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).