Seismic Modeling Analysis and Characterization of a Gas Hydrate and Free Gas Mixed System in Green Canyon, Gulf of Mexico

Abstract

Abstract

The Bottom Simulating Reflector (BSR) has been used extensively to interpret possible gas hydrate from seismic data. However, a BSR is not found in the Green Canyon (GC) 955 of Gulf of Mexico, where gas hydrate deposits were predicted and confirmed through drilling. The stratigraphic facies where gas hydrates are indicated by Logging Wile Drilling (LWD) drilling in GC955 have a complex amplitude response of high negative and high positive amplitudes showing both continous and discontinous character with different modes of stacking and complexity, e. g. low positive over high negative, high positive over high negative, etc. Rock physics-based modeling, with basic parameters guided by the existing well data, are used to decipher the complex amplitude signatures to discriminate gas hydrate sands from gas prone sands.

Field data are reprocessed conventional 3-D seismic data. Three scientific wells provide the basic constraints for the rock physics-based seismic modeling. The rock-physics modeling suggest that high positive amplitude and moderate to high peak/trough ratio are associated with high concentrations of gas hydrate; high negative amplitude and low peak/trough ratio will be associated with a gassy zone or a gas zone with low concentrations of gas hydrate above. The output of this modeling to the 3-D seismic data that traverse the scientific and industry well control points (wells) is consistent with drilled results and reveals variations in the depth of gas hydrate sands and gassy sands suggesting that there are localized non-equilibrium environments within the gas hydrate stability zone. The agreement with the drilled data suggests that a new and useful seismic attribute can delineate gas hydrate filled sands.

Introduction

A major challenge in gas hydrate exploration is the mapping of the distribution and concentration of gas hydrate deposits. As a gas hydrate indicator, the BSR has been used extensively in seismic interpretation of gas hydrate (Shipley et al., 1979; Hyndman and Spence, 1992). A BSR is parallel to the seafloor reflector and has the opposite polarity. Recent studies suggest that BSR is caused by a sharp drop in compressional velocity in the underlying gas zone (Singh et al., 1993; Mackay et al., 1994; Zhang and McMechan, 2006). Numerous expeditions have shown that the presence of a BSR does not correlate to high concentrations of gas hydrate above the BSR (Lu and McMechan, 2002; Tréhu et al., 2003). Thus, it is necessary to investigate better methods to image high concentrations of gas hydrate with seismic data. This paper reports a successful example of the application of rock physics based modeling to derive a seismic attribute, that when applied to conventional 3-D seismic data, identifies high concentrations of gas hydrate in Pliestocene clastic deposits in the Gulf of Mexico.