FIELD DATA CHARACTERIZATION

Leg 164 Downhole Logging Program

As discussed in the Initial Reports Volume for Leg 164 (Shipboard Scientific Party, 1996b, 1996c, 1996d), Holes 994C, 995B, and 997B were logged with the GLT (Fig. 1) (Collett and Ladd, Chap. 19, this volume). The geochemical log data included both neutron capture and stationary inelastic neutron log measurements. The inelastic neutron measurements consisted of 44 individual 5-min duration stationary measurements. The shipboard-acquired capture and inelastic neutron geochemical measurements require a significant amount of post-field reprocessing to correct for the effects of enlarged and irregular boreholes, fluids in the borehole, logging speed variations, and neutron activation of various elements in the formation. The GLT data processing is performed with a set of log-interpretation computer programs developed by Schlumberger, and the processing steps are described in Initial Reports Volume for Leg 164 (Shipboard Scientific Party, 1996a). In general, the neutron capture data is processed through a seven-step procedure in which (Step 1) the relative elemental yields are calculated by comparing the recorded spectral data to a series of standard spectra; (Step 2) the GLT logging runs are depth shifted to a reference log curve; (Step 3) the total natural gamma radiation and the concentration of thorium, uranium, and potassium in the formation are calculated; (Step 4) the aluminum concentration is calculated; (Step 5) the elemental yields are normalized to calculate the elemental weight percents; (Step 6) the elemental weight percents are converted to oxide percentages; and (Step 7) the statistical uncertainty of the elemental concentrations are calculated and presented in the form of an error log. The GLT logs from all three sites are significantly degraded by poor borehole conditions, but the neutron capture data from Holes 995B and 997B appear to yield useful information about the chemical composition of the formation. However, the neutron capture data from Hole 994C is severely degraded by the rugosity of the borehole, the effects of which could not be corrected. Therefore, the neutron capture data from Hole 994C has been disregarded in this study. Reprocessing of the GLT neutron capture and natural gamma radiation (NGT) data from Holes 995B and 997B yielded accurate estimates of the concentration of the following nine elements within the formation: calcium, iron, silicon, aluminum, potassium, uranium, thorium, gadolinium, and titanium (Shipboard Scientific Party, 1996a). During reprocessing of the GLT neutron capture data, it is was noted that sulfur occurred in concentrations below the resolution capability of the tool; thus, the GLT-derived sulfur concentrations were disregarded in this study. In addition, the configuration of the GLT as used in ODP does not allow the direct acquisition of hydrogen or chlorine concentrations (Shipboard Scientific Party, 1996a). As previously noted, the inelastic neutron geochemical data also required post-field reprocessing. The reprocessing of the inelastic neutron data from Leg 164 was performed by Schlumberger-Doll Research (Dr. Jim Grau, Schlumberger-Doll Research, Ridgefield, CT, pers. comm., 1997). The relative elemental yields from the acquired inelastic neutron data are calculated by comparing the recorded spectral data to a series of standard spectra. The occurrence of both gadolinium and titanium was also considered along with an "ITB" correction that accounts for the direct arrival of high-energy neutrons from the source. An activation standard (CACT) was also used in the analysis of the inelastic neutron data, which deals with the neutron activation of mostly oxygen in the formation. The shipboard measured and reprocessed COR for Holes 994C, 995B, and 997B are listed in Table 3. Data reprocessing and analyses revealed that nine of the inelastic neutron measurements from the Leg 164 drill sites are invalid (Table 3).

Mineralogy and Carbon-Oxygen Content of the Sediments on the Blake Ridge

Described in the response modeling portion of this report are a series of proposed equations that utilize GST-derived elemental concentrations to determine in situ gas hydrate saturations. Equation 6 is a modified version of a standard three-component carbon/oxygen hydrocarbon saturation equation (Eq. 2), which can be used to calculate gas hydrate saturations. Equation 6 accounts for all of the carbon and oxygen atoms associated with the borehole in a gas hydrate-bearing formation. As previously discussed in this report and as shown in Equation 6, the amount of carbon and oxygen measured by the GLT is not only controlled by the chemistry of the formation fluids. To use Equation 6 to calculate gas hydrate saturations we must first assess the carbon and oxygen content of the sediment matrix (including the organic carbon content), borehole fluids, and the boron sleeve on the GST.

In the following section, we have used the shipboard sedimentologic data (Shipboard Scientific Party, 1996b, 1996c, 1996d), shore-based powder X-ray diffraction (XRD) analysis of core samples from Hole 997B, and the GLT neutron capture data from Holes 995B and 997B to generate a comprehensive mineralogic model for the sediments (matrix) on the Blake Ridge and to assess the carbon and oxygen content of the sediments. Shipboard sedimentologic data was also used to assess the amount of organic carbon within the sediments (matrix) on the Blake Ridge (Shipboard Scientific Party, 1996b, 1996c, 1996d). We have also calculated borehole-correction factors needed to assess the effect of oxygen and carbon in the borehole fluids and within the boron sleeve on the GST.

Analysis of the cores recovered from Leg 164 indicate that the sediments on the Blake Ridge consist of a very homogeneous upper Miocene through Holocene hemipelagic accumulation of terrigenous clays and nannofossils, with subordinate amounts of diatoms and foraminifers. At Sites 994, 995, and 997, the sedimentary section was divided into three lithologic units based on observed mineralogic compositions (Shipboard Scientific Party, 1996b, 1996c, 1996d). The upper two lithologic units (lithologic Units I and II; 0 to ~150 mbsf) are Pleistocene and latest Pliocene in age and are characterized by alternating beds of dark greenish gray nannofossil-rich clay and more carbonate-rich beds of lighter greenish gray nannofossil-rich clay. Beds of coarse-grained foraminifer ooze and reddish brown terrigenous muds are rare, but indicate contour-current activity. Lithologic Unit III (from a depth of ~150 mbsf to the bottom of each hole at ~750 mbsf) is a monotonous dark greenish gray nannofossil-rich clay and claystone of late Pliocene to late Miocene age that is moderately to intensively bioturbated. Shipboard smear-slide and XRD analyses reveal that the dominant mineral phases within the cored sediments of the Blake Ridge are clay minerals, calcite, and quartz. Feldspars, dolomite, and pyrite are minor components of the sedimentary section. The clay-size fraction is made up mostly of clay minerals and nannofossils. The silt-size fraction is dominated by quartz; the estimated quartz abundance based on shipboard XRD data almost never exceeds 15 wt% (Shipboard Scientific Party, 1996b, 1996c, 1996d). Disseminated dolomite rhombs make up a few percent of the bulk mineralogy. The biogenic calcareous constituents consist of nannofossils and minor amounts of foraminifers. Siliceous fossils are present as diatoms and rare sponge spicules. Carbonate content is generally higher in lithologic Units I and II (0-150 mbsf; 20-60 wt% carbonate) than in Unit III (150-750 mbsf; 10-20 wt% carbonate). In general, the shipboard sedimentologic data from Leg 164 indicate that the cored sedimentary section on the Blake Ridge is characterized by a relatively uniform mineralogic assemblage of clay and quartz and a more variable calcite content. The effect of the variable calcite content on the bulk composition of the sediment matrix must be accounted for in the COR elemental ratio calculation of gas hydrate saturations. In addition, the composition of the clays must be further examined before proceeding with the proposed COR spectral analyses of gas hydrate saturations.

To further evaluate the mineralogic composition of the sediments on the Blake Ridge, we have conducted detailed XRD analysis of 21 core samples from Hole 997A (Table 4). The shore-based X-ray laboratory used for these analyses is maintained by the Department of Geology and Geological Engineering at the Colorado School of Mines. The X-ray laboratory is equipped with an automated Scintag XDS-2000 X-ray generator and diffractometer. The X-ray diffraction patterns were obtained with the following machine settings: X-ray generator = 40 kV and 40 mA; X-ray tube anode = copper; scatter slit width 1-2 mm; receiving slit width 0.3-0.5 mm (whole-rock samples) and 0.1-0.3 mm (oriented-clay samples); scan = continuous; scanning range = 2º-60º 2 (whole-rock samples), 2º to 40º 2 (oriented-clay samples), 2º-20º 2 (glycolated and heat-treated oriented samples); scanning rate = 2º 2 per minute; and the samples were rotated during analysis. Digital X-ray intensities were recorded and processed with Scintag's DMS X-ray diffraction processing program, which operates under a Windows-NT operating system. The processed digital data was corrected for background intensities (subtracting CuK2 contributions), and the position of each peak (2), d-spacing (Å), and intensity (counts per second above background) are calculated and displayed.

The 21 core samples from Hole 997A selected for XRD analyses are from near the depths at which the inelastic neutron measurements were made with the GLT in Hole 997B (Table 3, Table 4). Whole-rock samples and clay-sized separates were analyzed to identify the constituent minerals in the sediments. All of the core samples were air dried and ground by hand to a uniform texture. Randomly oriented whole-rock subsamples of each core sample were prepared for XRD analysis by packing the ground sediment samples into circular sample holders and sequentially X-raying the samples. The randomly oriented powders were not pretreated with any chemicals. The X-ray diffractograms of the whole-rock samples were examined to identify the major minerals in the core samples, which included quartz, calcite, and clays. Occasionally, the samples contained detectable pyrite, dolomite, siderite, and feldspars (both plagioclase and alkali feldspars).

Additional XRD analysis focused on semiquantitative identification of the type and amount of clays in the recovered core samples. Subsamples of each core sample were used to prepare oriented mounts of the clay-sized fraction. Using methods described in Moore and Reynolds (1989), mixtures of distilled water and crushed whole-rock core samples were centrifuged to separate the less than 2 mm fraction. Oriented mounts of the clay separates were prepared by a filter transfer method. X-ray diffractograms of the air-dried, glycolated (five days at 40ºC), heat-treated (550ºC for 2 hr), oriented clay mounts were examined to determine the mineral composition of the clays. Interpretation of clay mineralogy was based upon evaluating the spacing and relative intensity of the peaks on the X-ray diffractograms, which are controlled by the structure and composition of each mineral within the sample. Three clay minerals were identified within the core samples from the Blake Ridge: illite, smectite, and kaolinite. The method used to estimate the relative amount of illite and smectite (Table 4) in the mixed-layer clays was based on the work of Hower (1981). In this method, peak positions and low-angle diffraction characteristics of the glycolated samples are compared to standard diffraction profiles published by Hower (1981). Characterization of chlorite and kaolinite is made difficult because of peak overlaps and similarities in peak intensities in X-ray diffractograms of untreated samples. We have used standard heat treatment methods to identify the occurrence of chlorite and/or kaolinite in the core samples from Hole 997A. Heat-treating chlorite changes the diffraction pattern: the intensity of the 001 reflector increases and shifts to about 6.3 º2. However, heat treating did not reveal any chlorite peaks within the X-ray diffraction patterns of the Site 997 core samples. The completed XRD studies indicate that mixed-layer illite-smectite and kaolinite are the most abundant clay minerals in the sediments on the Blake Ridge.

In combination, the Leg 164 shipboard sedimentologic data and shore-based XRD analysis of core samples from Hole 997A reveal that clay minerals (mixed-layer illite-smectite and kaolinite), calcite, and quartz are the major mineral components within the cored sedimentary section on the Blake Ridge. Pyrite, dolomite, siderite, and feldspars constitute minor components within the Blake Ridge sedimentary section.

To further evaluate the carbon and oxygen content of the sediments on the Blake Ridge, we have used the results of the shipboard and shore-based sedimentologic studies (discussed above) along with GLT neutron capture data to construct detailed mineralogic models of the cored and logged sedimentary sections in Holes 995B and 997B. Schlumberger's ELAN-Plus (ELemental ANalysis) petrophysical program is a general-purpose inverse problem solving computer code that can be used to analyze GLT data (Quirein et al., 1986; Wendlandt and Bhuyan, 1990). With appropriate downhole log data, including GLT measurements, ELAN-Plus can explicitly define 20 or more mineral components. The ELAN-Plus analyses of GLT elemental yields from Holes 995B and 997B are discussed in the following section.

ELAN-Plus uses available GLT elemental yields and inverse log-analysis procedures to simultaneously quantify various minerals in the formation (Quirein et al., 1986). In general, the ELAN-Plus computer program solves inverse problems, in which log measurements (GLT elemental yields) and expected response equations (mineral compositions) are used to compute volumetric formation components (percentage by volume [vol%] of various mineral phases). ELAN-Plus solves an over-determined system of simultaneous linear response equations in which the number of equations exceeds or is equal to the number of unknowns (minerals). As discussed earlier in this section, nine GLT elemental yields (calcium, iron, silicon, aluminum, potassium, uranium, thorium, gadolinium, and titanium) were considered while modeling the input response equations. Eventually, five GLT elemental yields (calcium, iron, silicon, aluminum, and potassium) were selected to construct the mineralogical models for the sediments at Sites 995 and 997. The elemental yields of uranium, thorium, gadolinium, and titanium were not used in this quantitative interpretation because it is not clear how these minor elemental constituents are associated with the different mineral phases in the sedimentary section being modeled. Including trace elemental concentrations in the mineral response equations can have a significant effect on quantitative mineral calculations. The ELAN-Plus program also requires us to identify the expected minerals (response equations) within the formation. When considering GLT data, the response equations consist of the chemical formula (composition) of the expected mineral phases. As previously discussed in this section of the report, Leg 164 shipboard sedimentologic data and shore-based XRD analysis of core samples from Hole 997A have revealed the presence of eleven minerals within the sediments on the Blake Ridge: clay (illite, smectite, kaolinite), calcite, quartz, feldspars (albite, anorthite, and orthoclase), dolomite, siderite, and pyrite. Because the ELAN-Plus program can only solve over-determined systems, a maximum of only six mineral phases can be quantified within a single ELAN-Plus model when only five GLT elemental yields are available. Therefore, a series of different (six mineral phases) ELAN-Plus models were constructed to quantitatively analyze all eleven mineral phases within the sediments on the Blake Ridge. In addition, other log responses (including total gamma-ray radiation, bulk density, and neutron capture cross section) were considered in the ELAN-Plus mineral modeling effort; however, the additional log measurements were of little value. To further characterize the calcite composition of the sediments in the Blake Ridge boreholes, the photoelectric log from the density tool and the uranium concentration from the NGT were also considered, but both logs were severely degraded by poor borehole conditions. It was determined that the mineral assemblage that best achieved reasonable results (compared to core analyses results discussed later in this section of the report) consisted of smectite, illite, kaolinite, calcite, quartz, and pyrite. Given this mineral assemblage (Table 5), compositions of quartz and pyrite are assumed to be independent of solid solution. The composition of the clay minerals were assumed to be nearly ideal and were modified from the chemical analyses published by Herron and Herron (1988). The assumed chemical formula for the smectite is consistent with a montmorillonite clay type. The Blake Ridge sedimentary section is characterized by a complex carbonate mineral assemblage; principally calcite and siderite, with minor dolomite. We assumed the presence of a single hypothetical carbonate phase, an iron-bearing calcite containing 40.04 wt% calcium and 0.20 wt% iron. Various feldspars, including pure end members and mixed end members, were considered in the ELAN-Plus modeling effort; however, the relative amount of feldspar within the Blake Ridge sedimentary section never exceeded 2-3 vol%.

The ELAN-Plus-derived abundance of smectite (montmorillonite), illite, kaolinite, calcite, quartz, and pyrite in the logged sedimentary sections of Holes 995B and 997B are shown in Figure 7. In general, the mineralogy of the GLT-logged sedimentary section in Hole 995B is relatively uniform with depth (Fig. 7A). In Hole 995B, clays (montmorillonite, illite, and kaolinite) constitute about 60%-80% of the sedimentary section, with the amount of montmorillonite and illite being relatively similar throughout the hole; however, kaolinite appears to be less abundant. Pyrite occurs in Hole 995B as a widely disseminated minor component. The combined abundance of calcite and quartz ranges from about 10% to 40% according to the ELAN-Plus modeling estimates. In comparison, Hole 997B (Fig. 7B) is characterized by a more variable sedimentologic section. ELAN-Plus analyses indicate that clays comprise about 60%-100% of the bulk sediments in Hole 997B. The most notable mineralogic feature of Hole 997B is the obvious uphole increase in kaolinite starting near 250 mbsf. In addition, the ELAN-Plus analyses from Hole 997B indicate a downhole trend of increasing quartz that is not present in Hole 995B.

To evaluate the accuracy of the ELAN-Plus-derived mineralogic models, we compared the results of the ELAN-Plus modeling of the GLT data from Holes 995B and 997B with mineralogic models from both shipboard and shore-based XRD analysis of core samples (Fig. 8, Fig. 9). In Figure 8 the ELAN-Plus-determined quartz and calcite abundance for Holes 995B and 997B are compared to shipboard XRD analysis of core samples from Holes 995A and 997A (Shipboard Scientific Party, 1996c, 1996d). In comparison, the relative abundance of quartz in the sedimentary section at Sites 995 and 997, as determined by ELAN-Plus and shipboard XRD analysis, is similar. The ELAN-Plus-interpreted downhole trend of increasing quartz in Hole 997B was also observed in the shipboard XRD core analysis from Hole 997A. However, the ELAN-Plus-derived quartz abundance in both Holes 995B and 997B is characterized by a high-frequency variation, which is likely due to degraded GLT measurements. The relative abundance of calcite, as interpreted by ELAN-Plus and XRD analysis, at Sites 995 and 997 are also similar. The XRD-interpreted near-surface (<100 mbsf) increase in calcite abundance is not observed in the ELAN-Plus mineralogic models, because the upper 100 m of both holes were not logged with the GLT. However, both the ELAN-Plus and XRD analyses do suggest a downhole trend of increasing calcite abundance starting at a depth of about 200 mbsf. To further evaluate the apparent uphole increase of kaolinite in Hole 997B, we compared the ELAN-Plus modeling results with shore-based XRD analysis of 300 core samples from Hole 997A (Fig. 9). The additional 300 XRD analyses from Hole 997A were provided by Dr. Ryo Matsumoto, University of Tokyo, Japan. As shown in Figure 9, the relative abundance of kaolinite in Hole 997A, as determined by shore-based XRD analysis of core samples, is very similar to the kaolinite concentrations predicted by the ELAN-Plus mineralogic modeling in Hole 997B. In general, the mineralogic models for Sites 995 and 997 interpreted from available shipboard and shore-based XRD analysis of core samples compare favorably with the ELAN-Plus-derived mineralogic models for both drill sites.

In the following section the ELAN-Plus-derived mineralogic models for Holes 995B and 997B were used to calculate the carbon and oxygen content of the sediment matrix at the subsurface depths of the 35 inelastic neutron measurement stations that were determined to have yielded valid carbon/oxygen elemental ratios in Holes 994C, 995B, and 997B (Table 6). The concentrations of carbon () and oxygen () in the sediment matrix were calculated by multiplying the carbon-oxygen elemental concentration in each mineral assessed in Table 1 by the volume percent concentration of the ELAN-Plus-derived mineral compositions in Holes 995B (Fig. 7A) and 997B (Fig. 7B). The ELAN-Plus mineralogic model for Hole 995B was used to calculate the sediment matrix carbon-oxygen elemental concentrations for the nine GLT inelastic neutron stations in Hole 994C.

The next variable needed before we can calculate gas hydrate saturations with Equation 6 is the organic carbon content (C) of the sediments on the Blake Ridge. Fortunately, standard ODP core analysis includes the measurement of TOC content. On the Blake Ridge the TOC content of the sediments cored at Sites 994, 995, and 997 ranged from about 0.5 to 2.0 wt% (Shipboard Scientific Party, 1996b, 1996c, 1996d). In Table 6, we have included the shipboard-calculated total organic carbon content (C) of the sediments (converted to vol%) at the subsurface depths of the 35 "valid" GLT inelastic neutron stations at Sites 994, 995, and 997.

Borehole Carbon/Oxygen Corrections

The last two variables needed to calculate gas hydrate saturations with Equation 6 are the borehole correction factors for the presence of carbon and oxygen in the borehole fluids and within the boron sleeve on the GST. As discussed earlier in this report, each portion of the borehole (including the open-hole and tool sleeves) that contribute to the carbon (Cb) and oxygen (Ob) borehole correction factors can be individually quantified. Because ODP boreholes are drilled with seawater, the borehole carbon correction factor (Cb) relative to the carbon content of the borehole fluids can be disregarded. However, because the GST used on Leg 164 has a boron sleeve, the volume of carbon within the sleeve needs to be determined. The borehole carbon correction factor (Cb) for the boron sleeve was calculated by the method described by Roscoe and Grau (1988), in which the tool (GST) diameter (8.8265 cm) and sleeve thickness (0.6477 cm) are used to calculate the volume of the boron sleeve (Vc) actually measured by the GST. Assuming the carbon content of the boron sleeve is similar to the carbon content of oil (Dr. Jim Grau, Schlumberger-Doll Research, Ridgefield, CT, pers. comm. 1997) and by using the shipboard core-derived sediment porosities, it was possible to calculate the required Cb correction factors for the boron sleeve on the GST (Table 6). The borehole oxygen correction factor (Ob) is assumed to be the product of the drilling fluids (seawater) in the borehole. The volume of drilling fluids in the borehole is dependent on the diameter of the logging tool and the size of the borehole. In "conventional" boreholes, with diameters less than 30 cm, the borehole oxygen correction factor (Ob) is calculated in the same manner as the boron sleeve carbon correction factor (Cb) discussed above. In the borehole oxygen correction factor (Ob) calculations, the GST tool diameter (8.8265 cm) plus boron sleeve thickness (0.6477 cm) are used along with borehole caliper logs to calculate the volume of water (Vo) within the borehole measured by the GST. Assuming a water oxygen content of 0.055509 Avogadro units (or 3.342758 1022 atoms of oxygen per cubic centimeter of water; Table 1) and by using shipboard core-derived porosities, it is possible to calculate the required oxygen correction factors (Ob) for the drilling fluids in the portion of the boreholes that measured less than 30 cm in diameter (Table 6). However, the calculation of borehole oxygen correction factors (Ob) for boreholes exceeding 30 cm in diameter is more problematic due to the lack of GST calibration studies in severely enlarged boreholes. Because the diameter of the borehole greatly exceeded 30 cm at the depth of most of the GST inelastic neutron stations, a new method of calculating the required borehole oxygen correction factors (Ob) was developed. In this new method, GST measured carbon/oxygen ratios from three known water-saturated (no hydrocarbons) zones in Holes 994C and 995B were used to calculate borehole oxygen correction factors (Ob) for severely enlarged borehole conditions (borehole diameters of 36, 38, and 46 cm). The three water-saturated GLT calibration zones (Hole 994C: 248 and 263 mbsf; Hole 995B: 464 mbsf) were interpreted as 100% water saturated from available electrical resistivity and acoustic transit time borehole logs discussed in Collett and Ladd (Chap. 19, this volume). In Figure 10 the borehole oxygen correction factors (Ob) calculated from the GLT measured carbon/oxygen ratios in the known water-saturated zones are plotted along with the "standard" borehole oxygen correction factors (Ob) calculated for smaller (<30 cm) "conventional" boreholes. All of the borehole oxygen correction factors (Ob) in Figure 10 assume a sediment porosity of 55%. In Figure 10, the power function regression trendline projected through the "standard" and water zone calculated borehole oxygen correction factors (Ob) is used as a standard reference curve from which to calculate borehole oxygen corrections within severely enlarged boreholes. Listed in Table 6 for each of the 35 "valid" GST inelastic neutron stations in Holes 994C, 995B, and 997B are the borehole oxygen correction factors (Ob) as projected from the borehole oxygen correction trendline in Figure 10.

Carbon/Oxygen Calculated Gas Hydrate Saturations

The results of the carbon/oxygen calculated gas hydrate saturations from the 35 "valid" GST inelastic neutron stations at Sites 994, 995, and 997 are listed in Table 6 and displayed in Figure 11. The modified version of the standard three-component carbon/oxygen hydrocarbon saturation equation (Eq. 6) has been used to calculate the gas hydrate saturations in Table 6 and Figure 11. Table 6 also contains the values for most of the required variables in Equation 6. Listed in Table 6 for each of the GST neutron stations (Holes 994C, 995B, and 997B) are the station depth, reprocessed carbon/oxygen ratio (COR), core-derived sediment porosity (), total organic carbon content of the sediment (C), sediment matrix carbon () and oxygen content (), borehole carbon (Cb) and oxygen (Ob) correction factors, calculated gas hydrate borehole carbon (Cb) and oxygen (Ob) correction factors, and calculated gas hydrate saturations (Sh). The remaining variables in Equation 6 are assigned constant values: A = 0.75, ß = 0.007306, = 0.099908, = 0.055509, and µ = 0.044205 (the atomic concentrations for ß, , , and µ are listed in Avogadro units). The carbon/oxygen derived gas hydrate saturations in Figure 11 are shown as discrete values and are plotted along with downhole log traces of the resistivity-derived "standard" Archie (average core porosity) calculated gas hydrate saturations (from Collett and Ladd, Chap. 19, this volume). Each of the carbon/oxygen derived gas hydrate saturations in Figure 11 are also depicted with an error bar of 10%, which is the likely minimum error associated with the uncertainty in the calculated saturations (as previously discussed in this report).

The carbon/oxygen derived gas hydrate saturations in Hole 994C (Fig. 11A), Hole 995B (Fig. 11B), and Hole 997B (Fig. 11C) range from very low negative values (less than -10%) to relatively high values near 30%. The carbon/oxygen derived gas hydrate saturations that fall below 0% (or 100% water saturated), which is theoretically impossible, are likely caused by enlarged borehole conditions that have degraded the inelastic neutron measurements. This suggests that the borehole corrections developed in this study may not be able to account for all of the borehole conditions encountered on Leg 164. In comparison, the carbon/oxygen derived gas hydrate saturations in Hole 995B (relative to Holes 994C and 997B) appear to more closely match the resistivity derived saturations. This further demonstrates the dependence of the carbon/oxygen derived gas hydrate saturations on the size and rugosity of the borehole, because Hole 995B was characterized by relatively good borehole conditions. In general, the Leg 164 GLT measurements (ELAN and COR) were significantly degraded by poor borehole conditions. Within zones of relatively high-quality GLT measurements, the carbon/oxygen calculated gas hydrate saturations from the Blake Ridge compare favorably to the resistivity derived gas hydrate saturations. However, the inherent uncertainty associated with carbon/oxygen calculated gas hydrate saturations (Table 6; Fig. 11) remains an unresolved problem that limits the use of the GLT as a gas hydrate research tool.

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