Imaging through gas clouds: The application of CSI and FWI in Bohai, China

Abstract

Oil field A, situated in Bohai Bay, was discovered in 1999 and has been developed as one of the most productive oil assets in China. It continues to hold significant growth potential for the future. Though the field contains a large amount of resources remaining to be developed, seismic imaging has been challenging in area 5, resulting in structural uncertainty for reservoir interpretation and well planning. In the past three decades, several 2D and 3D seismic surveys have been acquired, processed, and reprocessed in this area. However, due to the existence of complicated gas clouds, which are shallow, multilayered, and extensive, obscured sub-gas-cloud images appear in all legacy seismic results, making fault interpretation under the gas clouds almost impossible. To improve the sub-gas-cloud image and overall structural interpretability, a narrow-azimuth full-field ocean-bottom cable (OBC) acquisition was conducted in field A during 2018 and 2019, and later, a compressive seismic imaging (CSI)-based full-azimuth and large-offset OBC infill survey was acquired in area 5, covering the widest gas cloud. Through high-fidelity signal processing, full-waveform inversion (FWI)-driven velocity model building, and imaging using both Kirchhoff migration and reverse time migration (RTM), the seismic image quality beneath complicated gas clouds is improved significantly. It is the first time that sub-gas-cloud faults and the Base of Guantao event have been imaged by seismic without significant dim zones. CSI acquisition, FWI, and RTM are the key elements to resolve gas-cloud-related challenges in area 5.