3. Results
3.1 Areal heterogeneity of sub-decimeter-sized samples
3.1.1 Elemental distribution
The results of powder-sized samples show the average mineral
compositions of the whole sample (Table 1). These results indicate that
the samples used in this study are carbonate-rich with small amounts of
quartz and a very small amount of pyrite & clay minerals. Therefore, in
the following μ-XRF mapping test, only the elemental composition data
related to the detected main minerals are presented, with tight elements
selected.
In XRF images (Figs. 2-3), the higher the concentration is, the brighter
the color is. Based on the mineral compositions provided by XRD results
for the powder sample, Ca being detected by the μ-XRF is present only in
calcite (CaCO3), Fe is in pyrite (FeS2),
Si is from siliceous minerals such as quartz (SiO2) and
clay minerals, and Al, Na & K are present in clay minerals. Since the
clay minerals only account for a very small amount of the total minerals
(Table 1), the Si signal will be mostly due to quartz, so the detection
of Al, Na, and K is used to characterize the clay minerals distribution.
Both Mg and Mn are used to reflect the potential presence of ankerite
[Ca(Fe, Mg, Mn)(CO3)2] or dolomite
[CaMg(CO3)2]. For both C and R
samples (Figs. 2-3), strong laminations are seen for high concentrations
of Ca, Si, and Fe, slight laminations are observed for Al and K, and no
laminations are present for Mg, Mn, and Na. To quantify the distribution
of three most abundant elements (Ca, Si, and Fe), the intensities of
scanned areas are normalized to the highest intensity observed for each
element (Fig. 4). In Fig. 4A, the normalized Ca distribution map of the
C sample shows that Ca is rich in all scanned areas with multiple
laminations across the sample. The difference in the relative intensity
(RI) between higher Ca areas (yellow color, ~80% RI)
and lower Ca (green color, ~60%) area is relatively
small. The area with higher Ca concentrations is in the upper two-thirds
of the sample, whereas the lower Ca concentrations occur in the bottom
portion of one-third (Fig. 4A). The normalized Si distribution map (Fig.
4B) shows that areas with higher Si concentration are located at the
sample bottom, and lower Si concentrations occur in the middle and upper
regions of the sample. The normalized distribution map for Fe (Fig. 4C)
shows very high intensity (~80-100% RI) at the sample
bottom, but only a low intensity (~0-20% RI) for the
rest of the sample.
For the normalized distribution maps of the R sample (Fig. 4D-F), the
areas with higher Ca concentrations (~60-100% RI)
appear in the middle of sample, while the areas with lower Ca
concentrations (~0-20% RI) are located on the left and
right sides (Fig. 4D). The higher concentration areas of Si
(~80-100% RI) and Fe (~60%-100 RI)
occur on the right-hand side (Fig. 4E-F). A comparison of these three
normalized maps shows that the area with high RI for Ca also has low
concentrations of Si and Fe; similarly, the area with low Ca
concentration has high Si and Fe concentrations.
Once the Ca, Si, and Fe data are normalized, the sedimentary features,
such as laminations and fractures, can be identified and marked on Figs.
4G and H for the C and R samples, respectively. In Fig. 4G, several
fractures cut through the laminations in the C sample and offset the
laminations on the right side by 3-5 μm. In the R sample (Fig. 4H),
fractures cut though the laminations and lead to an offset of
laminations.
3.1.2 Sedimentary textures
Thin section petrography was carried out to determine the textural and
mineral compositional changes across two C and R samples. Four positions
with different Si/Ca ratios (following the normalized map in Fig. 4) on
each sample were selected for petrographic microscopy (Fig. 5). Thin
sections of the small rectangular areas marked as A-H in Fig. 5 are
shown in Fig. 6A-H. Under a plane-polarized light, these sub-samples
show as either yellow (calcite) and brown-black color (quartz, clay,
pyrite, and organic matter). Unlike other fossil-rich Eagle Ford Shale
samples being collected from both wells and outcrop (Pommer and
Milliken, 2015; Lehrmann et al., 2019; Reed et al., 2019; Wang et al.,
2021a), fossils are only occasionally found in this sample. In the high
Ca regions (A and E), the yellow color is dominant, whereas in the high
Si regions (D and H), the proportion of brown-black color increases
dramatically. In other thin section views cutting across Ca-Si mixed
laminations (B, C, F, and G), the ratios of yellow/brown-black are
intermediate. Based on the thin section petrographic observations, the
samples do not show a lithological change at sub-cm scales, though there
is a variation in compositions, evidenced from different ratios of
yellow/black-brown colors.
3.1.3 Pore structure
The pore structure of the C and R samples was investigated with (U)SAXS
techniques. The porosity distribution and surface area for each sample
are presented in Fig. 7. Overall, the porosity of the C sample ranges
from 0.82 to 3.04%, with an average of 1.72±0.36% (N=132). The high
porosity region is mainly located at the bottom region. The porosity of
the R sample ranges from 0.93 to
2.50% and has an average of 1.61±0.35% (N=72). The high porosity
region occurs at the right and bottom regions. The surface area
distribution of these two samples is spatially mapped out as well from
(U)SAXS data. The surface area of the C sample is
1.51-14.1 m2/g
with an average of 6.52±1.74 m2/g (N=132), similar to
the R sample of 6.89±1.52 m2/g (N=72). While the
distributions of porosity and surface area are similar, there are still
some differences. For example, Positions 11 (coordinates: x=6 and y=6)
and 20 (coordinates: x=4 and y=6) have the lowest porosities on the C
sample, but their surface areas are not the lowest (Fig. 7 A and C). For
the R sample, Positions 27 (coordinates: x=2 and y=0) and 30
(coordinates: x=8 and y=0) do not show the highest porosity, but they
have the highest surface area (Fig. 7B and D)
3.2 Areal heterogeneity of selected sub-samples of large C and R samples
This section looks at the differences in mineral compositions, pore
types, organic matter quantity (TOC) and quality (pyrolysis), as well as
their influences on the pore size and surface area distributions. Six
sub-samples (1 cm×1 cm×0.8 mm) on both C and R samples with high
differences in Ca/Si intensity, porosity, and surface area were selected
and cut out for XRD and SEM analyses and then crushed to powder for TOC
and pyrolysis analyses. WAXS was conducted for the large-sized samples
before the rock chips were cut out to validate the XRD results at
different sampling scales (XRD: 1 cm × 1 cm; WAXS: 0.8 mm × 0.2 mm). For
the C sample, a total of six sub-samples were selected from Positions 3
(named C3), 10, 16, 22, 34, and 37. Similarly, six sub-samples from the
R sample were chosen from Positions 15, 22, 24, 26, 28, and 30 (Fig. 5;
Table 2).
3.2.1 Mineral compositions and organic matters
The results of mineral compositions, TOC, and pyrolysis of these 12
sub-samples are shown in Table 1. Due to their low abundance (0.7% from
powder sample), the clay minerals are neglected during the XRD mineral
composition calculation, as the calculated values for a very small
amount may have a large uncertainty. C3, C10, C16, and C21 have
relatively higher calcite (91.7 to 94.8 wt.%) and lower quartz (5.2 to
6.9 wt.%), whereas the C34 and C37 positions have relatively lower
calcite (85.6 and 87.0 wt.%) and higher quartz (10.8 and 11.7 wt.%).
Pyrite only appears in sub-samples ofC16, C21, and C37, with contents
ranging from 1.3 to 3.7 wt.%. R22 and R26 show a detection of
relatively low quartz (6.3 and 7.6 wt.%), high calcite (93.7 and 92.4
wt.%), and no pyrite. In contrast, R15, R24, R28, and R30 sub-samples
show higher quartz (11.1-15.5 wt.%) and lower calcite (82.7-88.9
wt.%). The higher quartz rock-chips are commonly found in sub-samples
with more abundant pyrite (ranging from 1.1 to 2.3 wt.%), with an
exception of R28. Even though the quantification function for mineral
composition from WAXS data has not been developed, its function can help
to determine what types of minerals appearing in the sampling location
of 0.8 mm × 0.2 mm. Results show that the higher calcite samples barely
contain pyrite along with a low intensity of kaolinite and quartz (Fig.
8A and C), whereas the lower calcite samples have higher intensities of
pyrite, kaolinite, and quartz (Fig. 8B and D). In addition, Table 1
shows that TOC values are higher in sub-samples with lower calcite for
both R and C samples. The pyrolysis results show that all of 12
sub-samples have low values of S1 (free hydrocarbons) and S3
(CO2 yield during pyrolysis from kerogen). Low S1 values
may be due to the fact that this being an outcrop sample, and free
hydrocarbons have evaporated off or been weathered away. The S2 values
(mass of hydrocarbons per gram of rock generated during pyrolysis)
increase with an increasing TOC content and, therefore, the hydrogen
index (HI) ranges from 538 to 769. The Tmax values
(434-435 oC) from the pyrolysis are very similar as
would be expected, indicating that this Eagle Ford Shale from outcrop is
likely in the very early oil generation stage (Yang and Horsfield,
2020).
3.2.2 Pore types
Fig. 9 presents the SEM images of six sub-samples taken from the C
sample (shown in Fig. 5). In the higher calcite sub-samples (C3, C10,
C16, and C21; Table 1), the pores are mostly interparticle between
calcite and quartz (Fig. 9A, B, and D). In some parts of the solid rock
matrix, there is a limited abundance of pores (Fig. 9C). In the
sub-samples with lower calcite contents (C34 and C37), the pores are
primarily interparticle and intraparticle in types. The interparticle
pores appear between calcite, quartz, pyrite, and clay minerals, whereas
the intraparticle pores are present inside pyrite framboids and clay
aggregates (Fig. 9E-F and H-I). Due to its low maturity, the organic
matter does not contain any pores (Fig. 9G). Similarly, Fig. 10 shows
the SEM images of six sub-samples from the R sample. The higher calcite
sub-samples (R22 and R26) contain dominantly interparticle pores, and
only a few pores appear in the solid matrix (Fig. 10B and D). In the
lower calcite sub-samples (R15, R24, R28, and R30), pores are mainly
observed between mineral crystals (Fig. 10A and G); in addition, pyrite
framboids and clay aggregates can provide pore spaces (Fig. 10E, H, and
I). The quartz in the SEM images is mainly secondary (formed as cement
during diagenesis) with a good crystal form, indicating that it was
probably from dissolved silica and precipitated as quartz cement (Fig.
9B and D; Fig 10E).
3.2.3 Pore structure
Fig. 11 shows the relationship of pore diameter with either incremental
porosity or surface area of six beam spots of highest porosity in each
subsample for C and R samples, whereas Table 2 gives the total porosity
and surface area as well as their distributions. Pores in the C sample
are dominated by diameters in the 100-1000 nm range. The higher calcite
locations (C3A, C10B, C16A, and C21B; all >91%) show two
major peaks in the 200-400 nm and 500-1000 nm ranges, whereas the lower
calcite locations (C34A and H37A; <87%) exhibit two major
peaks at 100-200 nm and 400-1000 nm. Unlike other locations, the C37A
shows two peaks in the 400-1000 nm range. The surface area of all
sampling locations mostly falls in the 1-10 nm range, with two main
peaks located at 1-4 nm and 5-7 nm. The higher calcite beam spots (C3A,
C10B, C16A, and C21B) show similar peaks in intensity and pore diameter
range; on the contrary, the lower calcite C34A spot has no peaks between
5-7 nm, and rather a high peak at 10-12 nm. Furthermore, for the R
sample, both higher calcite spots (R22A and R26A; >92%)
and lower calcite spots (R15A, R24C, R28A, and R30A; <89%)
have peaks at similar pore diameters, but they have different
intensities and widths. Moreover, the incremental surface areas of six
locations have similar peaks but different intensity as well (Fig. 11).