Julia Meyer Sheets

The current investigation aims to offer fundamental, molecular- to microscopic-level descriptions of methane gas inside natural source clay minerals. Texas montmorillonite (STx-1), Georgia kaolinite (KGa-2), and Ca²⁺-saturated Texas montmorillonite (Ca-STx-1, Ca-bentonite) were utilized as subsurface model clay minerals for elucidating nano-confinement behaviors of ¹³C-labeled methane gas. High-pressure magic angle spinning (MAS) nuclear magnetic resonance (NMR) was used to describe the interactions between methane and the clays by varying temperature and pressure. Proton-decoupled ¹³C-NMR spectra were acquired at 28.2 bar at 307 K, 32.6 bar at 346 K, 56.4 bar at 307 K, 65.1 bar at 346 K, 112.7 bar at 307 K, and 130.3 bar at 346 K. In the pure state, no significant thermal effect on the behavior of methane was observed. However, there was a perceptible variation in the chemical shift position of confined methane in the mixtures with the clays up to 346 K. Conversely, the ¹³C-NMR chemical shift of methane altered by varying pressure in a pure state, and the mixtures with clays, attributed to the interaction of methane with the clay surfaces or the nanopore network of the clay–silica mixed phase. Pressure-induced shifts in methane peak positions were observed: 0.25 ppm (28.2–56.4 bar) and 0.47 ppm (56.4–112.3 bar) at 307 K. For methane in a montmorillonite mixture, shifts were 0.32 ppm for bulk-like methane and 0.20 ppm for confined methane under similar conditions. At 346 K, increasing pressure from 65.1 to 130.3 bar caused shifts exceeding 0.50 ppm, with bulk-like methane showing a 0.64 ppm shift and confined methane a 0.57 ppm shift. There was only one ¹³C-NMR methane peak in the mixture with either kaolinite (KGa-2) or Ca-bentonite with line broadening compared to that of pure methane. Still, two peaks were observed in the mixture with STx-1, explained by the imbibition and mobility of methane in the pore network.

The present study aims to provide fundamental, molecular- to microscopic-level descriptions of methane gas within natural source clay minerals. Texas montmorillonite (STx-1), Georgia kao-linite (KGa-2), and Ca2+ saturated Texas montmorillonite (Ca-STx-1, Ca-bentonite) were used as subsurface model systems for elucidating nano-confinement behaviors of 13C labeled methane gas. High-pressure Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) was utilized to characterize the interactions between methane and the clays by varying temperature and pressure. In the pure state, no significant thermal effect on the behavior of methane was observed. However, there was a perceptible change in the chemical shift position of confined methane in the mixtures with the clays up to 346 K. Conversely, the 13C-NMR chemical shift of methane changed as a func-tion of pressure in a pure state, and the mixtures with clays, attributed to the interaction of me-thane with the clay surfaces or the nanopore network of the clay-silica mixed phase. There was only one 13C-NMR peak of methane in the mixture with either kaolinite (KGa-2) or Ca-bentonite with line-broadening compared to that of pure methane, but two peaks were observed in the mix-ture with STx-1, explained by the imbibition and mobility of methane in the pore network.

Asian aeolian dust is a primary factor in Northern Hemisphere atmospheric dynamics. Predicting past and future changes in atmospheric circulation patterns relies in part on sound knowledge of Central Asian dust properties and the dust cycle. Unfortunately for that region, data are too sparse to constrain the variation in dust composition over time. Here, we evaluate the potential of a Tibetan ice core to provide a comprehensive paleo-atmospheric dust record and thereby reduce uncertainties regarding mineral aerosols’ feedback on the climate system. We present the first datasets of the mineralogical, geochemical, and Sr-Nd isotope composition of aeolian dust preserved in pre-Holocene layers of two ice cores from the Guliya ice cap (Kunlun Mountains). The composition of samples from the Summit (GS; 6710 m a.s.l.) and Plateau (GP; 6200 m a.s.l.) cores reveals that the characteristics of the dust in the cores’ deepest ice layers are significantly different. The deepest GS layers reveal isotopic values that correspond to aeolian particles from the Taklimakan desert, contain a mix of fine and coarse grains, and include weathering-sensitive material suggestive of a dry climate at the source. The deep GP layers primarily consist of unusual nodules of well size-sorted grey clay enriched in weathering-resistant minerals and elements typically found in geothermal waters, suggesting that the dust preserved in the oldest GP layers originates from a wet and possibly anoxic source. The variability of the dust composition highlighted here attests to its relevance as a paleo-environmental messenger and warrants further exploration of the particularly heterogenous Guliya glacial dust archive.

Porosity and permeability are key variables that link the thermal-hydrologic, geomechanical, and geochemical behavior in rock systems and are thus important input parameters for transport models. Neutron scattering studies indicate that the scales of pore sizes in rocks extend over many orders of magnitude from nanometer-sized pores with huge amounts of total surface area to large open fracture systems (multiscale porosity). However, despite considerable efforts combining conventional petrophysics, neutron scattering, and electron microscopy, the quantitative nature of this porosity in tight gas shales, especially at smaller scales and over larger rock volumes, remains largely unknown. Nor is it well understood how pore networks are affected by regional variation in rock composition and properties, thermal changes across the oil window (maturity), and, most critically, hydraulic fracturing. To improve this understanding, we have used a combination of small- and ultrasmall-angle neutron scattering (U)SANS with scanning electron microscope (SEM)/backscattered electron imaging to analyze the pore structure of clay- and carbonate-rich samples of the Eagle Ford Shale. This formation is hydrocarbon rich, straddles the oil window, and is one of the most actively drilled oil and gas targets in the United States. Several important trends in the Eagle Ford rock pore structure have been identified using our approach. The (U)SANS results reflected the connected (effective) and unconnected porosity, as well as the volume occupied by organic material. The latter could be separated using total organic carbon data and, at all maturities, constituted a significant fraction of the apparent porosity. At lower maturities, the pore structure was strongly anisotropic. However, this decreased with increasing maturity, eventually disappearing entirely for carbonate-rich samples. In clay- and carbonate-rich samples, a significant reduction in total porosity occurred at (U)SANS scales, much of it during initial increases in maturity. This apparently contradicted SEM observations that showed increases in intraorganic porosity with increasing maturity. Organic-rich shales are, however, a very complex material from the point of view of scattering studies, and a more detailed analysis is needed to better understand these observations.

Porosity and permeability are key petrophysical variables that link the thermal, hydrological, geochemical, and geomechanical properties of subsurface formations. The size, shape, distribution, and connectivity of rock pores dictate how fluids migrate into and through micro- and nano-environments, then wet and react with accessible solids. Three representative samples of cap rock from the Eau Claire Formation, the prospective sealing unit that overlies the Mount Simon Sandstone, a potential CO2 storage formation, were interrogated with an array of complementary methods. neutron scattering, backscattered-electron imaging, energy-dispersive spectroscopy, and mercury porosimetry. Results are presented that detail variations between lithologic types in total and connected nano- to microporosity across more than five orders of magnitude. Pore types are identified and then characterized according to presence in each rock type, relative abundance, and surface area of adjacent minerals, pore and pore-throat diameters, and degree of connectivity. We observe a bimodal distribution of porosity as a function of both pore diameter and pore-throat diameter. The contribution of pores at the nano- and microscales to the total and the connected porosity is a distinguishing feature of each lithology observed. Pore:pore-throat ratios at each of these two scales diverge markedly, being almost unity at the nanoscale regime (dominated by illitic clay and micas), and varying by one and a half orders of magnitude at the microscale within a clastic mudstone. Individual minerals, primarily illite and glauconite, have unmistakable pore and pore-throat signatures and contribute disproportionately to connected reactive surface area. The pore types created or evolved during diagenesis mediate profound differences between bulk and pore-network-accessible mineral associations in the mudstones. Results of this study can ultimately be used to inform reactive-transport simulations of effective reactive surface area.