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.