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doi:10.2204/iodp.proc.311.103.2006

Downhole logging

Logging while drilling

Operations

Hole U1325A was spudded at 2212 mbrf (drillers depth) at 2315 h on 20 September 2005. LWD tools included the GeoVISION resistivity tool, the EcoScope tool, the SonicVISION tool, the TeleScope MWD tool, the ProVision nuclear magnetic resonance (NMR) tool, and the ADNVISION tool. For details on each tool and the measurements it takes, see "Downhole logging" in the "Methods" chapter.

Hole U1325A was the first hole drilled during Expedition 311, and to ensure that the LWD tools were powered up to provide real-time data for gas monitoring, it was decided to drill from the beginning with the pumping rate at a relatively high value of 280 gpm. Although it was recognized that such a high pumping rate was likely to wash out the hole near the seafloor, obtaining all the data needed for gas monitoring from the very start received higher priority in this first hole. The first 10 m of Hole U1325A was drilled with a rotation rate of 10 rpm and an ROP of 10–15 m/h. We then increased the rotation rate to 40 rpm while keeping other parameters unchanged until 30 mbsf and then increased this rate again to 60 rpm, keeping the instantaneous ROP below 60 m/h with depth. The target depth of 350 mbsf (2562 mbrf) was reached at 1930 h on 21 September. The LWD tool string was then brought back to the surface, and rig down and data download were completed by 0600 h on 22 September.

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

The LWD/MWD logs were acquired to plan coring and pressure coring operations in subsequent holes at Site U1325. As Hole U1325A 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 of >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 F51 shows the measured borehole fluid pressure profile in Hole U1325A after subtraction of the best-fit linear trend. The borehole fluid pressure shows only small fluctuations over the trend, except for a few positive pressure anomalies (20 psi or less) centered at 252, 277, 289, and 297 mbsf. These increases in the drilling fluid pressure coincide with major hole enlargements (>13 inches or 33 cm) as seen on the ultrasonic caliper log (Fig. F51). The caliper log also shows a sharp decrease in hole diameter below each washout (to <9.5 inches or 24 cm), suggesting that the washouts were generated above resistant intervals that were hard to drill. The observed increase in drilling fluid pressure was probably the result of an increase in pumping rates during drilling or to restriction of flow in the annulus by cuttings. The fluid pressure anomalies observed 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 fluid 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 F51 shows a well-defined fluid arrival with a slowness of ~200 ms/ft almost everywhere, which corresponds to the expected fluid velocity of ~1500 m/s. The only anomaly is a thin interval at 209–210 mbsf, where the fluid arrival suddenly disappears. This interval coincides with a 10 psi negative anomaly in the fluid pressure, suggesting that a small amount of gas may have entered the drilling fluid. 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 at depths around 210 mbsf, it had to be in low concentrations.

Logging quality

Figure F51 also shows the quality control logs for Hole U1325A. 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 per meter) in the GeoVISION resistivity, and no significant resolution loss was observed with variation in the 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. Because of the relatively high pumping rate, the hole is enlarged near the seafloor, with a diameter as large as 13 inches (33 cm). The hole diameter then decreases with depth to a value slightly larger than 10 inches (25 cm). Large washouts and hole restrictions in the interval 255–305 mbsf were noted earlier. The interval 170–235 mbsf also shows many small-scale hole irregularities with the hole diameter up to ~12 inches (31 cm). We will show later that the interval 170–235 mbsf contains a number of thin, alternating, high- and low-resistivity layers, where the high-resistivity layers are likely to have high gas hydrate concentrations. The small-scale hole enlargements may take place in unconsolidated, gas hydrate–rich sand layers that fail during drilling.

The density correction, calculated from the difference between the short- and long-spaced density measurements, generally varies from 0 to 0.2 g/cm³ despite the hole irregularities (Fig. F51), showing the good quality of the density measurements. Figure F52 is a summary of the main LWD logs in Hole U1325A with MAD density and porosity measurements from cores from Holes U1325B and U1325C 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 U1325A, the gamma ray logging pick for the seafloor was at a depth of 2203 mbrf, ~9 m above the initial depth estimated by the drillers.

Wireline logging

Operations

Hole U1325C was continuously cored to a depth of 300 mbsf, which was reached at 0715 h on 22 October 2005. As the ship's heave remained above 5 m, which exceeds the current wireline heave compensator limits, we decided to try the experimental Schlumberger heave compensator, which feeds heave information back to the winch and, in principle, has no limit on the amount of heave for which it can compensate. The Schlumberger heave compensation system had been successfully tested in two previous short repeat runs at Sites U1327 and U1328, but lacking an extended test we planned two logging runs with tools that had no protruding arms (i.e., calipers).

Wireline logging operations in Hole U1325C began with the deployment of an armless tool string consisting of the HNGS and the DIT. For details on the different tools, see "Downhole logging" in the "Methods" chapter. After hole preparation, the rig up of the tool string started at 1115 h and was completed at 1230 h on 22 October. Despite repeated attempts, we were not able to lower the tool string below a depth of 2465 mbrf (260 mbsf), where the LWD caliper log in Hole U1325A showed a borehole restriction. At 1515 h we decided to log up from this depth, and reached the seafloor at 1705 h, after some delay caused by problems reentering the drill pipe. The tool string was back on the rig floor at 1810 h.

At 1835 h we started rigging up the tool string for the second wireline run in Hole U1325C. This armless string consisted of the SGT, the DSI, and the Lamont-Doherty Earth Observatory (LDEO) high-resolution TAP tool. Rig up was complete at 2000 h. Again, it was impossible to lower the tool string past a hole restriction, this time located at 2391 mbrf (186 mbsf). At 2154 h, we started logging up, while the ship's heave reached 7 m. We completed a first pass at 2295 mbrf (90 mbsf), and lowered the tool for a second pass that we hoped to start at a deeper location than the first pass. The tool string, however, could not go deeper than 2388 mbrf (183 mbsf), and at 2241 h we started logging up for our second pass. The second pass was completed successfully, the tool string was brought back to the rig floor, and rig down was completed by 0130 h on 23 October.

Because we did not run a caliper log in Hole U1325C, we cannot clearly determine the effects of hole diameter on the wireline log measurements. The gamma ray log needs to be corrected for hole size, and the absence of a caliper log prevented us from applying this correction. This results in gamma ray values that are lower than the formation values, which could be the case for most of the section below 140 mbsf. There are a few intervals (e.g., 93–98 and 166–168 mbsf) where the gamma ray and the spherically focused and induction resistivities have very low values, which are likely caused by an enlarged hole (Fig. F53). The LWD/MWD ultrasonic caliper shows some hole irregularities near these intervals (Fig. F51). Apart from these possibly anomalous intervals, the gamma ray and induction logs are of good quality.

A comparison of the gamma ray logs measured by the HNGS tool in the first run and by the SGT tool in the second run shows an excellent correlation (Fig. F53), after some minor depth shifting of the data that is customarily needed to match different wireline runs. This result shows that the Schlumberger heave compensation system performed well when the ship's heave was as much as 7 m.

The acoustic data measured in Hole U1325C appear to be of good quality. The results of two passes are shown in Figure F53, and the acoustic waveforms and slowness-time coherence projections are shown in Figure F54. 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. The poor quality of the lower dipole waveforms below 140 mbsf, however, suggests that they were affected by borehole size.

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 at 2205 mbrf in Hole U1325C.

Logging-while-drilling and wireline logging comparison

Figure F55 shows a comparison of LWD (Hole U1325A) and wireline (Hole U1325C) data, using the gamma ray and resistivity logs. The LWD and wireline gamma ray logs have similar character and curve shapes, and correlate closely in the interval 70–145 mbsf. On the other hand, the LWD log gives higher readings (100 gAPI on average) than the wireline log (40 gAPI on average). The reasons for this discrepancy have not been fully determined, but it was recognized that the wireline measurements were consistent with values recorded during ODP Leg 146.

The comparison of resistivities shows a close match between the LWD and wireline logs. There are a few exceptions where the wireline logs measure resistivities that are less than those of the LWD logs, notably at 93–95 and 166–168 mbsf. As noted earlier, these low resistivities correspond to anomalously low gamma ray measurements in Hole U1325C, and are probably related to hole enlargements. The similarity between the LWD logs measured in Hole U1325A and the wireline logs measured in Hole U1325C points to high horizontal continuity at Site U1325 compared to the substantial lateral variability noted at Site U1327. The reason for this difference is that much of the interval drilled at Site U1327 was in deformed sediments of the Cascadia accretionary complex, whereas Site U1325 is located in a slope basin whose sedimentary sequence is laterally continuous. Although there is evidence for tilting of the sedimentary sequence in the seismic reflection data, Holes U1325A and U1325C were drilled along the structural strike and therefore should correlate well. Indeed, there is a remarkably close fit between LWD/MWD formation density from Hole U1325A and density measurements made in core samples from Holes U1325B and U1325C (Fig. F52).

Logging units

The logged section in Holes U1325A and U1325C can be divided into three logging units, based on obvious changes in the LWD/MWD and wireline gamma ray, density, electrical resistivity, and acoustic measurements (Figs. F52, F53, F54). The boundary between logging Units 1 and 2 corresponds to the 0.3 Ma age boundary noted at ~122 mbsf in "Biostratigraphy." These three logging units have no obvious correspondence to lithostratigraphic Units I, II, III, and IV, which were defined on the basis of changes in diatom abundance (see "Lithostratigraphy").

Logging Unit 1 (0–122 mbsf) is characterized by a well-defined gradual increase in density with depth (from 1.7 g/cm³ near the seafloor to 2.1 g/cm³ at 122 mbsf) and a decrease in porosity (from 70% near the seafloor to 40% at 122 mbsf). This increase in density is matched by a corresponding increase in resistivity with depth, from ~1 m near the seafloor to ~1.5 m at 122 mbsf. The boundary between Units 1 and 2 at 122 mbsf is marked by a sharp decrease in both density (from 2.1 to 1.7 g/cm³) and resistivity (from 1.5 to 1 m). This sharp decrease in resistivity is also well defined on the wireline induction data, and it correlates to a decrease in P-wave velocity from ~1800 to 1600 m/s (Fig. F53).

Logging Unit 2 (122–250 mbsf) is characterized by a uniform density that averages ~1.9 g/cm³. In contrast, the resistivity logs in Unit 2 show alternating, thin intervals of high and low resistivity, spanning the range 1–1.5 m. These thin intervals are especially well defined between 190 and 220 mbsf (Fig. F52). These high and low resistivities are likely to correspond to alternating layers that have high and low gas hydrate concentrations, respectively.

Logging Unit 3 (250–350 mbsf) displays uniform background resistivity (~1 m) and uniform density (~2 g/cm³). The most striking feature of this unit is the presence of several borehole enlargements (see the LWD caliper log in Fig. F51) that result in unreliable measurements of natural gamma ray radioactivity, density, and porosity (Fig. F52). Logging Unit 3 displays no obvious resistivity peaks that can be attributed to free gas in the pore fluid. As we are below the GHSZ in this unit, high resistivity should correspond to free gas.

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 F56 shows some of the images collected by the EcoScope and GeoVISION tools. It should be noted that the display in Figure F56 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, possibly sandy layers between 173 and 240 mbsf in Hole U1325A, where densities are generally low and high-resistivity, gas hydrate–rich layers (bright in the resistivity images of Fig. F56) alternate with low-resistivity layers (dark in the images of Fig. F56) that are likely to contain little or no gas hydrate. The images also show clearly a number of hole enlargements, which appear as dark bands in Figure F56 between 255 and 305 mbsf. These are the same washouts detected by the LWD ultrasonic caliper (Fig. F51).

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). The LWD/MWD density and neutron logs from Hole U1325A 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 2.75 g/cm³, which is the average grain density measured in core samples (see "Physical properties"). The density log–derived porosities range from ~60% at 20 mbsf to ~45% at 300 mbsf (Fig. F57). In several intervals (0–20, 70–100, and 255–305 mbsf), hole enlargements resulted in erroneously low values of density and corresponding porosity values that are too high.

The LWD/MWD neutron porosity log (Fig. F57) yielded porosities ranging from an average value of ~70% at 20 mbsf to ~55% 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 F57 is the "best thermal neutron porosity;" it has been corrected for the effect of clay, so that it is only marginally 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 F57 shows agreement throughout the logged interval, with the density porosities being slightly lower and the neutron porosities being slightly higher than the porosities obtained from core samples.

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 U1325A and U1325C show a number of positive resistivity anomalies without a corresponding decrease in porosity (Figs. F52, F53), suggesting that there are several intervals where gas hydrate may be present.

Water saturations from Archie's equation

To estimate the amount of gas hydrate at Site U1325, 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, and
• R_t = GeoVISION high-resolution button deep average resistivity, and
• n = saturation coefficient.

We used 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 values are generally lower than the EcoScope deep resistivities.

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

S_h = 1 – S_w.

The procedure to estimate S_w with Archie's relation is illustrated in Figure F58. 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 2.75 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 consists of two linear segments fitted to the data (0–20 mbsf and below 20 mbsf). At every logging depth, we combined the salinity value with a formation temperature obtained from the geothermal gradient estimated from the downhole temperature measurements (see "In situ temperature profile"), and used the formulas of Fofonoff (1985) to obtain the corresponding value of the water resistivity.

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 an "estimated m" coefficient 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 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_est curve in Figure F58.

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. F58). 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 F58.

Figure F58 shows a clear interval between 100 and 255 mbsf where the measured resistivity R_t occasionally exceeds the resistivity R₀ predicted for water-saturated conditions and where the inferred water saturations can be as low as ~40%. This interval encompasses the bottom of logging Unit 1 and all of Unit 2, and the high-resolution LWD porosity and resistivity logs used in Figure F58 show that it is heterogeneous, being composed by many alternating layers of highly variable porosity and resistivity. We interpret the 100–255 mbsf interval as a sequence of thin, gas hydrate–rich layers intercalated with layers that may contain little or no gas hydrate. This interpretation is in general agreement with the marked pore water freshening observed in sand layers compared to lower degrees of freshening in clay-rich layers (see the pore water salinity data in Fig. F58 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 F59 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 cold interval at 148–152 mbsf that corresponds to a high-resistivity interval on the LWD logs. Whereas the IR images show cold temperatures at 186–189 mbsf, in the hydrate-rich interval defined by high resistivities poor core recovery does not allow us to check how closely IR cold temperatures correlate with high resistivities.

Temperature data

The LDEO TAP tool was deployed on the DSI/SGT tool string in Hole U1325C (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 in situ formation temperature. Because we were not able to run a caliper log in Hole U1325C, we cannot relate temperature gradient changes in Figure F60 to changes in hole diameter. The most prominent feature in Hole U1325C (Fig. F60) is the sudden temperature step at 80 mbsf (in the downhole trip) and 75 mbsf (in the uphole trip), which take place when the TAP tool exits and enters the drill pipe.

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