| It is believed that there are huge amounts of gas hydrate in the subsurface of the continental slope. The identification and characterization of gas hydrate still remains a challenge for seismic interpreters, although the general features of bottom simulating reflector (BSR) that indicates gas hydrate with some degree of certainty have been well documented. In this study, quantitative interpretations integrated with a rock physics model, seismic modeling, and seismic inversion are shown to be a powerful tool for recognizing and mapping gas hydrate deposits. A rock physics gas hydrate model, providing a bridge between seismic data and geologic information, is developed by using Hertz-Mindlin contact theory, effective medium theory, and Gassmann fluid substitution. The model indicates that acoustic impedance may be used to detect and characterize subsurface gas hydrate-bearing sediments, the crossplot of normal incidence reflectivity and Poisson reflectivity may be used to distinguish free gas within the gas hydrate-bearing sediments, and density may be a good indicator to identify near-surface massive and nodular gas hydrate. Seismic modeling driven by the rock physics model demonstrates that the amplitude and waveform of seismic data are controlled by gas hydrate saturation, layer thickness, free gas, and their interactions. The modeling method is applied to 3D seismic interpretation for gas hydrate deposits in Green Canyon (GC) 955 of the Gulf of Mexico, suggesting a complex gas hydrate and free gas layer relationship in the area. Seismic acoustic inversion in combination with the rock physics model is used to predict the distribution and concentration of gas hydrate deposits throughout a 3D seismic volume in GC474 of the Gulf of Mexico. The predicted concentration in GC474 is up to 20% by volume, which is consistent with the estimation from logging data in GC955. The distribution features of gas hydrate interpreted from 3D seismic data in GC474 and GC955 and logging data in GC955 are associated with structure and stratigraphy. The results suggest that the combination of qualitative and quantitative interpretation is an effective interpretation strategy for gas hydrate deposits. |
