2. Methods
2.1 Sample collection and preparation
A Cretaceous-aged Eagle Ford Shale sample was collected from an outcrop outside of Del Rio city, TX. First the outcrop was cored as a cylinder at 10.2 cm in diameter and 10 cm in height, and then two samples perpendicular to each other were cut at a thickness of 0.8 mm (Fig 1A); both samples are perpendicular to the bedding plane of the original outcrop. The circular sample (C sample in later texts) shown in Fig. 1B has a diameter of 10.2 cm, and the rectangular sample (R sample) shown in Fig. 1C has a dimension of 10.2 cm×6.1 cm. Samples were placed in an oven at 60 C° for 2 days to remove the moisture in the connected pore space before analyses. Both μ-XRF mapping and X-ray scattering were first carried out on these C and R whole samples, and then selected locations were cut into 1cm×1cm×0.08 mm sub-samples (orange squares in Fig. 1B-C) for X-ray diffraction (XRD) analyses and scanning electron microscopy (SEM) imaging, and crushed to powder for the analyses of total organic carbon (TOC) and pyrolysis. The rock chips shown in blue rectangular (Fig. 1B-C) were prepared to make four thin sections for petrography. The rest of rock samples in orange squares and blue rectangles were ball-milled as powders for bulk XRD analyses.
2.2 X-ray scattering
Both ultra-small angle X-ray scattering (USAXS) and small-angle X-ray scattering (SAXS) were conducted on the C and R samples to investigate the areal heterogeneity of porosity, pore size distribution, and surface area distribution over a pore-diameter scale of 1-1000 nm, and wide-angle X-ray scattering (WAXS) was used to determine the mineral types. USAXS/SAXS/WAXS analyses were conducted at 9-ID beamline of Advanced Photo Source (APS) at Argonne National Laboratory. The detailed beamline parameters, fundamental principles, analyses procedure, and data processing follow the work of Ilavsky and Jemian (2009), Ilavsky et al. (2018), and Wang et al. (2021b). For the C sample, a total of 37 positions were scanned (Fig. 1B), with each position scanned at 3-4 beam spots (labeled as A to C-D) following the sequence in Wang et al. (2021b) for a total of 132 beam spots being scanned. For the R sample, there was 30 positions (Fig. 1C), and each position was scanned at 2-3 beam spots for a total of 72 beam spots. Each beam spot only took 90, 10, and 10 seconds to complete the sequential runs of USAXS and SAXS (for pore structure) & WAXS (for mineral composition). Hereafter individual beam spots are described as, for example, Position 1A or Position 2B, where the former refers to Position 1 & beam spot A, and the latter to Position 2 & beam spot B. In brief summary, the USAXS and SAXS are combined to be (U)SAXS to determine the porosity, pore diameter distribution, and surface area, at beam spots of 0.8 mm×0.8 mm, and the WAXS is used to determine the type of minerals, at the same beam spots but a smaller area of 0.8 mm×0.2 mm.
2.3 Micro-X-ray Fluorescence Mapping
μ-XRF mapping is a fast and non-destructive method that can quantitatively measure the spatial elemental distribution on the surface of mm to cm-sized samples (Nikonow and Rammlmair, 2016; Birdwell, et al., 2018). Our data were obtained from a μ-EDXRF spectrometer M4 Tornado manufactured by Bruker, and the chemical elements Al, Ca, Fe, K, Mg, Mn, Na, and Si were selected to be monitored at 50 kV. The maximum resolution of μ-XRF scanning is 14 μm, and the scanning time depends on the sample size and resolution. For the sub-dm-sized samples used here with the best resolution, the scanning time is around 20 hours.
2.4 Petrographic microscopy
To investigate the changes in sedimentary textures for C and R samples, a rectangular area of ~2 cm × 10 cm on both samples was cut out for petrographic microscopy (Fig. 1B-C), and the photos of prepared thin sections were taken under the Leica DM 750P polarizing microscope.
2.5 X-ray diffraction
After X-ray scattering and μ-XRF mapping for pore structure and elemental composition of the large-sized C and R samples, six sub-samples as rock chips (1 cm×1 cm×0.8 mm) from areas showing large variations were selected and cut out from each sample, in order to determine the mineral compositions by XRD on the intact sample. In addition, a ball-milled powder sample from the rest of rock chips after cutting of twelve sub-samples was prepared to investigate the average mineral composition of the sample. The XRD analyses were conducted on a Shimadzu MaximaX XRD-7000, and the 2θ was set to be 20 to 70 degrees. The Jade 9 analysis program was used to determine the mineral compositions from the raw spectral data.
2.6 Field emission-scanning electron microscopy
Six selected sub-samples on each of C and R samples were gradually polished with sandpapers ranging from 200, 400, 800, 1200, 2000, and 3000 grits. After polishing, samples were coated with Au/Pt in CrC-100 Sputtering system and then directly examined by Hitachi 4800 SEM to investigate the pore types. In this study, images were collected with the secondary electrons (SE) mode under 15 kV for minerals and their related pores and 1.5 kV for organic matters-hosted pores.
2.7 TOC and pyrolysis
When the previous analyses were finished, these 12 selected sub-samples from C and R samples were crushed to be powders (<75 μm) to examine the organic matter content (TOC) by LECO C230 Carbon Analyzer, as well as quality and thermal maturity through HAWK pyrolysis manufactured by Wildcat Technologies.