Micro- to nano-scale areal heterogeneity in pore structure and mineral
compositions of a sub-decimeter-sized Eagle Ford Shale
To improve the understanding of spatial heterogeneity in fine-grained
shale, methods of microscale X-ray fluorescence (μ-XRF) mapping,
(ultra-) small-angle x-ray scattering [(U)SAXS] and wide-angle X-ray
scattering were used to determine elemental and pore structure
variations in sizes up to ~10 cm on two samples prepared
at circular (8 cm×8 cm×0.8 mm in width×length×thickness) and rectangular
(5 cm×8 cm×0.8 mm) orientations from a piece of Eagle Ford Shale outcrop
in South Texas. Thin section petrography and field emission-scanning
electron microscopy, X-ray diffraction (XRD), total organic carbon, and
pyrolysis were also utilized to investigate the potential spatial
heterogeneity of pore types, mineral and organic matter compositions for
both samples. Overall, the siliceous-carbonate mineral contents in these
carbonate-rich Eagle Ford Shale vary between laminations at mm scales.
For the circular sample, porosity and surface area variations range from
0.82 to 3.04% and 1.51 to 14.1 m2/g, respectively.
For the rectangular sample, values for porosity and surface area vary
from 0.93 to 2.50% and 3.95 to 10.8 m2/g. By
analyzing six selected sub-samples on each of two samples with X-ray
scattering and XRD techniques, nm-sized pores are mainly interparticle
ones in the higher calcite regions, where the porosity is also
relatively lower, while the lower calcite regions consist of both
interparticle and intraparticle pore types with higher porosity.
Finally, the μ-XRF and (U)SAXS are combined to generate porosity
distribution maps to provide more insights about its heterogeneity
related to the laminations and fractures at our observational scales.