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.