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doi:10.2204/iodp.proc.311.104.2006

Downhole logging

Logging while drilling

Operations

Hole U1326A was spudded at 1839 mbrf water depth (drillers depth) at 1320 h on 22 September 2005. LWD tools included the GeoVISION resistivity tool, the EcoScope tool, the SonicVISION tool, the TeleScope MWD tool, the ProVision nuclear magnetic resonance tool, and the ADNVISION tool. For details on each tool and the measurements it makes, see "Downhole logging" in the "Methods" chapter.

Similar to the other sites, the uppermost 10 m of the hole was drilled with a rotation rate of 10–15 rpm, a pump flow rate of 190 gpm, and an ROP of 10–15 m/h. We then increased these rates to 40 rpm and 220 gpm until 30 mbsf, and then to 60 rpm and 270 gpm (to start the MWD telemetry), keeping the instantaneous ROP below 50 m/h with depth. The target depth of 300 mbsf (2139 mbrf) was reached at 0410 h on 23 September. Considering the proximity between Holes U1326A and U1327A (~8 nmi apart), it was decided to raise the drill string above the seafloor and move by DP to the location of Hole U1327A without tripping back to the surface.

Gas monitoring with real time logging-while-drilling/measurement-while-drilling data

The LWD logs were acquired to plan coring and pressure coring operations in subsequent holes at Site U1326. As Hole U1326A was drilled without coring, the LWD/MWD data had to be monitored for safety to detect gas entering the borehole. As explained in "Downhole logging" in the "Methods" chapter, the primary measurement used for gas monitoring was annular pressure while drilling, which is measured by the EcoScope tool in the borehole annulus. We looked for sudden decreases >100 psi in the annular pressure, which could be caused by low-density gas entering the borehole. We also monitored pressure increases of the same magnitude, which could be the result of fluid acceleration caused by a gas kick (Aldred et al., 1998).

Figure F49 shows the measured borehole fluid pressure profile in Hole U13265A after subtraction of the best-fit linear trend. The borehole fluid pressure shows only small fluctuations over the trend, except for a positive pressure anomaly (~30 psi) centered at 182 mbsf. Whereas positive drilling fluid pressure anomalies at other sites were correlated with hole enlargements, the ultrasonic caliper log (Fig. F49) shows no hole diameter anomaly near 182 mbsf; thus, the reason for the pressure increase is not clear. One possibility is that a partial borehole collapse somewhere above the LWD string caused a sudden influx of cuttings in the annulus. The observed fluid pressure anomalies were well below the 100 psi level that would have required preventive action.

We also monitored the coherence of the sonic waveforms acquired by the SonicVISION tool, focusing on the velocity of the fluid wave in the borehole. Loss of coherence in the waveforms and a slower velocity for the drilling fluid indicate the presence of gas. The sonic waveform coherence image in Figure F49 shows almost everywhere a well-defined fluid arrival with a slowness of ~200 µs/ft, which corresponds to the expected fluid velocity of ~1500 m/s. The only anomalies are at two intervals (80–87 and 258–261 mbsf), where the fluid arrival suddenly disappears. As will be shown later, these two intervals are characterized by high resistivity. The shallower one is presumably a gas hydrate–rich interval where some gas hydrate dissociation may have generated free gas during drilling. The deeper interval is located below the BSR depth, estimated from seismic reflection data (234 mbsf), and may contain free gas. Even if the low coherence of the drilling fluid arrival was caused by free gas, it is important to stress that the fluid pressure anomalies were small and that if gas was in the drilling fluid it had to be in low concentrations.

Logging data quality

Figure F49 also shows the quality control logs for Hole U1326A. The ROP was generally 60 m/h or less in the interval from the seafloor to TD. This is sufficient to record one measurement every 4 cm (~25 measurements/m) in the GeoVISION resistivity, and no significant resolution loss was observed with variation in ROP. The ultrasonic caliper log, which is a direct measurement of the borehole diameter recorded by the EcoScope tool, is our best indicator of borehole size. It shows a large washout near the seafloor (10–20 mbsf), where the hole diameter reached 13 inches (33 cm), and many smaller hole enlargements throughout the rest of the hole. In several cases, borehole diameter restrictions (<9 inches or 23 cm) can be observed immediately below these enlargements (e.g., at 110, 206, and 226 mbsf), suggesting that the washouts were generated above resistant intervals that were hard to drill. Except for these enlargements and restrictions, the hole diameter was generally just above 10 inches (25 cm).

Despite these hole irregularities, the density correction, calculated from the difference between the short- and long-spaced density measurements, generally varies from 0 to 0.2 g/cm³ (Fig. F49), showing the good quality of the density measurements. Figure F50 is a summary of the Hole U1326A LWD gamma ray, density, resistivity, and resistivity image logs with density and porosity measurements from cores from Holes U1326C and U1326D superimposed (see "Physical properties").

The depth relative to the seafloor was fixed for all LWD logs by identifying the step change in the gamma ray log associated with the seafloor. For Hole U1326A, the gamma ray logging pick for the seafloor was at a depth of 1838 mbrf.

Wireline logging

Operations

Hole U1326D was drilled to a depth of 300 mbsf, which was reached at 1330 h on 26 October 2005. The weather conditions were favorable (ship's heave was ~3 m), but because of a poor weather forecast that required that we leave the site by midday on 27 October, it was decided to run a single, armless tool string that included the most critical log still missing (sonic velocity). The tool string consisted of the SGT, the DSI, and the DIT. For details on the different tools, see "Downhole logging" in the "Methods" chapter.

After hole preparation, rig up of the tool string started at 2210 h and was completed at 2308 h on 26 October. The bottom of the hole was reached without complications at 2139 mbrf (302 mbsf), and we started logging a first pass at 0034 h on 27 October. The first pass was completed at 1936 mbrf (99 mbsf), with the top of the 34 m long tool string just below the drill pipe. We lowered the tool string to the bottom of the hole again and started a second pass at 0127 h. The tool string reached the seafloor at 0235 h, and was pulled back onto the rig floor at 0345 h. Rig down of all wireline equipment was completed at 0505 h on 27 October.

Logging data quality

As we did not run a caliper log in Hole U1326D, we cannot clearly determine the effects of hole diameter on the wireline logging measurements. There are a few intervals (e.g., ~100 and 180 mbsf) where both the gamma ray and the induction resistivities have very low values, which could be the result of an enlarged hole (Fig. F51). Apart from these anomalous intervals, the gamma ray and induction logs are of very good quality.

A comparison of the gamma ray, resistivity, and acoustic logs measured in the two passes show an excellent correlation (Fig. F51). The acoustic waveforms and slowness-time coherence projection are shown in Figure F52. The velocity profiles acquired in the two passes are very similar, and almost no additional processing was required to derive reliable compressional (V_P) and shear (V_S) wave velocities.

Despite the absence of caliper data, the analysis of the sonic waveforms and coherences in Figure F52 suggests that the borehole was in good condition. The very clear P-wave arrivals in the monopole waveforms generate a strong coherence peak that makes V_P labeling extremely robust over the entire hole. In addition to the clear P-wave arrivals, the monopole waveforms also display a strong arrival at ~2 ms, which likely corresponds to a fluid wave with a uniform velocity that creates a second coherence "track" at a nearly constant velocity of ~1500 m/s. In the interval between ~70 and 80 mbsf, where both velocity and resistivity are very high, the monopole waveforms display low amplitudes that are characteristic of very high gas hydrate concentrations. The low waveform amplitudes for most of the lower dipole and a portion of the upper dipole could possibly result from the attenuating influence of disseminated gas hydrate. This effect would be accordingly stronger on the higher frequency waveforms from the lower dipole. This effect could also be the result of poor hole conditions, but the high quality of the monopole waveforms suggests otherwise.

The depths relative to seafloor for all of the wireline logs were fixed by identifying the step change in the gamma ray log associated with the seafloor. The gamma ray pick for the seafloor was at 1837 mbrf in Hole U1326D.

Logging-while-drilling and wireline logging comparison

Figure F53 shows a comparison of LWD (Hole U1326A) and wireline (Hole U1326D) data, using the gamma ray and resistivity logs. The LWD gamma ray log gives higher readings (100 gAPI on average) than the wireline log (40 gAPI on average), and the curves do not have features that can be easily correlated. Part of the reason for this difference may be that the wireline gamma ray log has not been corrected for the effect of hole size, as we could not run a caliper log. The ultimate reason for this discrepancy is not clearly understood. However, wireline data are consistent with data recorded during Leg 146.

The comparison of resistivities shows similar values measured by the LWD and wireline logs. The prominent, asymmetric resistivity high that is clearly visible in the LWD logs at 72–100 mbsf (Fig. F50) is found with the same shape in the wireline resistivity logs, only displaced upward by ~12 m to 60–88 mbsf. Hole U1326D, where the wireline logs were measured, was drilled ~20 m southwest of LWD Hole U1326A. As described in "Logging-while-drilling borehole images," borehole images show layers steeply dipping to the north-northeast. A northward-dipping structure is consistent with finding the same stratigraphic horizon in a shallower position in Hole U1326D compared to Hole U1326A. The resistivity peak centered around 260 mbsf in the LWD resistivity logs, on the other hand, does not clearly correlate to a similar shallower peak in the wireline logs; the wireline logs instead show four peaks between 195 and 255 mbsf that do not have any obvious counterpart in the LWD logs. Finally, as noted earlier, the very low resistivities measured in the wireline logs at ~100 and 180 mbsf are likely to be the result of an enlarged hole.

Logging units

The logged section in Holes U1326A and U1326D can be divided into three logging units, based on obvious changes in the LWD and wireline gamma ray, density, electrical resistivity, and acoustic measurements (Figs. F50, F51, F52). These three logging units have no obvious correspondence to the lithostratigraphic units described in "Lithostratigraphy." We defined the depth intervals of the logging units mainly on the basis of the more complete suite of LWD logs acquired in Hole U1326A. As noted above, the wireline logs from Hole U1326D show similar horizons in a position that is shallower by ~12 m, and the depth intervals of the logging units need to be correspondingly adjusted in Hole U1326D.

Logging Unit 1 (0–72 mbsf in Hole U1326A) is characterized by a resistivity trend that steadily increases from ~1 m near the seafloor to ~1.5 m at the bottom of the unit. This increase in resistivity with depth is matched by an increase in density (from 1.7 g/cm³ near the seafloor to 2 g/cm³ at 120 mbsf) and a decrease in porosity (from 70% near the seafloor to 50% at 120 mbsf). This unit shows only a few small resistivity peaks (e.g., at ~52 and 60 mbsf in the LWD logs; Fig. F50) that may be attributed to gas hydrate.

Logging Unit 2 (72–240 mbsf in Hole U1326A; 60–228 mbsf in Hole U1326D) is characterized by uniform density and porosity trends, with values around 2 g/cm³ and 45%, respectively. Resistivity shows a prominent high at 72–107 mbsf in Hole U1326A, with peaks above 40 m. This interval of high resistivity displays a number of alternating, thin intervals of very high and low resistivity, spanning the range 2–40 m. These thin intervals are especially well defined between 77 and 90 mbsf (Fig. F50). These high and low resistivities are likely to correspond to alternating layers with high and low gas hydrate concentrations, respectively. Below this high-resistivity interval at the top, the rest of Unit 2 (107–240 mbsf in Hole U1326A; 95–228 mbsf in Hole U1326D) has a constant background value of ~1.5 m. The P-wave velocities (V_P) measured by wireline logging in Hole U1326D (Fig. F51) show highly variable values (between 1750 and >3000 m/s) in the same interval (60–90 mbsf) where the wireline log resistivities reach high values and are variable. The high V_P values are consistent with high concentrations of gas hydrate, as pure gas hydrate has V_P values of 3800–4000 m/s (e.g., Pearson et al., 1983). For the rest of this unit, V_P increases from ~1700 m/s below 90 mbsf to 2000 m/s at the bottom of Unit 2 (228 mbsf in Hole U1326D).

The top of logging Unit 3 (240–300 mbsf in Hole U1326A; 228–288 mbsf in Hole U1326D) shows a subtle increase in density, from ~2 g/cm³ at the top to 2.1 g/cm³ at 255 mbsf, and remains constant to 300 mbsf. This density trend is matched by a corresponding decrease in porosity (from 40% at the top to 35% at 255 mbsf). The background resistivity also increases slightly and displays a clear peak that reaches 5 m at 255–261 mbsf (Fig. F50). This resistivity peak could be caused by gas hydrate or free gas. Free gas seems more likely, as this interval is located below the projected BSR depth estimated from seismic reflection data (234 mbsf), and should be below the base of the GHSZ. On the other hand, V_P is at least 1700 m/s in Unit 3 (Fig. F51), and there is no sign of the low velocities expected if free gas were present. The relatively high velocities observed could lead to an increase in the depth of the BSR estimated from reflection traveltimes.

Logging-while-drilling borehole images

The GeoVISION, ADNVISION, and EcoScope LWD tools generate high-resolution images of borehole log data. The ADNVISION and EcoScope tools produce images of density and hole radius (computed on the basis of the density correction, which depends on the borehole standoff). The GeoVISION tool produces a gamma ray image and shallow, medium, and deep depth of investigation resistivity images.

Figure F54 shows some of the LWD images collected by the EcoScope and GeoVISION tools. It should be noted that the display in Figure F54 is highly compressed in the vertical direction. The unwrapped images are ~90 cm wide (for an 11 inch diameter borehole), and the vertical scale is compressed by a factor of ~37:1. These high-resolution images can be used for detailed sedimentological and structural interpretations and to image gas hydrate distribution in sediments (e.g., in layers, nodules, or fractures). Gas hydrate–bearing sediments exhibit high resistivities within intervals of uniform or low bulk density. Layers with high resistivities and high densities are likely to be low-porosity, compacted, or carbonate-rich sediments.

The images suggest that there may be high concentrations of gas hydrate in thin layers between 81 and 100 mbsf in Hole U1326A, where densities are generally low and high-resistivity, gas hydrate–rich layers (bright in the resistivity images of Fig. F54) alternate with low-resistivity layers (dark in the images of Fig. F54) that are likely to contain little or no gas hydrate. The images also clearly show a number of hole enlargements, which are imaged as dark bands in Figure F54 at depths of ~110, 206, and 225 mbsf. These are the same washouts detected by the LWD ultrasonic caliper (Fig. F49).

When the images are examined with less vertical exaggeration than in Figure F54, they clearly show that the layers identified in Hole U1326A are dipping to the north-northeast with dips between 45° and 85° (Fig. F55). This is in agreement with the west-northwest–east-southeast strike of the uplifted ridge where Hole U1326A was drilled and with evidence for landward-dipping reflectors in seismic profiles. It should be pointed out that reflections on the seismic profiles across this ridge show dips of 15° or less, but seismic reflection data cannot image steeply dipping structures. Also, the observation of steep dips in Hole U1326A should be interpreted carefully. In a complex tectonic structure (e.g., a faulted anticline or a stack of imbricated thrust sheets) the dips measured along a vertical line may vary significantly over short horizontal distances, and their interpretation can be variable (for examples of structural interpretation of dips from borehole images, see Luthi, 2001).

Logging porosities

Sediment porosities can be determined from analyses of recovered cores and from downhole measurements (see "Physical properties" and "Downhole logging," both in the "Methods" chapter). Data from the LWD density and neutron logs from Hole U1326A were used to calculate sediment porosities. Core-derived physical property data, including porosities (see "Physical properties"), were used to both calibrate and evaluate the log-derived sediment porosities.

The LWD log–derived density measurements were used to calculate sediment porosities () using the standard density-porosity relation

= (_g – _b)/(_g – _w).

We used a constant water density (_w) of 1.03 g/cm³ and a grain/matrix density (_g) of 2.76 g/cm³, which is the average grain density measured in core samples (see "Physical properties"). The density log–derived porosities range from ~60% near the seafloor to ~40% at 300 mbsf (Fig. F56). In several intervals (e.g., 10–20, 105–110, and 205–210 mbsf) hole enlargements resulted in an erroneously low value of density, and corresponding porosity values that are too high.

The LWD neutron porosity log from Hole U1326A (Fig. F56) yielded sediment porosities ranging from an average value of ~70% near the seafloor to ~50% at 350 mbsf. As noted earlier, porosities measured by the neutron log are expected to be higher than those computed from the density log in clay-rich sediments because the neutron log essentially measures hydrogen abundance, and hydrogen in clay minerals is counted as porosity. The EcoScope neutron porosity shown in Figure F56 is the "best thermal neutron porosity," which has been corrected for the effect of clay, so that it is only slightly higher than the density porosity. Like the density porosity, the neutron porosity log also gives values that are too high where the hole is enlarged.

The comparison of core- and LWD-derived porosities in Figure F56 shows good agreement throughout the logged interval, with the density porosities being slightly lower and the neutron porosities slightly higher than the core porosities.

Gas hydrate and free gas occurrence

As previously discussed (see "Downhole logging" in the "Methods" chapter) the presence of gas hydrate is generally characterized by increases in electrical resistivity and acoustic velocity that are not accompanied by a corresponding decrease in porosity. A decrease in porosity alone in a water-saturated sediment can result in an increase in resistivity and acoustic velocity. Resistivity logging data from Holes U1326A and U1326D show a number of positive resistivity anomalies without a corresponding decrease in porosity (Figs. F50, F51), suggesting that there are several intervals where gas hydrate may be present.

Water saturation from Archie's equation

To estimate the amount of gas hydrate at Site U1326, we used the Archie relation (e.g., Collett and Ladd, 2000)

S_w = [(a x R_w)/(^m x R_t)]^1/n,

where

• S_w = water saturation,
• a = tortuosity coefficient,
• R_w = formation water resistivity,
• = density porosity computed from the ADNVISION enhanced resolution bulk density,
• m = cementation coefficient,
• R_t = GeoVISION high-resolution button deep average resistivity, and
• n = saturation coefficient.

We use the button deep resistivity instead of the ring resistivity because the button deep resistivity matches the resistivities with the greatest depth of investigation measured by the EcoScope tool (except for having a higher resolution), whereas the ring resistivity is generally lower than the EcoScope deep resistivities.

Gas hydrate saturation (S_h) is the percentage of pore space in sediment that is occupied by gas hydrate, which is the complement of the water saturation S_w :

S_h = 1 – S_w.

The procedure followed to estimate S_w with Archie's relation is illustrated in Figure F57. We first computed porosity from the density log as described above with a water density _w of 1.03 g/cm³ and a grain/matrix density _g of 2.76 g/cm³ (see "Physical properties").

To estimate the formation water resistivity (R_w), we started by constructing a salinity versus depth function based on IW salinity measurements (see "Interstitial water geochemistry"). This salinity versus depth function simply consists of a constant value of 34. At every logging depth, we combined the salinity value with a formation temperature obtained from the geothermal gradient estimated from the downhole formation temperature measurements (see "In situ temperature profile") and used the formulas of Fofonoff (1985) to obtain the corresponding value of the water resistivity R_w.

To estimate the water saturation S_w we also need to choose values for the Archie coefficients a and m. One way to do this is to choose a logged interval where the sediments can be assumed to be water saturated and to fit a and m to a plot of measured resistivity versus porosity, known as a "Pickett plot" (e.g., Doveton, 1994). In marine sediments, however, the range of porosity is relatively small and it is not possible to obtain a robust estimate of both a and m. We prefer to set a = 1, which is physically the most realistic value because it gives a resistivity equal to the formation water resistivity when the porosity is 100%. We then compute an "estimated m" coefficient by

m_est = –log(F)/log(f),

where F = (R_t/R_w) is the formation factor. This m_est curve should give the appropriate value to be used in Archie's equation in water-saturated intervals and will give anomalously high values in intervals that contain hydrocarbons. A reasonable value of m = 2.5 can be chosen from the baseline trend of the m curve shown in Figure F57.

The next step is to compute the resistivity R₀ predicted by Archie's equation for a water-saturated formation of a given porosity, which is given by

R₀ = (a x R_w)/^m.

Using Archie coefficients of a = 1 and m = 2.5, we computed an R₀ curve that generally follows the measured resistivity R_t, but is significantly lower than R_t in several intervals, mostly in logging Unit 2 (Fig. F57). Finally, we computed the water saturation S_w using a saturation coefficient n = 2.

As noted earlier, several hole enlargements resulted in logging density values that are erroneously low and approach water density. In the Archie interpretation, these low densities translate into artificially high porosities, low values for the predicted water-saturated resistivity R₀, and low values of water saturation S_w. These anomalous intervals should be ignored and are marked by gray overlays in Figure F57.

Figure F57 shows a clear interval between 60 and 100 mbsf where the measured resistivity R_t generally exceeds the resistivity R₀ predicted for water-saturated conditions, and where the inferred water saturations can be as low as ~20%. This interval encompasses the bottom of logging Unit 1 and the top of Unit 2, and the high-resolution LWD porosity and resistivity logs used in Figure F57 show that it is very heterogeneous, being composed of many alternating layers of highly variable porosity and resistivity. We interpret the 60–100 mbsf interval as a sequence of thin, sandy, gas hydrate–rich layers intercalated with clay-rich layers that may contain little or no gas hydrate. This interpretation is in general agreement with the pore water freshening observed in sand layers at Site U1326 (see the pore water salinity data in Fig. F57 and "Interstitial water geochemistry").

Comparison with infrared images

The IR core images taken on the catwalk measure the core liner temperature, and cold anomalies in the IR images caused by gas hydrate dissociation give an independent "map" of gas hydrate concentrations (see "Physical properties"). Figure F58 shows a comparison of the LWD and wireline resistivity logs with the IR images. Some of the IR cold temperature anomalies correlate with layers of high resistivity. For example, the IR images show a thin cold interval centered at 247 mbsf that corresponds to a high-resistivity peak (~3 m) in the wireline logs). Other cold intervals in the IR images are harder to correlate to high-resistivity peaks in the wireline logs because of incomplete core recovery. The correlation between the IR images (taken on cores from Holes U1326C and U1326D) and the LWD logs (Hole U1326A) needs to be done with care because the dip of the sedimentary structure noted earlier will require a depth shift.

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