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doi:10.2204/iodp.proc.311.109.2006

Gas hydrate concentration estimates and
no-hydrate, no-gas baselines

Regional concentrations of gas hydrate beneath the northern Cascadia continental slope off Vancouver Island have been estimated earlier using MCS (Yuan et al., 1996, 1999), seafloor CSEM (Yuan and Edwards, 2001; Schwalenberg et al., 2005), and Leg 146 downhole data, including pore water salinity, electrical resistivity, and sonic velocity data (Hyndman et al., 1999, 2001; Spence et al., 2000a). The estimated concentrations of between 15% and 30% of pore saturation in a 100 m thick layer above the BSR are much higher than estimated elsewhere, especially the Blake Ridge (Paull et al., 1996) and central Cascadia off Oregon during Leg 204 (Tréhu, Bohrmann, Rack, Torres, et al., 2003). Although both of these other studies involved different sediment environments, a careful reevaluation of the northern Cascadia estimates and their uncertainties was carried out by Riedel et al. (2005). They concluded that the data and model uncertainties allowed a wide range of concentrations.

Electrical resistivity

Gas hydrate concentrations may be estimated from downhole electrical resistivity (e.g., Collett and Ladd, 2000; Hyndman et al., 1999) because, compared to the saline pore water, gas hydrate is a nearly perfect insulator. The resistivity from downhole logs in the Cascadia Basin Site 888 is ~1.0 m to a depth of several hundred meters, typical of deep-sea ocean-bottom sediments. In contrast, the resistivity at gas hydrate Site 889 is much higher (~2.0 m) (Westbrook et al., 1994). The critical unknown in using this resistivity to estimate gas hydrate concentrations is the no–gas hydrate reference resistivity, which is largely controlled by pore fluid salinity. Pore fluid has substantially lower salinity than normal (near seawater) in the recovered cores at Site 889, but this may be due to dissociation of in situ gas hydrate after recovery. Hyndman et al. (1999) showed that it is possible to estimate both the in situ pore fluid salinity and the in situ gas hydrate concentration using downhole log resistivity data and porosity and resistivity data from recovered cores. An important uncertainty is the relation between resistivity and porosity described by Archie's relation (Archie, 1942), which was estimated from core measurements.

Archie's relation states:

_f = ^mS_wⁿ_w/a,

where

• _f = formation conductivity,
  
• = porosity,
• S_w = fraction of pore volume filled with water,
• _w = conductivity of the pore water,
• a = a constant, typically in the range 1 a 2.5,
• m = sediment cementation factor in the range of 1.5 m 3, and
• n = saturation exponent in the range 1.5 n 2.2.

The parameters a and m are determined from purely water-saturated sediments. The parameters may be estimated from core measurements or from downhole resistivity and porosity logs (i.e, gamma ray density or neutron porosity). For logging data, they are typically estimated from a log-log crossplot of formation factor as a function of porosity, generally known as a "Pickett plot" (e.g., Serra, 1984). The parameter n can be estimated by averaging values for different lithologies (Pearson et al., 1983; Collett, 2000). Spangenberg (2001), however, showed that the parameter n is dependent on the grain-size distribution and gas hydrate concentration itself; therefore, there is no "global" value for n that can be used with confidence for the entire sediment column under investigation. Using Leg 146 core data at ODP Sites 888 and 889/890, Hyndman et al., (1999) determined the parameters to be a = 1.4, m = 1.76, and n = m. Using the Site 889 resistivity and neutron porosity logs, Collett (2000) estimated significantly different parameters of a = 1.0, m = 2.8, and n = 1.94. There are significant uncertainties in both log and core approaches. Core samples may have been significantly disturbed upon recovery, including the effect of gas hydrate dissociation, and there are sampling biases. In turn, the available downhole logs are of low quality because of hole enlargements, and the logs only measure porosity indirectly. The differences in these empirical parameters result in substantially different gas hydrate concentrations. Using the Archie parameters of Hyndman et al. (1999) yield the highest concentrations, which are on average 30% of the pore space in an interval of 100 m above the BSR. Alternatively, using the Collett (2000) parameters yields much smaller concentrations that are on average only 5% of the pore space.

Pore water chlorinity/salinity

Dissociation of gas hydrate releases methane and freshwater; therefore, pore water in recovered cores that is fresher than the assumed background level is commonly taken as an indication of in situ presence of gas hydrate that has dissociated upon core recovery. This process has been used widely to estimate gas hydrate concentrations (e.g., Hesse and Harrison, 1981; Hesse, 2003; Kastner et al., 1995; Ussler and Paull, 2001). The limitation of the method, however, is the problem of estimating the background (or reference) in situ pore fluid chlorinity/salinity before gas hydrate dissociation upon recovery. The in situ chlorinity/salinities may be strongly affected by the formation of gas hydrate (i.e., values higher than seawater) and by a number of geochemical processes, such as clay dehydration, that give values lower than seawater (Torres et al., 2004). Torres et al. (2004) argued that much of the low salinity in cores from Site 889 was due to geochemical processes not caused by dissociation of in situ hydrate. Figure F7 shows pore water salinities from Site 889/890 in comparison to data from Site 888 and Leg 204 (Holes 1244C, 1251B, and 1252A). The two extreme models for the reference are

1. A strongly decreasing smoothed profile through the Site 889 data such that only the local deviations represent freshening by gas hydrate that dissociated upon recovery, similar to the assumption by Ussler and Paull (2001) for the Blake Ridge; or
2. The chlorinity/salinity from the Cascadia Basin well Site 888, which is close to that of seawater.

Use of core pore fluid salinity data along with electrical resistivity data allows independent calculation of both the in situ reference salinity and the gas hydrate concentrations (Hyndman et al., 1999). However, as discussed above, the results are strongly dependent on the estimated Archie parameters. Within the estimated range of Archie's parameters the derived in situ pore water salinities (i.e., before gas hydrate dissociation in recovered cores) either fall on a decreasing salinity reference profile, similar to that observed in the recovered core, or on a near-seawater reference, as at Cascadia Basin reference Site 888. Consequently, Site 889/890 gas hydrate concentrations can either range between 5% and 10% or 30% and 40%, depending on the no–gas hydrate reference salinity used (Riedel et al., 2005).

Seismic P-wave velocity

Seismic velocities from ODP sonic logs, downhole vertical seismic profiles (VSPs), and MCS data can be used to calculate gas hydrate concentrations, if an appropriate no–gas hydrate velocity-depth profile can be estimated. All the empirical methods to obtain gas hydrate concentrations from velocity data depend on a no-gas, no–gas hydrate reference velocity-depth profile. There are several different approaches to define this reference relation. The first approach is to determine the velocity-depth from MCS or downhole log data in a sediment section with a similar composition and state that contains no gas hydrate (e.g., Site 888). However, the porosity-depth and velocity-depth profiles for no gas hydrate may be quite different in the accretionary prism because of tectonic shortening and overpressure. The second approach is to use the deeper velocity-depth section from MCS data in the area well below the BSR where no gas hydrate or gas is expected and interpolate smoothly to near-surface velocities, where again no regional gas hydrate is expected. Yuan et al. (1996) applied this method to Site 889/890; we recently carried out more extensive semblance velocity analyses in the area (Fig. F8B), and the results confirm the general velocity-depth trends observed by Yuan et al. (1996) but added new detailed information, especially just below the BSR. A third approach is to use downhole log gamma density or neutron porosity data from the hydrate section. These parameters, which are expected to be little affected by the presence of gas hydrate, may be converted to no–gas hydrate velocity using normal no–gas hydrate velocity-density or velocity-porosity relations. The same approach can use ODP core porosity or density data, after dissociation of any gas hydrate). A fourth approach is to employ rock physics modeling by using core porosity and grain composition to calculate a theoretical velocity (e.g., Helgerud et al., 1999; Dvorkin and Nur, 1993).

Depending on which no–gas hydrate/no-gas velocity baseline is used, estimated gas hydrate concentrations at Site 889/890 range from as low as 5% to >25% saturation (Fig. F8). The various velocity baselines shown in Figure F8 were calculated from smoothed downhole core porosity data fitted using Athy's law (Athy, 1930). Background profiles based on the smoothed porosity data were calculated using the Lee et al. (1993) weighted mean equation, the Hyndman et al. (1993) porosity-velocity relation, and an empirical porosity-velocity relation by Jarrad et al. (1995). All these core-based baselines are substantially above the reference profile by Yuan et al., (1996), based on seismic interval velocities, and therefore result in lower gas hydrate concentration estimates of <10% of the pore space.

In spite of having four nearly independent methods of estimating gas hydrate concentrations, Riedel et al. (2005) concluded that the data still allow regional concentrations above the BSR that range from <5% to >25% saturation (3%–13% of sediment volume). The uncertainties come mainly from the difficulty of obtaining no–gas hydrate/no-gas reference profiles with depth, (i.e., velocity, resistivity, and pore fluid salinity).

Free gas concentrations

To estimate the amount of free gas below the BSR, the main constraint is the reduction in P-wave velocity. Again, a no-gas velocity reference is needed. In the case of gas, however, this is less a source of error than for gas hydrate because even a small amount of gas decreases the velocity substantially, and velocities much below that of water are not expected except in the presence of gas. The amount of free gas can be estimated from velocity data using the Biot-Gassmann theory (Desmons, 1996), which includes the effect of pressure (P) and temperature (T) on gas compressibility. Both parameters can be estimated from the general gas hydrate phase diagram. P-wave velocity was calculated as function of free gas concentration for the Site 889 region (Desmons, 1996). P-wave velocity drops to values as low as 800 m/s, but all major changes occur within the first 5%–10% gas saturation. There are two constraints from VSP and regional MCS data showing decreasing P-wave velocity values that can be used to estimate the amount of free gas below the BSR near Site 889 (Fig. F8). The VSP data show a decrease to values of ~1480 m/s, and newly determined interval velocities from MCS Line 89-08 show an even further decrease to values ~1400 m/s below the BSR. These velocities are equivalent to a gas concentration of <1% of the pore space.

Using the effective medium theory by Helgerud et al. (1999), the P-wave velocity for unconsolidated marine sediments was calculated for the presence of free gas at Site 889 (Riedel, 2001). The amount of ~1% of free gas results in a seismic P-wave velocity of ~1480 m/s, similar to the lowest values determined from the VSP experiment at Site 889 (MacKay et al., 1994).

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