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.