The ODP Logging Database contains the majority of the logging data collected during ODP and all of the downhole logging data collected during Leg 204; it can be accessed and searched through the ODP Logging Services Web site: www.ldeo.columbia.edu/BRG/ODP/DATABASE/DATA/search.html. The Web site provides convenient methods for downloading large amounts of data, as well as information about the applications of logging data to scientific problems. In addition, logging data are also distributed on a CD-ROM included in the Leg 204 Initial Reports volume (Tréhu, Bohrmann, Rack, Torres, et al., 2003).
The NMR LWD data from Leg 204 as presented on the ODP Logging Service Web site is stored in two ASCII files.
Because gas hydrates are solids consisting of weakly interacting host and guest molecules, NMR methods of analysis, which are sensitive to the mobility of the guest and host molecules, are useful in establishing the presence of gas hydrates in high-resolution laboratory studies (Davidson and Ripmeester, 1984). Moreover, the high-resolution capabilities of modern NMR laboratory devices can provide valuable information about clathrate structures. No laboratory experiments, however, have been conducted to analyze the response of wellbore NMR devices to the presence of gas hydrate.
There are numerous studies in which laboratory apparatuses have been used to characterize the nuclear magnetic properties of gas hydrates. Results of these laboratory NMR studies were summarized by Ripmeester and Ratcliffe (1989), from which most of the following discussion has been obtained. In Davidson et al. (1986), NMR line shapes were obtained from a Gulf of Mexico gas hydrate sample. These laboratory experiments clearly showed that the sample contained substantial amounts of gas hydrate. Davidson et al. (1986) did not report any gas hydrate relaxation times or free-fluid indexes (FFIs); however, they did publish several gas hydrate NMR spectrums from which it is possible to obtain relaxation times.
If published NMR line shapes for Structure I methane hydrates are assumed to be Gaussian in nature, it can also be assumed that the free induction decay is also Gaussian and the second moment (or mean square line width) is then inversely related to T2 (J.A. Ripmeester, pers. comm., National Research Council Canada, 1999). Thus, if a NMR second moment of ~33 Gauss is assumed, T2 of the water molecules in the Structure I gas hydrate is ~0.01 ms.
The example free-fluid induction decay signal plot in Figure F4 shows that the gas hydrate clathrate T2 of 0.01 ms is very similar to the relaxation times of other solids such as the rock matrix. T2s on the order of 0.01 ms are sufficiently short to be lost in the "dead time" (below the detectable limit of the tool) of standard NMR borehole instruments. Gas hydrates, therefore, cannot be directly detected with today's downhole NMR technology. It is possible, however, that existing NMR well logs could still yield very accurate gas hydrate saturation data. In theory, due to the short T2s of the water molecules in the clathrate, gas hydrates would not be "seen" by the NMR tool and the in situ gas hydrate would be assumed to be part of the solid matrix. Thus, the NMR-calculated FFI and associated porosity estimate in a gas hydrate–bearing sediment would be apparently lower than the actual porosity. With an independent source of accurate in situ total porosities, such as density or neutron porosity logging measurements, it would be possible to accurately estimate gas hydrate saturations by comparing the apparent NMR-derived porosities with the actual total porosities. The above-described calculations were originally presented by Collett (2000); more recently Kleinberg et al. (2003) have been able to demonstrate the use of this approach with acquired field data.
Data from the LWD NMR logs have been used to calculate sediment porosities in all nine Leg 204 holes surveyed with the proVision tool (Fig. F5A, F5B). Core-derived physical property data have also been used to both calibrate and evaluate the proVision-derived sediment porosities. The sediment porosities derived by the proVision in all nine holes ranged from ~80% near the seafloor to ~35% near the bottom of one the deepest holes on Hydrate Ridge (Hole 1245A).
In studies of downhole logging data it is common to compare porosity data from different sources to evaluate the results of particular measurements. The comparison of core-derived and proVision log-derived porosities in Figure F5A and F5B reveals that the proVision porosities are generally similar to the core-derived porosities. Dissimilarities occur in the upper portion of several holes where the proVision porosity log is degraded by washouts. In numerous cases, the proVision porosity logs also exhibit anomalous low-porosity zones within the interval of expected gas hydrate stability. The differences between the proVision porosity log and core values and other porosity logs (not shown) may also be due to the effects of elastic rebound (e.g., Hamilton, 1976; Goldberg, 1997), excessive lateral motion of the tool (Horkowitz et al., 2002), and the distinctive physical properties of gas hydrates (e.g., Collett, 2000; Kleinberg et al., 2003). The response of the proVision porosity log in the gas hydrate–bearing sediments on Hydrate Ridge was one of the primary focuses of the postcruise research activities as discussed below.
The presence of gas hydrate at most of the sites drilled during Leg 204 was documented by direct sampling, with pieces of gas hydrate being recovered from cores (Shipboard Scientific Party, 2002). Gas hydrates were also inferred to occur in every hole drilled on Hydrate Ridge during Leg 204 based on geochemical core analyses, infrared image analysis of cores, and downhole logging data. Gas hydrate occurrences are generally characterized by increases in logging electrical resistivities and acoustic velocities and apparent reductions in NMR porosities (Collett, 2000; Kleinberg et al., 2003). Most of the Leg 204 downhole logging data from Hydrate Ridge show that the sedimentary section above the expected depth of the bottom-simulating reflector (BSR) is characterized by distinct zones of elevated values of electrical resistivity and acoustic velocity. In addition, the proVision NMR tool often reveals low-porosity zones within the same high-resistivity intervals, suggesting the possible occurrence of gas hydrate. By quantifying this difference, the NMR logging data have been used to estimate the amount of gas hydrate at each LWD site drilled on Hydrate Ridge during Leg 204 (Fig. F6). For the purpose of this discussion, it is assumed that the relatively low NMR porosities in comparison to the density-derived porosities above the depth of the BSR at each site (Fig. F6) are due to the presence of gas hydrate. The simple relationship below between density porosity (DPHI) and NMR porosity (TNMR) was used to calculate gas hydrate saturations in each LWD hole on Hydrate Ridge:
A more rigorous result can be obtained by further assessing the effect of gas hydrate on the density log measurement (Kleinberg et al., 2003).
As shown in Figure F6, the assumed gas hydrate saturations in the section above the BSR ranges from –10% (erroneous negative values) to values approaching 50% at Site 1249. However, the scatter of the NMR-derived values in intervals where free gas and gas hydrate are not present (Sh = 0) ranges from –10% to +10%, which probably represents a measure of the relative of uncertainty of the NMR porosity measurement. Although large spikes in the NMR-derived gas hydrate concentration reaching 20%–30% are observed above the BSR at most of the Leg 204 sites, it appears that the NMR-derived gas hydrate saturations on Hydrate Ridge are lower on average than those calculated by the Archie method (Collett et al., 2003).
The response of the NMR logging tool to free gas is similar to the response in gas hydrate (Kleinberg et al., 2003). Both show an apparent reduction in the measured NMR porosity. In Figure F6, we also see "apparent" gas hydrate–bearing zones below the depth of the BSR at several sites, a result that is theoretically impossible. Combining Archie resistivity, density, and neutron porosity logging analyses has confirmed the occurrence of free gas below the BSR on Hydrate Ridge. These zones are also characterized by low wireline logging acoustic velocities (Shipboard Scientific Party, 2002), another strong indication of the presence of free gas. Free gas saturations in some of these localized zones have been estimated to exceed 50%, such as in the Horizon A turbidite sequence (Collett et al., 2003; Tréhu et al., 2004). In Figure F6, the estimation of gas hydrate concentration from the NMR logs is therefore only valid above the BSR.