Mineralized fracture in a mudrock shale


Publications

  1. Mineralized fracture in a mudrock shale>
    . Strategies for sealing fractures to increase the security of geologic CO2 storage: Lessons learned from a multiscale multimodal imaging study of a syntaxial vein in a shale. Geochimica et Cosmochimica Acta. .

    Abstract — Veins recovered from deep sedimentary formations offer insights about mineral precipitates and processes that lead to sealing of underground fractures. These processes are of interest in the context of subsurface negative emissions technologies such as geologic carbon storage, where fractures are potential pathways for unwanted fluid migration. In this study, we characterized minerals and porosity of a syntaxial vein in a shale sample from the Wolfcamp formation in Texas. The original fracture has an aperture of 5 mm and is filled with distinct zones of minerals and vuggy regions. Thin-sections from cuts across the vein were examined at micron scale resolution using scanning electron microscopy, energy dispersive X-ray spectroscopy, QEMSCAN, and polarized light microscopy. Larger-scale analyses were done using synchrotron X-ray fluorescence. Collectively, these methods reveal elongated crystals of dolomite as large as 900 microns, which are overlain with a mixture of smaller crystals including calcite and ferroan dolomite. Precipitation of SiO2 is found to fill some of the void space. Mineral identification was further corroborated using powder x-ray diffraction. Quantitative analysis of a 3D image from X-ray computed tomography indicate that the vein volume contains 62% elongate dolomite crystals, 33% mixed ferroan dolomite and calcite, 1% silica, and 4% vuggy void space. Synchrotron X-ray scattering reveal that the vein mineral precipitates have porosity of ~1% and this is much less than the shale matrix porosity. The findings in this study suggest that as the formation formed and subsided, fracture fluids migrated vertically and experienced pressure changes causing exsolution of CO2. A geochemical model simulation demonstrated how this could have led to carbonate precipitation in the veins. A fundamental understanding of the sequence of vein mineral precipitation and the associated reduction in porosity may inspire strategies designed to induce fracture sealing, thus preserving the integrity of underground CO2 storage. Examples that would cause carbonate supersaturation include increasing the pH, adding divalent cations, relying on vertical migration for solution depressurization, and heating to reduce carbonate solubility.

  2. Mineralized fracture in a mudrock shale>
    . Quantification of mineral reactivity using machine learning interpretation of micro-XRF data. Applied Geochemistry. .
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    Abstract — Accurate characterization of mineral reactivity requires quantification of spatial heterogeneity, mineral abundances, elemental makeup, and the extent to which minerals are accessible. In this work, sedimentary rock samples were characterized using a neural network approach for 2D mineral mapping based on synchrotron micro x-ray fluorescence (µXRF) data. The approach is called Synchrotron-based Machine learning Approach for RasTer (SMART) mapping, which reads µXRF scans and outputs mineral maps of millimeter size. The SMART mineral classifier is trained on coupled µXRF and micro-x-ray diffraction (µXRD) data which is what distinguishes it from existing mapping tools. The resulting classifier has accuracy of > 96%. Here we demonstrate the SMART classifier on new samples including consolidated shales from the Eagle Ford, Green River, Upper Wolfcamp, Haynesville, and New Albany formations. Scans were obtained using an x-ray microprobe at beamline 13-ID-E (GSECARS) at the Advanced Photon Sources (APS). Individual mineral maps generated by the SMART classifier well-captured distributions of both dominant and minor phases. Quantification of trace elements in pyrite grains of the Eagle Ford shale revealed zinc concentrations up to 4.2 wt.%, along with minor copper and arsenic copresence. Predicted median pyrite grain size is 3.17 µm in diameter, with 62% of the grains predicted to be smaller than 4 µm. Mineral accessibility was examined by contact with other, more reactive phases, and was quantified using a new type of image called an adjacency map. Adjacency analyses revealed that of the total pyrite surface present in the Eagle Ford shale, 28% is in contact with calcite. The adjacency maps are useful for quantifying the likelihood that a mineral could be exposed to fluids after dissolution of a reactive phase like calcite. Lastly, the ability to pool data from different samples and increase the number of identifiable minerals was demonstrated by training a classifier using two different sets of coupled µXRF-µXRD data. Results demonstrate that this approach is promising for extending applications to a wider suite of rock samples.