Abstract

We describe an integrated seismic evaluation for the Longmaxi shale gas potential in the central part of the Sichuan Basin, onshore China, which benefits from innovations in high-density 3D seismic, borehole-based vertical seismic profiling (VSP), and passive seismic recording for microseismic monitoring. The geology of the basin has a complex history that creates several geophysical challenges, including strong multiple contamination and the requirement for high-resolution data to enable detailed structural and stratigraphic mapping. For the 3D surveys, we adopted a new wide-azimuth and high-density acquisition design. A large part of the processing was devoted to noise removal, optimizing surface-consistent processes including deconvolution, and thorough testing of data-driven internal multiple attenuation. Prestack depth imaging is important for accurate geosteering of long horizontal trajectories within a narrow target window, and our resulting images in depth provide a more planar target structure than with (more routine) time imaging. The more planar true structure was confirmed by drilling. For the borehole seismic calibration, vibroseis was used as the VSP source in preference to the standard use of explosives. This resulted in greater consistency of the waveforms and better amplitude and timing of events with which to constrain the 3D seismic processing and imaging. Surface microseismic monitoring of the stimulation was achieved using a new and bespoke design. The detected microseismic activity helped support and optimize the real-time fracture operations, mitigate casing deformation, obtain in-situ stress fields, and understand the control of preexisting geologic features. The innovations in surface seismic, VSP, and microseismic, along with the integration of other disciplines, contributed to the project's overall success.

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