4 Discussion
In this study, the results of μ-XRF patterns show that the Ca abundance is inversely correlated with Si, Fe, Al, and K (Figs. 2-3) at sub-dm scales. The (U)SAXS results illustrate that the porosity and surface area vary across the whole sample area (Fig. 7). Shales are fine-grained rocks being deposited and accumulated under low-energy aqueous environments, and the depositional process of laminations is a function of the in situ water energy and sediment supply (O’Brien 1996; Fr´ebourg et al., 2016; Yawar and Schieber, 2017). In a similar deposition environment, the only difference between adjacent laminations is the type of sediment supply. Fig. 12 shows cross-plots of calcite from XRD vs. porosity [from (U)SAXS], surface area [from(U)SAXS], TOC (from LECO), and S2 (from pyrolysis) of 12 selected sub-samples from the C and R samples. It shows a negative relationship between calcite and porosity (correlation coefficient R2=0.7852; N=12), surface area (R2=0.4748), TOC (R2=0.7358), and S2 (R2=0.8641). This negative relationship between calcite and porosity indicates that calcite contents are inversely related to pore space. In addition to pores being observed between the mineral crystals, Louck et al. (2012) reported that pores are also presented in pyrite framboids and clay aggregates. From the XRD results and SEM images in this study, the pyrite and clay minerals are only present in the lower calcite regions. Within the pyrite framboids and clay aggregates, those intraparticle pores provide extra pore spaces and surface area in these lower calcite locations, which lead to higher porosity and surface area compared with higher-calcite spots. Reed et al. (2009) and Frebourg et al. (2016) also pointed out that the recrystallization of calcite will eliminate its original texture and the calcite overgrowth will fill the pore networks, and therefore, porosity will decrease in the higher calcite content regions. In our thin sections photos (Fig. 6 A, C, and H), the calcitic-fossils were dissolved and reprecipitated to be with crystalline calcite and only a couple of molded fossils are visible. Therefore, in the higher calcite spots, the porosity will be expected to be lower than that in the lower calcite spots.
Many other studies also reported that the organic matter can provide a certain amount of porosity to organic matter-rich shale as a result of petroleum generation (Curtis et al., 2012; Ko et al., 2016; İnan et al., 2018; Wang et al., 2020; Bai et al., 2021). However, in this Eagle Ford Shale sample, pyrolysis results indicate that organic matter is at best in the early oil window, and SEM observations show that no pores show up within the organic matter particles. Therefore, the contribution of organic matter-hosted pores to porosity and pore surface area is negligible in this sample. Overall, for this carbonate-rich Eagle Ford Shale, the increase in siliceous minerals and pyrite will lead to high porosity and surface area. Several studies have reported that high porosity is not necessarily related to high silica content (Yang et al., 2016; Wu et al., 2019; Shu et al., 2021), but might be related to clay mineral contents (Ross and Bustin, 2008; Chen et al., 2016). Calcite also shows a weak negative relationship to the surface area, but the R2 is only 0.4748. It’s been recognized that clay minerals have a much higher surface area than calcite and quartz (Clouter et al., 2001; Michot and Villieras, 2006; Montes-Hernandez et al., 2008; Kuila and Prasad, 2013). However, our Eagle Ford Shale sample has a very limited amount of clays (less than 1%) from the XRD analyses. In addition, TOC and S2 show a good negative relationship (R2= 0.7358 and 0.8641) with calcite. Many previous studies suggested that Fe can stimulate organic matter productivity (Tribovillard et al., 2015; Frebourg et al., 2016; Zhang et al., 2017). As with the clay minerals, however, the composition of pyrite cannot be accurately calculated due to its low abundance in this sample.
Since the cross-plot of calcite and porosity shows a good correlation (R2=-0.7852), the Ca signals on the μXRF maps can be directly correlated to the porosity. Based on the normalized Ca intensity data, the porosity of each lamination can be calculated and shown in Fig. 4G and H. Therefore, the sedimentary textures map (Fig. 4G and H) can be filled with calculated porosity of each lamination to generate a porosity-lamination map to reflect the porosity changes among laminations (Fig. 13). Although the porosity does not show a large change within the same lamination, the fractures often related to local tectonic activities can offset the lamination and lead to porosity variation. The local tectonic movement will stimulate the generation of fractures which can offset the a lamination by micrometers to a meter (at scales larger than meters, the fracture is called a fault). In this Eagle Ford sample, the lamination offset is limited to μm-mm scales, and it will not impact porosity measurements at the 2.5 cm-diameter core plug scale, but will affect petrophysical analyses at sample sizes at sub-mm scales (e.g., 100s μm used for gas physisorption, a common approach to determining pore size distribution). However, other studies (Gillen et al., 2019; Zhang et al., 2019; Xu et al., 2020) reported that some lamination offsets can be at the cm-m scale; this large-scale offset will lead to variable results from two cores which are distanced at meters apart.