This study presents the latest insights into the reservoir structure and hydraulic fracture geometry at Utah FORGE by using Distributed Acoustic Sensing (DAS) recorded microseismic data for subsurface imaging. To monitor injection activities and evaluate reservoir response, two fibers were permanently deployed in wells [16B(78)-32] and [78(B)-32]. The DAS systems recorded abundant microseismic data during the 2023 circulation test, and the 16A(78)-32 and 16B dual-well stimulation sequence in 2024. In addition to the direct P and S arrivals, DAS microseismic data also capture reflected waves from nearby fractures, faults, and the granitoid-alluvium interface of this area. The rich wavefields recorded by DAS offer the potential to use microseismic events as high-frequency sources near the reservoir, enabling high-resolution imaging of the intricate structures within the bedrock. We developed an imaging technique that leverages DAS microseismic events as imaging sources, with each fiber channel functioning as a receiver. We apply prestack Kirchhoff migration to each individual source following wavefield separation, then stack hundreds of sources to generate a 3D reflectivity volume. Although reflected S-waves are commonly visible in our DAS microseismic data, we conducted a careful event selection based on data quality and location uncertainties, balancing the trade-off between data quantity and imaging quality. The developed imaging workflow produces a high-resolution map of the granitoid contact and, more importantly, reveals internal structures within the heart of the geothermal reservoir that have not been previously well described. By correlating well-log data, core analyses, and geological evidence, we identify a lithological interface located just below and nearly parallel to the granitoid contact. Key findings also reveal two potential natural fractures near the stimulation zone, visible prior to stimulation, which may accommodate the injected fluid, affecting the hydraulic fracturing efficiency. For example, low-frequency DAS (LF-DAS) suggested that fluid injected into 16A stimulation stage 8 propagate upward more than 200 m along the wellbore and generate a heart-shaped ‘fracture’ opening response exactly at one of the imaged natural fractures, and cause intensive microseismic activities. Those internal structures are hardly accessible by typical surface sources since the granite-alluvium interface of this region is strong and not easily penetrated by seismic waves. Although the fiber in well 16B is shorter than the wellbore, and the LF-DAS data cannot capture the frac hits from the early stages beyond the fiber end, microseismic reflections are still visible and enable imaging of the potential fracture azimuth. Time-lapse imaging of the hydraulic fracture is conducted and integrated with LF-DAS to construct a more comprehensive fracture geometry. In conclusion, the 3D fracture volume produced by DAS microseismic imaging deepens our understanding of geothermal reservoir dynamics, potentially enhancing geothermal exploitation strategies.

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