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doi:10.2204/iodp.proc.311.107.2006

Downhole logging

Logging while drilling

Operations

LWD operations in Hole U1329A began with rigging up of the BHA at the rig floor at 1100 h on 25 September 2005. LWD tools included the GeoVISION resistivity tool, the EcoScope tool, the SonicVision tool, the TeleScope MWD tool, and the ProVISION nuclear magnetic resonance tool. Because the EcoScope tool had been running well in the previous four holes and was collecting essentially the same data as the ADNVISION tool, we decided not to use the ADNVISION tool to save time and to spare one more manipulation of a radioactive source. For details on each tool and the measurements it collects, see "Downhole logging" in the "Methods" chapter. Hole U1329A was spudded at 1840 h at 970 mbrf water depth (drillers depth). Similar to the other sites, the first 6 m was drilled with a rotation rate of 10–15 rpm, a pump flow rate of 100 gallons per minute (gpm), and a rate of penetration (ROP) of 10–15 m/h. We then increased these rates to 40 rpm and 220 gpm until 30 mbsf, and then again to 60 rpm and 270 gpm (to start the MWD telemetry), keeping the instantaneous ROP below 50 m/h with depth. The deepest part of the hole proved to be surprisingly hard to drill, with a significant increase in resistivity at 1140 mbrf raising concern for the presence of free gas. Because the density was increasing as well, the annulus pressure was stable, and the sonic waveform coherence was strong, it was concluded that the change in resistivity was controlled by the lithology, and drilling proceeded, although very slowly, for the last few meters. The target depth of 220 mbsf (1190 mbrf) was reached at 0805 h on 26 September. The drill string and tools were brought back to the surface. Final rig down started at 1100 h and was completed by 1330 h. All data were downloaded and the rig floor cleared by 1430 h on 26 September.

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 U1329. As Hole U1329A was drilled without coring, the LWD data had to be monitored for safety reasons to detect gas entering the borehole. As explained in "Downhole logging" in the "Methods" chapter, the primary measurement we used in gas monitoring was annular pressure while drilling measured by the EcoScope tool in the borehole annulus, the space between the drill string and the borehole wall. We looked for sudden decreases of >100 pounds 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 F52 shows the measured borehole fluid pressure profile in Hole U1329A after subtraction of the best-fit linear trend. The borehole fluid pressure increases by a maximum of ~200 kPa (~29 psi) over the general trend at a depth of ~200 mbsf. As noted earlier, this interval was hard to drill, and the logs show sediments of high density, low porosity, and high resistivity. The observed increase in the drilling fluid pressure is probably caused by an increase in pumping rates during drilling or to the cuttings restricting flow in the annulus. The borehole fluid pressure anomalies observed in Hole U1329A were well below the 100 psi level that required preventive action. Moreover, the sonic waveform coherence image in Figure F52 does not indicate any loss of coherence that could be attributed to free gas.

Logging quality

Figure F52 also shows the quality control logs for Hole U1329A. Except for a short excursion to ~70 m/h at ~140 mbsf, the ROP was generally 40 m/h or less in the interval from the seafloor to TD. This is sufficient to record one measurement every 4 cm (~25 measurements per meter) 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. Most of the hole had a diameter slightly larger than 10 inches (25 cm), with larger washouts up to 12 inches (30 cm) restricted to the uppermost 30 m of the hole. 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. F52), showing the good quality of the density measurements. Larger corrections (>0.25 g/cm³) are only seen in the uppermost 30 m of the hole and between 200 mbsf and TD. Figure F53 is a summary of the main LWD logs with density and porosity measurements from cores from Holes U1329C and U1329E 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 U1329A, the gamma ray logging pick for the seafloor was at a depth of 964 mbrf. This pick turned out to be deeper by 8 m than the wireline pick in Hole U1329D (956 mbrf). Because the wireline gamma ray log showed the seafloor more clearly, we corrected the reference seafloor depth for all LWD logs to 956 mbrf.

Wireline logging

Operations

Wireline logging was conducted in Hole U1329D, which had been drilled as a dedicated hole for logging, except for one XCB core that was taken in the lowermost 10 m to reach the target depth of 210.5 mbsf. During the previous day and night, time was spent waiting on weather, which was unsafe for any drilling or logging operations. At the start of logging operations at 1215 h on 30 September 2005, the ship's heave was still ~3 m. The hole was displaced with a barite and sepiolite mud mixture, and after the wireline was spooled, rigging of the tool string for the first run was completed by 1400 h. Wireline logging operations began with the deployment of the triple combo tool string (resistivity, density, and porosity measurements), which consists of the Hostile Environment Gamma Ray Sonde, the phasor Dual Induction Tool, the Hostile Environment Litho-Density Tool, the Accelerator Porosity Sonde, and the Lamont-Doherty Earth Observatory (LDEO) high-resolution Temperature/Acceleration/Pressure (TAP) tool. For details on the different tools, see "Downhole logging" in the "Methods" chapter. The tool string went down without difficulty, and TD (1166 mbrf; 210 mbsf) was reached at 1535 h. When the wireline heave compensator was activated before starting to log up, the recorded heave was still 3 m. Logging uphole occurred without problem despite two tight spots at 1144 and 1128 mbrf (188 and 172 mbsf), with caliper readings of ~6 inches (15 cm). The first pass was finished at 1635 h when we logged the seafloor at 956 mbrf. Because of the enlarged hole condition, it was agreed not to run a second pass, and the tool was brought back to the surface. Rig down was completed by 1800 h.

For the second wireline run, we deployed the FMS-sonic tool string, which consists of the FMS, the General Purpose Inclinometer Tool, the Scintillation Gamma Ray Tool, and the Dipole Sonic Imager. We reached TD (1166 mbrf; 210 mbsf) without problems, and the first pass started at 2005 h. We had difficulties passing the tight spot at 1128 mbrf, requiring us to close the FMS arms and to apply a tension of up to 5500 lb in order to proceed. It was decided not to go below this depth on our second pass, which recorded uphole from 1125 mbrf (169 mbsf). Difficulties in reentering the drill pipe and significant heave forced us to close the caliper arms and finish logging at 1036 mbrf (80 mbsf). The tool string was back on deck at 2235 h, and rig down was completed by 2330 h on 30 September.

Logging quality

Wireline logging data from the triple combo and FMS-sonic tool string runs are compromised to some extent by poor hole conditions (Fig. F54). The hole was enlarged between 129 and 172 mbsf, with the density tool caliper measuring a diameter >16 inches (41 cm) for most of this interval. Outside of this interval, the wireline log quality is generally very good. In the enlarged hole, however, several measurements were affected:

• Densities were too low.
• Porosities were too high.
• Spherically focused resisitivity log gave noisy readings at the shallow depth of investigation.
• Coherence of the sonic waveforms was reduced.
• Poor contact of the FMS pads with the borehole wall resulted in variable quality images.

Despite the enlarged hole conditions, the acoustic waveforms and slowness-time coherence projection (Fig. F55) show that the acoustic data are of good quality. Only limited reprocessing was required to extract reliable compressional (V_P) and shear (V_S) wave velocities. As expected in these shallow and poorly consolidated sediments, both velocities are very low, and V_P is only slightly higher than the borehole fluid velocity (~1500 m/s) over most of the logged interval. V_P and V_S increase sharply below ~180 mbsf, coinciding with similar increases in resistivity and density seen near the bottom of the hole. Above this depth, the dominant low frequency of the flexural dipole waveforms is typical of a large hole diameter (Harrison et al., 1990).

The depth relative to the seafloor for all wireline logs was fixed by identifying the step change in the gamma ray log associated with the seafloor. The gamma ray pick for the seafloor was identified at 956 mbrf for all wireline runs. As noted earlier, we used the same seafloor depth for the LWD data obtained in Hole U1329A because the seafloor gamma ray pick from the LWD run was less reliable than the wireline pick.

Logging-while-drilling and wireline logging comparison

Figure F56 shows a comparison of LWD (Hole U1329A) and wireline (Hole U1329D) data, using the gamma ray, neutron porosity, density, and resistivity logs. In general, the LWD and wireline data match relatively well, exhibiting similar curve shapes and absolute logging values. The exception is the gamma ray log, where the LWD log gives higher readings (100 gAPI on average) than the wireline log (50 gAPI on average). Moreover, the LWD and wireline gamma ray curves have different shapes and there is no obvious correlation. The cause of this discrepancy is not fully understood; however, the wireline data are consistent with the values recorded during Leg 146.

The wireline neutron porosity and density contain several intervals where the neutron porosity readings are anomalously high and the density readings are anomalously low compared to the LWD logs. Most of these anomalies are in the interval where Hole U1329D was enlarged (125–170 mbsf) and are probably caused by poor contact of the tool pad with the borehole wall. Finally, the resistivities measured by wireline and LWD tools are very similar.

Logging units

The logged section in Holes U1329A and U1329D 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. F53, F54, F55, F56).

Logging Unit 1 (0–130 mbsf) is characterized by a uniform low resistivity of ~1 m (small fluctuations around this average are clearly visible on the resistivity images), uniform low densities of ~1.8 g/cm³, and porosities between 50% and 60%. Logging Unit 1 corresponds to lithostratigraphic Units I and II (see "Lithostratigraphy").

Logging Unit 2 (130–183 mbsf) is characterized by a small increase in resistivity and density compared to logging Unit 1. From the top to the bottom of logging Unit 2, resistivity increases from ~1.1 to 1.5 m, and density increases from ~1.8 to 2 g/cm³. In Hole U1329D, logging Unit 2 corresponds to an interval where the hole diameter is mostly >16 inches. The top of this unit corresponds to the bottom of lithostratigraphic Unit II, which is an unconformity separating Pleistocene from upper Miocene sediments (see "Biostratigraphy").

Logging Unit 3 (183 mbsf to TD) is characterized by an abrupt increase in resistivity, density, and P-wave velocity. Resistivity and density keep increasing with depth, from ~3 m and 2.3 g/cm³ at the top of the unit to ~12 m and 2.5 g/cm³ at TD, indicating an increasingly consolidated formation with porosities possibly as low as 15% at TD. This unit was sampled only with Cores 311-U1329C-22X (178.9–188.5 mbsf), 23P (188.5–189.5 mbsf), and 311-U1329D-1X (201.0–210.5 mbsf; 1 m recovery). Logging Units 2 and 3 correspond to lithostratigraphic Unit III (see "Lithostratigraphy").

Logging-while-drilling and wireline 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. Finally, the wireline FMS tool produces an electrical resistivity image. The resolution of the resistivity images from the GeoVISION tool is considerably lower (5–10 cm) than the resolution of the images from the FMS (0.5 cm). The GeoVISION tool, however, provides 360° coverage of the borehole wall, whereas FMS images cover only ~30% of the borehole wall.

Figure F57 shows some of the LWD images collected by the EcoScope and GeoVISION tools. It should be noted that the display in Figure F57 is vertically compressed. 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. If the concentration of gas hydrate is very high, there may be a small decrease in density. On the other hand, high resistivities and high densities are likely to correspond to low-porosity, compacted, or carbonate-rich sediments. This is the case for the interval below 183 mbsf in Figure F57 (logging Unit 3). There is also evidence of a thin layer containing high-resistivity and high-density streaks around 130 mbsf.

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 were used to calculate sediment porosities from Hole U1329A. 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 2.73 g/cm³, which is the average grain density measured in the core samples (see "Physical properties"). The density log–derived porosities range from ~60% at 15 mbsf to ~15% at 220 mbsf (Fig. F58).

The LWD neutron porosity log (Fig. F58) yielded sediment porosities ranging from an average value of ~60% at 15 mbsf to ~23% at 220 mbsf. Porosities measured by the neutron log are expected to be higher than those computed from the density log in sediments containing clay because the neutron log essentially measures hydrogen abundance, and hydrogen in clay minerals is counted as porosity. The EcoScope neutron porosity shown in Figure F58 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.

The comparison of core- and LWD-derived porosities in Figure F58 reveals that the log-derived porosities agree with the core-derived values in logging Units 1 and 2 (0–183 mbsf), with the density porosities being slightly lower and the neutron porosities slightly higher than the core porosities. Porosities from the density and neutron logs, however, are systematically lower than the core-derived porosities below ~183 mbsf (logging Unit 3).

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 measured electrical resistivities and acoustic velocities 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. Resistivities logged in Holes U1329A and U1329D do not show any obvious anomalies characteristic of gas hydrate (Figs. F53, F54). The presence of gas hydrate is known to increase sonic velocity and attenuation, and the absence of any significant velocity anomaly or energy dissipation in the waveforms (Fig. F55) is in agreement with the general inference of very low to no gas hydrate occurrence at this site.

To estimate the amount of gas hydrate that might be present at Site U1329, 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.

Gas hydrate saturation (S_h) is the percentage of pore space in sediment 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 F59. We 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 2.73 g/cm³ (see "Physical properties").

To estimate the formation water resistivity (R_w), we first constructed a salinity versus depth function based on IW salinity measurements (see "Interstitial water geochemistry"). This salinity versus depth function consists of three linear segments fitted to the data (0–30, 30–177, and 177–226 mbsf). 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 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 a log of "estimated m" given by

m_est = –log(F)/log(),

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 law 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_est curve in Figure F59. A value of m = 2.0 might be more appropriate in logging Unit 3. Variations in the cementation exponent m have been related to changes in particle shape (Jackson et al., 1978), and therefore it is not necessarily expected to remain constant in a heterogeneous sequence of marine sediments.

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 closely follows the measured resistivity R_t, confirming that little or no gas hydrate is present at Site U1329 (Fig. F59). Finally, we computed the water saturation S_w using a saturation exponent n = 2. Water saturation is ~1 throughout Site U1329 (Fig. F59).

There are two intervals that have a water saturation <100%, and therefore, suggests the possible presence of gas hydrate or gas between ~112 and 120 mbsf and between ~145 and 165 mbsf. The former interval is in the GHSZ, and a pressure core taken at ~115 mbsf does show a layer with a high P-wave velocity, which could indicate gas hydrate (see "Pressure coring"). The 145–165 mbsf interval is below the GHSZ, and it may contain a small amount of free gas, as gas hydrate and free gas are effectively electrical insulators, the resistivity analysis done here is applicable to both. A degassing experiment of pressure Core 311-U1329C-23P (~188 mbsf) suggests the presence of free gas (see "Pressure coring"). This pressure core is in logging Unit 3 (below 183 mbsf), which in Figure F59 shows water saturations >100%, because the predicted water-saturated resistivity R₀ is actually greater than the measured resistivity. As noted earlier, the sediments in logging Unit 3 are more consolidated or cemented than those of the shallower units and need a cementation exponent m of ~1.8 to get a consistent value of S_w. We repeated our water saturation analysis focusing on logging Unit 3 and using m = 1.8 but could not find convincing evidence for the presence of gas. If free gas is present in logging Unit 3, its saturation must be low.

Temperature data

The LDEO TAP tool was deployed on the wireline triple combo tool string in Hole U1329D (Fig. F60). During the process of coring and drilling, cold seawater is circulated in the hole, cooling the formation surrounding the borehole. Once drilling ceases, the temperature of the fluid in the borehole gradually rebounds to the in situ equilibrium formation temperature. Thus, the temperature data from the TAP tool cannot be immediately used to assess the formation temperatures. The temperature profile in Figure F60, however, reveals a few gradient changes that were caused by borehole temperature anomalies. Specifically, the sudden temperature increase at 187 mbsf during the downhole trip and the sudden decrease at 173 mbsf during the uphole trip correspond to two borehole restrictions clearly visible on the caliper log (Fig. F54). The smaller step decrease in temperature during the uphole trip at 131 mbsf is also likely to be related to a corresponding decrease in borehole radius as seen on the caliper log (Fig. F54).

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