Estimation of methane hydrate quantities from marine seismic data and physical modeling (time-average method)

Marine seismic reflection data collected from offshore southwestern Taiwan show that prominent seismic bottom simulating reflectors (BSRs) are observed that indicate the existence of gas hydrate in the seafloor sediment with free gas zone underneath. We apply a theoretical rock physics model to analyze 2D marine seismic data to determine gas hydrate and free gas saturation. High-porosity marine sediment is modeled as a granular system where the elastic wave velocities are linked to porosity; effective pressure; mineralogy; elastic properties of the pore-filling material; and water, gas and gas hydrate saturation of the pore space. To apply this model to seismic data, we first obtain interval velocity from migration velocity analysis. Next, all input parameters to the rock physics model, except porosity and water, gas and gas hydrate saturation, are estimated from geological information. The saturation estimations are extremely sensitive to the input velocity values. Therefore, accurate velocity determination is crucial for correct reservoir characterization. We apply pre-stack depth migration and amplitude versus angle (AVA) methods to analyze two seismic lines (EW9509-35 and EW9509-46) located in the key area offshore southwestern Taiwan. A more advanced approach to velocity analysis is through the use of iterative migration, also known as migration velocity analysis (MVA). These techniques can overcome the restriction of lateral homogeneity and retrieve information about the velocity. Further more, the incidence and emergence angles of any reflection can be approached more accurately when using ray-tracing methods in depth domain, rather then ray-path approximations based on RMS velocities in time domain. Thus, efficient amplitude versus angle analysis is available within the process of pre-stack depth migration with little additional computing time, and that enables us to quantify further acoustic and shear-wave velocities in the subsurface. Velocity structures derived from pre-stack depth migration and from analyzing the wide-angle seismic data show that the hydrate-bearing sediments generally have velocity ranges from 1750 to 2000 m/s, with most values around 1900 m/s. Low velocity zones observed beneath the gas hydrate bearing sediments clearly indicate the presence of free gas below.