Seismic characteristics of gas hydrate system at the Hydrate Ridge, offshore Oregon

Abstract

Seismic wave velocities vary in the presence of gas hydrate and free gas in the sediments. Seismic properties (velocities) of the gas-hydrate bearing sediments allow us to identify the presence of gas hydrates, to study their character, formation and distribution, and to estimate the amount of gas hydrate and/or free gas that may be present in the sediments. Accuracy in the estimation of distribution and saturation of gas hydrates and free gas depends on the interval velocities of Pand S-waves. We have carried out an interactive velocity analysis of Pand converted S-waves in the tau-p (intercept time – ray parameters) domain, which directly gives the interval velocities. This requires multicomponent seismic data. A two-ship seismic experiment was carried out (to record multicomponent seismic data) in summer 2002 at the Hydrate Ridge to map the gas hydrate. Our approach to multicomponent velocity analysis comprises three steps: 1) P-wave velocity analysis, 2) PP to PS event correlation, and 3) Swave velocity analysis. PP to PS correlation is performed using synthetic seismograms. Observed velocities are matched with modeled velocities to estimate gas hydrate saturation. Pand S-wave velocities are modeled with a “Modified Wood equation” which is a modification of Wood equation with a rock physics model and an empirical relation, respectively. We present results from the multicomponent ocean bottom seismometer data recorded at the Hydrate Ridge, offshore Oregon. The Pwave velocity is found to be more sensitive to the saturation of gas hydrates and free gas than S-wave velocity. Gas hydrate is estimated to be upto 7% of rock volume (12% of pore space). The S-wave velocity does not show an anomalous increase in the hydrate-bearing sediments. Thus we conclude that hydrate does not cement sediment grains enough to affect shear properties. It is more likely that the hydrates are formed within the pore space in this region. Introduction Gas hydrate is an ice-like substance that contains low molecular weight gases (mostly methane) in a lattice of water molecules (Sloan, 1998). In marine environments, methane hydrates are usually stable at temperatures in the range of 0C to 150C, water depth greater than 500 m, and sediment depths up to 300 meters below sea floor. Formation of gas hydrate is favorable in the deeper ocean (methane gas, water and thermodynamic conditions are available), which means huge amount of gas (about 15000 giga tones) may be present in the offshore gas hydrates (Kvenvolden, 1999). Free gas is usually present below the gas hydrates. Hydrates and free-gas make a strong acoustic interface, which is evident in seismic section as a Bottom Simulating Reflector (BSR). Offshore gas hydrates system has got attention from the geoscientist because of its potential to be 1) a drilling hazard, 2) a future energy source, 3) continental slope stability, and 4) a factor affecting global carbon cycle. Many of these relations are not well established, which can be better constrained if distribution and saturation of gas hydrates and free gas are known. Gas hydrates and free-gas are generally detectable with seismic methods (Yuan et al., 1996) since the seismic velocity, in general, increases in the presence of gas hydrates and decreases in the presence of free gas. The velocity also increases if the hydrate content in the pore spaces increases (Hyndman and Spence, 1992). Therefore, seismic velocity has been widely used to estimate the saturation of gas hydrates and free gas (e.g., Lee et al., 1996; Helgerud et al., 1999; Lu and McMechan, 2002). Other methods gives local estimation of gas hydrates using resistivity data from well logs, chloride measurement from core data, infra-red image of cores, and pressure core sampler (Dickens et al., 1997; Trehu et al., 2004). Hydrate Ridge (HR) is a 25-km long and 15-km wide accretionary ridge in the Cascadia convergent margin (MacKay, 1995) formed as the Juan de Fuca plate subducts obliquely beneath the North American plate (Figure 1). Hydrates and its seismic proxies (BSR, and amplitude blanking) appear to be well developed beneath HR (Trehu et al., 1999). A two-ship seismic experiment was conducted at the HR during summer 2002 with the seismic ship Maurice Ewing and the drilling ship JOIDES Resolution. The cruise was designed to acquire surface seismic (streamer recording, MCS) and subsurface seismic [Vertical seismic profiles (VSP) and Ocean bottom seismometer (OBS)] data to map the distribution and saturation of gas hydrates which is possible with accurate estimation of elastic properties (Pand S-waves velocities) of gas hydrateand free gasbearing sediments. Seismic velocity analysis is normally performed in the offset-time ( t x − ) domain, which gives root mean square (RMS) velocity. The resultant RMS velocity is converted to interval velocity using the Dix equation (Dix, 1955). Interval velocity can be directly estimated with data analysis in the tau-p (intercept time – ray parameters) domain (Stoffa et al., 1981). We have carried out an interactive velocity analysis for Pand S-waves in the taup domain. S-wave in marine setting is a converted shear wave, and can be recorded with multicomponent receivers as OBS, and VSP geometry. We use the OBS data recorded on the HR. Presence of weak anisotropy due to the hydrate veins has been reported by Kumar et al. (2004), however we will consider isotropy model in this study. Once interval velocities are estimated, they are