Constraints on Characteristics and Distribution of Gas Hydrate and Free Gas Using Broad-Band Processing of Three-Dimensional Seismic Data

Gas hydrate drilling expeditions in the Pearl River Mouth Basin, South China Sea, have identified concentrated gas hydrates with variable thickness. Moreover, free gas and the coexistence of gas hydrate and free gas have been confirmed by logging, coring, and production tests in the foraminifera-rich silty sediments with complex bottom-simulating reflectors (BSRs). The broad-band processing is conducted on conventional three-dimensional (3D) seismic data to improve the image and detection accuracy of gas hydrate-bearing layers and delineate the saturation and thickness of gas hydrate- and free gas-bearing sediments. Several geophysical attributes extracted along the base of the gas hydrate stability zone are used to demonstrate the variable distribution and the controlling factors for the differential enrichment of gas hydrate. The inverted gas hydrate saturation at the production zone is over 40% with a thickness of 90 m, showing the interbedded distribution with different boundaries between gas hydrate- and free gas-bearing layers. However, the gas hydrate saturation value at the adjacent canyon is 70%, with 30-m-thick patches and linear features. The lithological and fault controls on gas hydrate and free gas distributions are demonstrated by tracing each gas hydrate-bearing layer. Moreover, the BSR depths based on broad-band reprocessed 3D seismic data not only exhibit variations due to small-scale topographic changes caused by seafloor sedimentation and erosion but also show the upward shift of BSR and the blocky distribution of the coexistence of gas hydrate and free gas in the Pearl River Mouth Basin.