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doi:10.2204/iodp.proc.317.103.2011

Downhole logging

Operations

Two holes were logged at Site U1351. Hole U1351B was logged after the completion of coring, whereas Hole U1351C was drilled as a dedicated logging hole in order to increase the chances of obtaining high-quality logs and a vertical seismic profile (VSP) at this site.

Hole U1351B

Logging operations in Hole U1351B began by conditioning the hole for logging immediately after coring was completed at 1616 h on 24 November 2009 (all times are local ship time, UTC + 13 h) to a total depth of 1163.3 m DRF (1030.6 m DSF). After the hole was swept with 50 bbl of high-viscosity mud and displaced with 420 bbl of logging gel, the bit was raised to the logging depth of 215 m DRF (81 m DSF). Rig up of the triple combo tool string (natural gamma ray, bulk density, electrical resistivity, and porosity) began at 2245 h, and the tool string was run into the hole (RIH) at 0015 h on 25 November. While the tool string was being lowered at a speed of 3500–4000 ft/h, gamma ray and resistivity data were recorded from the seafloor to a few meters above the total depth of 1163 m WRF, where a first logging pass was started at 0135 h. This repeat pass was completed at 0203 h (1023 m WRF), and the tool string was run back down to total depth for the full pass, which started at 0225 h at a speed of 900 ft/h. At 0540 h (258 m WRF), the caliper was closed for reentry into the pipe, and the pass was completed at 0615 h when the seafloor was identified by a drop in the gamma ray log at 132 m WRF. The tool string was back on deck at 0630 h and rigged down at 0730 h.

By 0830 h, the FMS-sonic tool string had been rigged up and RIH at a speed of 1500 ft/h to record sonic velocity on the way down. After a difficult exit from the drill pipe, the tool string met an obstruction at 618 m WRF, and after several attempts to go deeper we concluded that the borehole was rapidly deteriorating. At 1050 h, we decided to begin logging up with both the FMS and sonic tools from the deepest depth reached (620 m WRF) at a speed of 1200 ft/h. Because of the apparent collapse of the hole, no second pass was attempted, and the FMS calipers were closed at 255 m WRF before the top of the tool string was pulled into the pipe. Data acquisition concluded at 1200 h, when the sonic log identified the bottom of the drill string. The tool string was back at the surface at 1215 h and rigged down completely at 1315 h. The initial logging plan included a VSP; however, because of the poor hole condition, this was postponed until the next hole, and the rig floor was cleared to resume drilling operations.

Hole U1351C

Hole U1351C was drilled without coring as a dedicated logging hole in order to provide the best possible conditions for acquiring a complete set of high-quality logging data at Site U1351. The hole was drilled with a 9⅞ inch tricone bit to a total depth of 1100 m DRF (966 m DSF), which was reached at 1815 h on 27 November. After circulating seawater and sweeping the hole with 50 bbl of high-viscosity mud, the bit was released, the hole was displaced with logging gel, and the bit was raised to the logging depth of 218 m DRF (84 m DSF). Rig-up of the triple combo tool string started at 0350 h on 28 November, and the tool string was RIH at 0500 h. At 0515 h, the recording of gamma ray and resistivity logs was initiated as the tools were lowered downhole at a speed of 1500 ft/h. At 0700 h, with the bottom of the tool string at 934 m WRF, the readings of wireline surface tension dropped, but tool head tension remained normal. To restore surface tension, the wireline had to be reeled in to a reading of 915 m WRF. This indicated that the wireline had been spooled over while the tool was not moving. The normal head tension and zero-acceleration of the tool suggested that some section of the upper hole had collapsed and fallen in on the wireline and that the tool string was being held underneath. After several attempts to move up and down, pulling several times up to the limit of the wireline before trying to slack off and apply moderate tension in the hope of wiggling the tool free, we decided that the only way to recover the tool was to go down with the pipe to clear the obstruction and release the tool. The wireline was cut and held from the surface with an assemblage of T-bars. The drill string was lowered around the wireline and trapped tool string, one stand at a time, while circulating in an attempt to clear the obstruction. After a ~36 h effort, the entire tool string was brought to the surface at 1830 h on 29 November, showing only damage to the wireline immediately above the tool, likely from the process of trying to bring the tool inside the pipe. At this point, the logging tools were laid down, and full circulation and rotation capability were restored to the drill string, which was then tripped back to the surface after cementing and abandoning Hole U1351C. As a result, the only logs recorded in Hole U1351C were the resistivity and gamma ray measurements made on the way downhole.

Data quality

Figures F66, F67, and F18 show a summary of the main logging data recorded in Hole U1351B. These data were converted from the original field records to depth below seafloor and processed to match depths between different logging runs. The resulting depth scale is WMSF (see "Downhole logging" in the "Methods" chapter).

The first indicators of the overall quality of the logs are the size and shape of the borehole measured by the calipers. The hole size measured by the Hostile Environment Litho-Density Sonde (HLDS) caliper during the triple combo run and by the FMS arms are shown in the left-hand columns of Figures F66 and F67, respectively. Both sets of calipers show that the hole diameter is almost uniformly larger than 18.5 inches (>46 cm) above 500 m WMSF, suggesting that none of the tools were able to make good contact with the formation above this depth and that density and porosity data in particular should be used with caution. Below 500 m WMSF, the hole is apparently less enlarged, but it becomes very irregular. Many anomalously low density readings below 600 m WMSF are likely indicative of multiple narrow washouts that significantly affect the quality of the density readings.

The quality of the logs can also be assessed by comparing log data with measurements made on cores from the same hole. Figure F66 shows a comparison of gamma ray and density logs with NGR and GRA bulk density track data and with MAD measurements made on cores recovered from Hole U1351B (see "Physical properties" for description of core measurements). The different sets of measurements generally agree well, even in the upper half of the hole where the calipers indicate an enlarged borehole. Because of the in situ nature of the density log, its values should be equal to or slightly higher than measurements made on the recovered core. The fact that this is verified over most of the interval logged is one more indication that, with the exceptions of the obvious anomalies in the deeper section of the hole, the density log was not seriously affected by the hole conditions and should provide reliable constraints for the generation of a synthetic seismogram and detailed seismic–well correlations.

The very good agreement shown in Figure F19 between the logs recorded in Holes U1351B and U1351C shows that the gamma ray and resistivity logs were not significantly affected by the enlarged hole in the upper part of Hole U1351B. None of the low gamma ray and resistivity peaks below 620 m WMSF in Hole U1351B are matched in Hole U1351C, suggesting that they result from the irregular hole in the deeper section of Hole U1351B. Although the logging tool string could not go deeper than 770 m WMSF in Hole U1351C, the overall good agreement between the two holes above that depth suggests that similar low gamma ray and resistivity peaks at greater depths in Hole U1351B are also the result of irregular hole size.

The high coherence in sonic waveforms indicated by distinct red areas in the V_P and V_S tracks in Figure F67 suggests that, despite the enlarged hole, the Dipole Sonic Imager was able to capture the compressional and flexural wave arrivals and should provide reliable compressional and shear velocity values. However, in some intervals the automatic labeling of the wave arrivals (black curve) failed to recognize the compressional wave and selected the stronger but slower "mud" arrival instead. This is particularly clear between ~165 and 210 m WMSF, where the high velocity in the sandy intervals is shown by coherence peaks for velocity values >2000 m/s, whereas the arrival automatically labeled corresponds to velocities near ~1500 m/s (i.e., the velocity of sound in the borehole fluid). Additional postcruise processing will be required to correct these profiles and will also likely reduce the variability of V_P and V_S in some intervals.

Logging stratigraphy

The combined analysis of gamma ray, resistivity, density, and velocity logs allows for the identification of several logging units defined by characteristic trends. Because of the uniformity of the sediments at this site (see "Lithostratigraphy"), these units are mostly defined by subtle changes in trends and correlations rather than by indications of significant changes in the formation. The downhole logs were used to define three logging units.

Logging Unit 1 (83–260 m WMSF) is characterized by relatively high amplitude variations in gamma ray values, generally increasing with depth. The low peaks in gamma ray in this unit between ~170 and 210 m WMSF correspond to intervals with high resistivity and sonic velocity values, which are likely sand-rich layers consistent with the poor core recovery in this interval. The overall logging signature of this unit is indicative of alternating sand-rich and clay-rich beds. One of the most striking features in this unit is a 7 m thick layer between ~248 and 255 m WMSF with the highest uranium readings in the interval logged (Fig. F18), associated with low gamma ray counts and high resistivity, density, and sonic velocity. This layer coincides generally with Core 317-U1351B-30X, where the lithologic observations that potentially explain these log values are the occurrence of calcareous concretions and glauconitic sandy mud.

Logging Unit 2 (260–510 m WMSF) is defined by low-amplitude variability and decreasing trends with depth in gamma ray and resistivity. Three distinct intervals of increasing-upward gamma ray within this unit suggest fining-upward, transgressive subunits. Caliper readings consistently larger than 18.5 inches in Units 1 and 2 show that the formation has little cohesion.

The top of logging Unit 3 (510–1032 m WMSF) is defined by a significant increase in gamma ray, which is accompanied by increases in density and resistivity. Below a ~50 m thick interval with high gamma ray, density, and resistivity values, the logs are variable within this unit and without clear trends. Resistivity increases slightly with depth, whereas natural radioactivity generally decreases downhole. Logging Unit 3 corresponds to an interval where the borehole diameter is slightly smaller (12–18 inches) than in the upper units, suggesting more consolidated or cohesive sediments.

Core-log correlation

Although the full integration of coring and logging data will be the object of various postcruise efforts, some preliminary correlations can be made at this stage to illustrate the complementary nature of these data sets.

Gamma ray logs through the pipe

Natural gamma ray values were logged through the drill pipe in Holes U1351B and U1351C. Gamma ray measurements are highly attenuated when the tool is inside the BHA and the drill pipe (above 81 and 84 m WSF in Holes U1351B and U1351C, respectively). Although the signal was attenuated by a factor of about four, the correlation between gamma ray from logs and NGR from cores is still very good (Fig. F68). Gamma ray from Hole U1351C has a closer fit with core data, suggesting a less enlarged borehole than Hole U1351B. Sandy layers of meter- and submeter-scale thickness described in the cored interval of Hole U1351B correspond to low peaks in the gamma ray log, suggesting that even an attenuated gamma ray signal can be used as a proxy for lithology. Two additional peaks (at ~68 and ~80 m WSF) in the gamma ray log may indicate the presence of sand layers that were not recovered in cores.

Formation MicroScanner images

When drilling with low recovery, FMS images can be used to recognize features in the cores that help identify similar features in the images recorded in intervals without recovery. Some of the lithologies recovered at Site U1351 with recognizable signatures in the FMS images are shown in Figure F69. The fine sand layers inferred from the FMS images in Figure F69A were not recovered but present the same character as thicker sand layers that were recovered deeper in the hole. The authigenic carbonates observed in Section 317-U1351B-27X-4 can be seen in Figure F69B as bright, sharp layers, as well as more subtle resistive features distributed along the image, whereas the shell-rich intervals distributed in Section 317-U1351B-28X-2 appear as more dispersed, bright resistive patterns in Figure F69C.

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