Abstract

The Marcellus Shale sits in a geologically complicated region, characterized by faults and salt-cored compressional folds. Large lateral velocity variations associated with this complex geology make seismic imaging difficult. Recently acquired 3D seismic with wide azimuthal coverage and long offsets have poorly imaged folds and faults within the Marcellus Shale. Wells drilled using the existing 3D seismic volumes often encounter incorrect bed dips and folds mistakenly interpreted as faults (or vice versa) because of the poor imaging quality. We carefully examined each processing step in the seismic processing and imaging workflow and identified a few key drivers that could potentially lead to improvements in subsurface imaging. Starting from field seismic records, we identified and corrected geometry errors. To improve signal-to-noise ratio, we applied advanced noise attenuation practices including land surface-related multiple elimination. We carefully tested and validated an extension patch in the 5D seismic data interpolation that improved the offset coverage in the crossline direction. Furthermore, we performed orthorhombic prestack time migration (PSTM). An orthorhombic velocity model fits the geology better than the traditional vertical transverse isotropic (VTI) model for layered subsurface because a dominant set of orthogonal fracture sets is present in the basin. We achieved a significant improvement of subsurface imaging by implementation of these processing steps as key drivers. The new orthorhombic PSTM correctly images steeply dipping (75° and above) faults that were elusive or absent on the previous VTI migration. Additionally, previously interpreted faults are now clearly imaged as folds, small and large in scale. These imaging improvements have enabled the accurate drilling of lateral wells in the target that otherwise would have been drilled out of zone. Some large faults, which pass through the Marcellus, Mahantango, and Tully Formations, are now well-imaged but were previously barely visible. Successful imaging of such large faults helps identify and avoid geohazards. The dramatic improvement in the subsurface imaging and amplitude-preserving processing enhance the reservoir description of the Marcellus Shale.

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