1. Introduction
Successful and economical recovery of gas and oil from shale has become
one of the hottest geological topics in recent years. Compared to
conventional sandstone reservoirs, shale is well known for its complex
pore systems and high heterogeneity from nanometer to basin scales (Peng
et al., 2017; Borrok et al., 2019; Ma et al., Mighani., 2019; Zhang et
al., 2019; Huang et al., 2020). Complementary with wireline log and
seismic data, core analyses are a direct measurement of rock properties.
However, they are limited by their resolution and sample holding
capacity of various measurement approaches and associated
instrumentation. Cores are usually analyzed after plugging, cutting,
and/or crushing which will lead to the issues of inability of studying,
and altering, the heterogeneity at the dm-scale. With the improvement in
instrumentation, a variety of non-destructive techniques is available to
assess the pore structure and geochemical properties of core-sized
(commonly 2.5 cm in diameter) samples. Microscale X-ray fluorescence
(μ-XRF) mapping has been applied in core-sized samples to visualize the
elemental and laminar distribution (Reed et al., 2019; Birdwell et al.,
2019; Barker et al., 2020a; 2020b). Wang et al. (2021b) recently applied
a rapid and high-precision (ultra) small-angle X-ray scattering
[(U)SAXS] technique to characterize the porosity, pore size
distribution, surface area and their distribution in a Barnett Shale
sample across an area of several tens of cm2.
The Cretaceous Engle Ford Shale is a prolific petroleum reservoir in the
central to southwest Texas region (U.S. Energy Information
Administration, 2022). The organic matter-rich beds of the Eagle Ford
Formation have been extensively studied with regard to depositional
environment, diagenesis, mineral composition, organic matter type, pore
types and pore-size distribution, and water-rock interaction (e.g.,
Pommer and Milliken, 2015; Frebourg et al., 2016; Alnahwi and Loucks.,
2018; Wang et al., 2021a). These studies indicate a high degree of
heterogeneity across multiple observational scales. At the field scale
across several counties, the organic matter (OM)-rich beds are
interbedded with OM-poor limestones, and the proportion of OM-rich shale
vs. limestone negatively influences the formation fracability and
positively the petroleum production (Breyer et al., 2015). On the core
scale of centimeters, the Eagle Ford Shale shows a lithologic variation
from coccolith-rich pellets to siliceous-argillaceous seams, and
foraminifera (Reed et al., 2019), and there are still many remaining
needs for further investigation into larger-sized samples for the
decimeter-scale variability in properties such as pore structure,
mineral and OM composition.
This work has employed μ-XRF mapping, (U)SAXS, and wide-angle X-ray
scattering (WAXS) to investigate the spatial heterogeneity of the
elemental & mineralogical composition and pore structure on two Eagle
Ford Shale wafers taken perpendicular to each other. Then, six locations
on each sample (1 cm×1 cm×0.8 mm) showing large variations in pore
structure and elemental composition were selected and cut into chips.
These sub-samples were processed at different sizes for analyses by
petrographic microscopy, field emission-scanning electron microscopy
(FE-SEM), X-ray diffraction (XRD), total organic carbon content (TOC),
and pyrolysis to investigate the spatial heterogeneity of sedimentary
structure, mineral composition, pore types, organic richness, and
thermal maturity.