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doi:10.2204/iodp.proc.324.105.2010

Downhole logging

Downhole logging data obtained from Hole U1348A included natural and spectral gamma ray, density, photoelectric factor (PEF), and electrical resistivity measurements from three depths of investigation. Interpretations of gamma ray and electrical resistivity downhole logs were used to identify 15 logging units in Hole U1348A, with one in the section covered by the bottom-hole assembly (BHA), five in the sedimentary sequences in the open hole interval, and nine in the volcaniclastic section.

Operations

A wiper trip was completed throughout the open hole, and the RCB bit was released at the bottom of the hole using the mechanical bit release (MBR) before the start of wireline logging operations. The hole was displaced using 86 bbl of barite mud, and the drill pipe was set at 101 m wireline matched depth below seafloor (WMSF). Logging operations in Hole U1348A consisted of two tool string deployments and wireline heave compensator testing that took place in good sea conditions with ship heave fluctuating between 0.5 and 1 m. Downhole logging operations began at 1700 h on 6 October 2009 and were concluded at 0645 h on 7 October after the tools from the second tool string were rigged down.

Tool string deployment

HNGS-HLDS-GPIT-DITE

The wireline tool string deployment consisted of a 22 m long modified triple combo tool string that included a logging equipment cable head (LEH-QT), digital telemetry cartridge (DTC-H), Hostile Environment Natural Gamma Ray Sonde (HNGS), Hostile Environment Natural Gamma Ray Cartridge (HNGC), Hostile Environment Litho-Density Sonde Cartridge (LDSC), Hostile Environment Litho-Density Sonde (HLDS), digital telemetry adapter (DTA-A), General Purpose Inclinometry Tool (GPIT), and the Dual Induction Tool model E (DITE). During the initial rig up, problems powering up the DITE were encountered. The tool string was taken apart on the rig floor to test the DTC-H, DTA-A, GPIT, and DITE combination. The tests revealed that the previously modified GPIT was not working properly in this tool combination. The GPIT was removed from the tool string, and after the remaining tools passed the surface check, the tool string was lowered at ~1160 m/h. During the descent, one of the two depth encoders failed, and the logging operations continued with only one operational depth encoder. A cable-stretch correction of 3.4 m was applied to the wireline depth to account for the water depth and the weight of the tool string.

Downhole logs were recorded in a downlog pass from seafloor to 322 m WMSF at 550 m/h. Uplog Pass 1, from 326 to 101 m WMSF, was recorded at 275 m/h, and repeat uplog Pass 2, from 326 m WMSF to seafloor, was recorded at 550 m/h.

HNGS-DSI-GPIT-FMS

The second wireline tool string deployment consisted of a 34.39 m long FMS-sonic tool string that included an LEH-QT, DTC-H, HNGS, HNGC, Dipole Sonic Imager (DSI), DTA-A, GPIT, and FMS. Downhole logs were recorded in a downlog pass, from seafloor to 326 m WMSF at 550 m/h, and in uplog Pass 1, from 329 to 66 m WMSF.

After completion of the Pass 1 uplog and as the FMS caliper arms were entering the pipe, a tension increase of ~800 lb was recorded. The uplog continued until 65 m WMSF while the tool string was checked and FMS caliper arm closure was confirmed. The tool string checks did not reveal any problems, and the prevailing thought at the time was that the bottom of the tool string contained some mud or sediment that produced the recorded tension. The tool string was lowered back into the hole to begin the repeat uplog Pass 2.

After the tool string had been lowered completely outside the pipe, head and surface tension measurements revealed that the tool string was not moving. The tool string was raised and lowered once more to get past what was perceived as a hole obstruction. After encountering the same results, the deployment was terminated and the tool string was raised into the pipe. At this point, the bottom of the tool string became stuck inside the pipe. The FMS caliper arms were reading a fully closed position, and several attempts were made to enter the pipe with ~5 m of play going in and out of pipe. The caliper arms were opened and closed again, and a final attempt did not solve the problems.

At this point, pumping operations began to clear any potential debris that could have been keeping the FMS caliper arms from closing. After steadily pumping for 30 min, increasing the pump rates to a maximum of 50 strokes/min, and trying to reenter the pipe while pumping, the results were the same: an inability to move up or down. The last resort for recovering the tool string consisted of pulling up in increments of 1250 lbf above normal logging surface tension readings of 5500 lbf. Unsuccessful pulling attempts were made at 6750, 8000, and 9300 lbf surface tension readings. A maximum of 9650 lbf (5500 lb head tension), which is the Schlumberger maximum safety limit for pulling with this type of wireline, was also unsuccessful.

After the unsuccessful retrieval operations described above, the Kinley crimper and cutter were deployed to sever the wireline and recover the tool string while tripping pipe (see "Operations"). The wireline was spooled after it was severed, and pipe tripping operations took place for ~7 h. The tool string was recovered and retrieved from the bottommost sub of the BHA. Results showed that three FMS arms were significantly damaged, and the tool string was held inside the pipe by the mangled arms and springs protruding through the BHA holes for the MBR.

Interpretation of downhole acceleration, tool speed, tension, downhole force, FMS button responses, and caliper measurements suggest that the tool string experienced downward motion during Pass 1 and that the C1 FMS caliper arms suddenly closed partially before fully opening once again. A closer examination of the lowermost part of the damaged FMS tool showed that the inner and outer linkages, concentric shafts that give uphole readings of the caliper status, were damaged. This suggests that the trunion that allows the arms to operate was not making contact with at least two of the arms. This would cause the arms to remain open, whereas the movement on the linkages would suggest that they were operating normally. Therefore, the FMS caliper arms most likely opened as soon as the hole was reentered and were severely damaged, possibly flipping backward, when holding the weight of the entire tool string in a narrower part of the hole.

Data processing

Logging data were recorded onboard the JOIDES Resolution by Schlumberger and archived in digital log information standard (DLIS) format. Data were sent by satellite transfer to the Lamont-Doherty Earth Observatory-Borehole Research Group, processed there, and transferred back to the ship for archiving in the shipboard database. Processing and data quality notes are given below.

Depth shifting

In general, depth shifts are applied to logging data by selecting a reference (base) log (usually the total gamma ray log from the run with the greatest vertical extent and no sudden changes in cable speed), and features in equivalent logs from other passes are aligned by eye. The downhole logs were first shifted to the seafloor based on the logger's seafloor depth of 3267 m wireline depth below rig floor. This depth differs 8 m from the drillers bottom-felt depth. The depth-shifted logs were then depth matched to those of HNGS-HLDS-DITE tool string Pass 2 (Table T10).

Data quality

The quality of wireline logging data were assessed by evaluating whether logged values are reasonable for the lithologies encountered and by checking consistency between different passes of the same tool. Gamma ray logs recorded through the BHA should be used only qualitatively because of the attenuation of the incoming signal. The thick-walled BHA attenuates the signal more than the thinner walled drill pipe. PEF measurements are strongly affected by the use of heavier mud.

A wide (>30.5 cm) and/or irregular borehole affects most recordings, particularly those like the HLDS that require eccentralization and good contact with the borehole wall. The density log roughly correlates with the resistivity logs, but it is largely affected by the hole conditions. The hole diameter measurements recorded with the hydraulic caliper on the HLDS (LCAL) show a very irregular borehole. Good repeatability was observed between Pass 1 and Pass 2, particularly for measurements of electrical resistivity, gamma ray, and density.

The DSI was operated in P&S monopole and upper dipole modes for both downlog and Pass 1 (all with standard frequency). The slowness data from delta-time compressional (DTCO) and delta-time shear upper dipole logs (DT2) are generally of good quality for these passes and thus were converted to acoustic velocities (VCO and VS2, respectively). Reprocessing of the original sonic waveforms, to be performed at a later date, is highly recommended to obtain more reliable velocity results.

The FMS images are generally of good quality below 151 m WMSF because of the relatively good hole condition (hole size < 35.6 cm) and of intermediate quality above 151 m WMSF because of the large borehole size (26–41 cm). The irregular and possibly elliptical shape of the borehole occasionally prevented some FMS pads from directly contacting the formation, resulting in poor-resolution or dark images. Hence, the FMS images (and the high-resolution resistivity logs) should be used with caution in this depth interval.

Preliminary results

Electrical resistivity measurements

Three electrical resistivity curves were obtained with the DITE. The spherically focused resistivity (SFLU), medium induction phasor-processed resistivity (IMPH), and deep induction phasor-processed resistivity (IDPH) profiles represent different depths of investigation into the formation (64, 76, and 152 cm, respectively) and different vertical resolutions (76, 152, and 213 cm, respectively). Downhole electrical resistivity measurements covered 225.4 m of the open hole sedimentary and volcaniclastic lithostratigraphic sequences drilled in Hole U1348A (Fig. F28). The DITE was the only tool that reached the bottom of the logged interval in Hole U1348A because it was the bottommost tool in the logging tool string (Fig. F28).

In the upper 99 m of the logged interval the IMPH values range from 1 to 5.8 Ωm, the IDPH values range from 1.0 to 4.7 Ωm, and the SFLU values range from 0.6 to 7.8 Ωm. In the volcaniclastic stratigraphic units (III–VI) the IMPH measurements range from 1.3 to 24.8 Ωm, the IDPH values range from 1.3 to 21.7 Ωm, and the SFLU values range from 0.8 to 14.6 Ωm (Fig. F28).

Gamma ray measurements

Standard, computed, and individual spectral contributions from 40K, 238U, and 232Th were part of the gamma ray measurements obtained in Hole U1348A with the HNGS. The total gamma ray measurements through the BHA show one anomaly (logging Unit Ip), a peak between 93.2 and 93.7 m WMSF (Fig. F28).

Downhole gamma ray measurements in open hole covered 99 m of the bottommost sedimentary sequences (logging Units Is–Vs) and 109 m of the volcaniclastic units (logging Units Iv–IXv). Total gamma ray measurements in the bottommost sediments of Hole U1348A are moderately variable, ranging from 0.8 to 58.8 gAPI with a mean of 8.2 gAPI. Potassium values are relatively low, with values between 0 and 2.4 wt% and a mean of 0.14 wt% (Fig. F29). Uranium values are mostly between 0 and 4.9 ppm, with a mean of 0.4 ppm. Thorium values range from 0.14 to 4.1 ppm, with a mean of 0.7 ppm.

Total gamma ray measurements in the volcaniclastic units (IIIv–VIv) are higher than the overlying sediments units (Is–IIs), with values between 9.2 and 65.4 gAPI (Fig. F28). Potassium values are relatively high in the volcaniclastics, with values between 0.12 and 2.6 wt% (Fig. F29). Uranium values are mostly between 0.0 and 3.2 ppm (Fig. F29). Thorium ranges from 0.0 to 2.8 ppm, with a mean of 0.7 ppm (Fig. F29).

Density measurements

Density values range from 1.2 to 2.8 g/cm3 over the sediment section of the open hole (Fig. F30). In the volcaniclastic section, density values are between 1.7 and 2.5 g/cm3. A comparison between discrete physical property sample results and the downhole density log shows that discrete sample data (MAD) are consistent with the downhole data (Fig. F30).

Sonic velocity measurements

Downhole velocity data were obtained for the open hole interval between 101 and 308.9 m WMSF (Fig. F30). In the upper sediment section (stratigraphic Units I and II), velocity is highly variable, averaging ~2.5 km/s. In the volcaniclastic section velocities are even more variable, between 1.8 and 6.4 km/s. A comparison with discrete sample measurements of P-wave velocity shows that the core data are consistent with the downhole measurements (Fig. F30).

Magnetic field measurements

Measurements of total magnetic moment, magnetic inclination, magnetic intensity, and hole deviation were obtained with the GPIT (Fig. F56). The mean magnetic inclination and total magnetic moment from 101 to 328 m WMSF are 49° and 0.43 Oe, respectively. The magnetic intensity is 0.32 Oe on the z-axis and varies between –0.27 and 0.30 Oe on the x- and y-axes.

Formation MicroScanner images

FMS images were obtained for the open hole interval between 105 and 328 m WMSF. The diameter of hole from the FMS calipers varied between 20 and 40 cm. High-quality FMS images were obtained in sections of the hole with a diameter <35.6 cm. FMS images from the sediment section show numerous bright, highly resistive layers throughout the unit that are most likely chert layers. FMS images from the volcaniclastic section show sections with subhorizontal contacts, moderately dipping layers, and vesicular or brecciated textures (Fig. F57). Preliminary structural analysis of the lower borehole intervals shows dipping contacts striking northeast to southwest with southeast-oriented dips ranging from 25° to 30° in magnitude that correlate to structures observed in the cores recovered from Hole U1348A (Fig. F58).

Lithostratigraphic correlations

Preliminary interpretation of the downhole log data divided Hole U1348A into 15 logging units within 3 main sections, the section covered by the BHA, the sedimentary sequences in open hole, and the volcaniclastic sequences (Figs. F28, F29, F30). Logging units in the section covered or partially influenced by the BHA were interpreted on the basis of the gamma ray downhole logs, and only intervals that showed significant anomalies were characterized as logging units. Logging units within the open hole section that contained sedimentary sequences were also interpreted on the basis of the gamma ray fluctuations, whereas the volcaniclastic sequence was characterized using both the gamma ray and resistivity logs.

One logging unit was qualitatively identified in the section covered by the BHA (Fig. F28):

  • Logging Unit Ip (93.1–97.7 m WMSF) is characterized by a significant increase in gamma ray measurements. A large contribution from thorium is apparent in the spectral gamma ray measurements (Figs. F28, F29).

Five logging units were identified in the upper sedimentary sequence (stratigraphic Units I and II) in the open hole below the BHA based on gamma ray downhole logs (Fig. F28):

  • Logging Unit Is (101.1 to 165.7 m WMSF) is characterized by gradual increases in resistivity and gamma ray measurements. At the base of this unit, both resistivity and gamma ray values decrease slightly.

  • Logging Unit IIs (165.7–170.4 m WMSF) is defined by a sharp increase in total gamma ray measurements that is mostly associated with an increase in uranium (Fig. F29).

  • Logging Unit IIIs (170.4–187.4 m WMSF) is defined by a decrease in gamma ray values with respect to Unit IIs. Gamma ray and resistivity measurements show small variations within this unit, but overall the values are fairly consistent (Fig. F29).

  • Logging Unit IVs (187.4–193.5 m WMSF) is characterized by a sharp increase in gamma ray measurements and a decrease in resistivity (Fig. F28). The gamma ray anomaly is mostly caused by high contributions from thorium and potassium (Fig. F29).

  • Logging Unit Vs (193.5 to 204.3 m WMSF) is defined by a decrease in gamma ray measurements with respect to Unit IVs. The gamma ray values are consistent throughout the unit. Electrical resistivity measurements show an increase at the top of the unit, followed by a decrease in values in the lowermost part of the unit.

The volcaniclastic sequence below 154.93 m WMSF is divided into nine logging units using the downhole resistivity and natural gamma logs (Fig. F28):

  • Logging Unit Iv (204.3–210.5 m WMSF) is defined by an increase in resistivity to a maximum value of ~25 Ωm. Gamma ray measurements sharply decrease, whereas density and compressional wave velocities increase (Fig. F30).

  • Logging Unit IIv (210.5–214.6 m WMSF) is characterized by a sharp decrease in resistivity and a sharp increase in gamma radiation. Spectral gamma ray measurements indicate contributions from thorium, uranium, and potassium (Fig. F29). Decreases in both density and compressional wave velocity also define this unit (Fig. F30).

  • Logging Unit IIIv (214.6–245.8 m WMSF) has consistent resistivity and gamma ray values throughout the unit. Gamma ray values are mostly controlled by potassium, which averages 1.29 wt% throughout the interval (Fig. F29).

  • Logging Unit IVv (245.8–260.1 m WMSF) is defined by sharply increasing resistivity values (Fig. F28). Total gamma ray, potassium, and compressional wave velocity also decrease.

  • Logging Unit Vv (260.1–267.5 m WMSF) is characterized by sharply increasing resistivity and gamma ray values. Potassium increases, whereas uranium and thorium have low values. Density decreases through the unit, whereas compressional wave velocities are higher than in the surrounding units.

  • Logging Unit VIv (267.5–273.0 m WMSF) is characterized by low resistivity values and a large peak in gamma radiation. Spectral gamma ray measurements indicate a large contribution from uranium in this unit, with lower contributions from thorium and potassium. Bulk density and compressional wave velocity are low throughout the interval.

  • Logging Unit VIIv (273.0–287.2 m WMSF) is defined by sharply increasing resistivity values in the upper part of the unit. NGR decreases sharply at the top of the unit and then increases in the lower part of the unit (Fig. F28).

  • Logging Unit VIIIv (287.2–291.3 m WMSF) is defined by sharply increasing resistivity values, whereas natural gamma ray values decrease. Spectral gamma ray data indicate that potassium, thorium, and uranium are low (Fig. F29). Density and compressional wave values increase sharply at the top of the unit.

  • Logging Unit IXv (291.3–326.6 m WMSF) is characterized by lower resistivity values than the overlying unit, and clear cycles are present within the unit. NGR data only partially cover the unit and values decrease sharply at the top of the unit and then increase slowly (Fig. F28). Potassium, thorium, and uranium values are low, as are density and compressional wave velocity.