-SEM, Mn contents is consistent with hot-water dolomite (Gasparrini indicating the homogeneous element composition of crystals, et al., 2006). The degree of cation ordering of dolomite in sam- ple 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 ce- ments in the shale samples are mainly burial dolomite.

4.1.2 Quartz cements

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 miner- als (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 miner- als 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 asso- ciation of quartz cements and clay minerals (Fig. 2), the quartz cements were probably formed by precipitation of SiO2 re- leased 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.

4.2 Formation Time of Cements The relative timing for the formation of the two distinct types of cements in the Longmaxi Formation shales was deter- Figure 10. Multifractal spectra describing the spatial distribution patterns of mined by calculating the fluid volume required for the forma- cements. (a) Multifractal spectra of quartz cements in sample L3, based on tion of Fe-bearing dolomite using the method of Land (1985). Fig. 7; (b) multifractal spectra of quartz cements in sample L7, based on Fig. At least 443.8 unit volumes of formation water (assumed as 8; (c) multifractal spectra of Fe-bearing dolomite cements in sample L7, seawater composition) are required per unit volume of based on Fig. 9.

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

888 Wenda Zhou, Shuyun Xie, Zhengyu Bao, Emmanuel John M. Carranza, Lei Lei and Zhenzhen Ma

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

4.3 Distribution of Cements in Shales The two distinct types of cements in the Longmaxi For- mation shales have different distribution patterns. The results of FE-SEM and EPMA show that the Fe-bearing dolomite ce- ments exist in the form of micron-sized crystal annuli. In con- trast, 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. 7–8). Multifractal results help us differentiate the difference of distribution of the cements. Usually, the width of the multifrac- tal 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 val- Figure 13. Scatterplot of Δα versus Δf(α). ue of Δf(α) can show the distribution patterns of relatively con-

tinuously or dispersedly distributed cements (Ouyang et al., which are formed with stable and continuous supplement of the 2015). Generally speaking, the smaller the Δf(α) value is, the source element. This reflects the high permeability of the shale larger the proportion of continuously distributed cements is. In primary inorganic pore structure during the formation of order to discern the relationship between the parameters, a Fe-bearing dolomite cements. In contrast, considering the per- scatterplot of Δα versus Δf(α) is shown in Fig. 13. It is clear in meability of the SQ part of the Longmaxi Formation shales, 1 Fig. 11 that the Δα values are smaller and the Δf(α) values reach which have only the quartz cements (Wang et al., 2013), it the maximum for quartz cements in sample L3, quartz cements would be difficult for such large amount of formation water to in sample L7 with medium Δα and Δf(α) values, whereas flow through. However, no significant micron-scale pores were Fe-bearing dolomite cements in sample L7 have larger Δα and found in sample L7 by FE-SEM, indicating that the inter- smaller Δf(α) values. In this aspect, with the weakest multifrac- connected pores were not blocked by the quartz cements during tality, the Fe-bearing dolomite cements in sample L7 distribute the formation of Fe-bearing dolomite cements; allowing the relatively homogeneously, but the quartz cements in samples existence of some micron-scale pores. Therefore, quartz ce- L7 and L3 are more irregularly distributed in micro-space. The ments were formed after Fe-bearing dolomite cements. Because homogeneous distribution of Fe-bearing dolomite cements, the Mg2+ cations in formation water from deposition are low which occupy micron-size primary pore spaces, affected the (assumed to be normal seawater composition), the Mg2+ re- illitization of smectite and inhibited the release of quartz, lead- quired in the formation of Fe-bearing dolomite cements would ing to the irregular distributions of quartz cements in the SQ have come mainly from illitization of smectite (Boles and 1 and SQ parts of the Longmaxi Formation shales. Franks, 1979) and the SiO required for quartz cements forma- 2 2 tion would also have come from this process. Thus, the forma- 4.4 Influence of Cements on Primary Inorganic Pore tion of Fe-bearing dolomite cements in the Longmaxi Forma- Structure of Shales tion shales was only slightly earlier than that of the quartz ce- 4.4.1 Influence of Fe-bearing dolomite cements on prima- ments, both occurred during diagenesis. ry inorganic pore structure of shales

EPMA analyses (Fig. 2) show that calcium in sample L7 is

Chemical Compositions and Distribution Characteristics of Cements in Longmaxi Formation Shales, Southwest China 889 highly concentrated in the micron-size carbonate minerals. The the authors want to express their sincere appreciations for the nitrogen adsorption analyses yield no peak at 300 nm of pore English polishing and constructive suggestions of the anonymous sizes in samples L6 to L10 (Fig. 12), but a clear peak at 300 nm reviewers during the peer review. The final publication is availa- of pore sizes in samples L1 to L5 (Fig. 11). These results show ble at Springer via https://doi.org/10.1007/s12583-019-1013-7. that, due to the presence of Fe-bearing dolomite cements, micro- pores in samples L6 to L10 are substantially filled. From the REFERENCES CITED FE-SEM results, most Fe-bearing dolomite crystals are euhedral Al-Aasm, I. S., Packard, J. J., 2000. 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