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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).

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