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doi:10.2204/iodp.proc.317.105.2011

Downhole logging

Operations

Logging operations at Site U1353 began with drilling the dedicated logging Hole U1353C on 26 December 2009. This hole was drilled using the same 11 inch APC/XCB bit used for Hole U1353B with a center bit installed. The hole was drilled to a total depth of 625 m DRF (529 m DSF), which was reached at 2230 h on 27 December (all times are ship local time, Universal Time Coordinated [UTC] + 13 h). To prepare the hole for logging, the center bit was retrieved and the hole was swept with 50 bbl of high-viscosity mud. The hole was then displaced with 300 bbl of heavy mud (~10.5 ppg), and the bit was raised to the logging depth of 201 m DRF (105 m DSF). Rig up of a modified triple combo tool string began at 0340 h on 28 December. Because of the difficult drilling conditions and poor recovery at this site as well as our experience from logging at the previous sites, no radioactive source was used in this hole. The modified tool string consisted of the Hostile Environment Natural Gamma Ray Sonde (HNGS), the Hostile Environment Litho-Density Sonde (HLDS; without source, for the caliper only), the General Purpose Inclinometry Tool (GPIT), and the Dual Induction Tool (DIT). The tool string was rigged up by 0410 h and run into the hole at 0416 h at a speed of 3000 ft/h. While the tool was being lowered, natural gamma ray and resistivity data were recorded from the seafloor to a total depth of 622 m WRF, where a first logging pass was started at 0503 h. This repeat pass was completed at 550.1 m WRF, and the tool string was run back down to total depth for a full pass, starting at 0530 h at a speed of 900 ft/h. The caliper was closed at 235 m WRF for reentry into the pipe, and the pass was completed at 0703 h when the seafloor was identified in the gamma ray log at 94.5 m WRF. The tool string was back at the surface at 0750 h and rigged down completely by 0815 h.

The density sonde caliper showed a good borehole with a diameter ranging from 12 to >19.5 inches. We decided that enough daylight remained to run both the FMS-sonic and the Versatile Seismic Imager (VSI) tool strings, so the FMS-sonic was chosen for the second run. By 0850 h, the tool string had been rigged up and run into the hole at a speed of 2500 ft/h to record sonic velocities on the way down. After a difficult exit from the pipe at 201 m WRF, the tool string met with an obstruction at 343 m WRF that could not be passed. After multiple attempts to pass below the obstruction, we decided to begin the first pass. At 1020 h, the FMS calipers were opened, and a first pass was recorded at a speed of 1000 ft/h between 343 and 235 m WRF. This pass proceeded without additional difficulty, with the exception of an increase in tension between ~291 and 288 m WRF that forced the caliper to be closed. When the tool string was sent down for a second pass, it was not able to reach deeper than 305 m WRF, indicating that the formation was gradually collapsing. The second pass started at 1107 h and was interrupted several times by instances of high wireline tension that required the FMS calipers to be closed. The top of the tool string was brought inside the pipe at 1128 h, but the tool string was raised only ~7 m before high tension indicated that something was impeding its progress. During the following 2 h, efforts to bring the tools back on deck were complicated by the formation apparently closing in on the tool string and the drill pipe. Coordinated pulls on the drill string and on the wireline, along with periods of intense circulation, eventually managed to bring most of the tool string inside the pipe by 1340 h. We then decided to recover the tools using a reverse cut and thread operation, with two T-bars alternately supporting the wirelines while the drill string was tripped back stand by stand. The base of the BHA reached the drill floor at 1750 h, with ~10 m of tool string extending below the bit. Visual inspection immediately confirmed that the two centralizers of the logging string had failed to close properly, likely because formation material had accumulated behind them and prevented their normal collapse. The repeated pull on the wireline during recovery operations eventually bent the centralizers in a way that blocked any further upward motion. The tool string was completely removed from the pipe and rigged down at 1925 h. Further inspection established that no section of the logging string other than the centralizers had suffered any damage.

Data quality

Figures F47, F48, and F49 show a summary of the main logging data recorded in Hole U1353C. 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 wireline log matched depth below seafloor (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. Hole size measured by the HLDS caliper during the triple combo run and by the FMS arms is shown in Figures F47 and F48, respectively. Although both sets of calipers indicate an enlarged and irregular hole and operations showed that the borehole was gradually collapsing, all calipers maintained mostly good contact with the formation above ~350 m WRF, which suggests that all recorded data should be of good quality. Below this depth, hole size is close to the maximum reach of the HLDS caliper (>17 inches), but the only measurements made (gamma ray and resistivity) are not significantly affected by large hole size. The quality of the resistivity log is indicated by the good overlay between the two resistivity measurements with deep and medium depths of penetration into the formation (Fig. F47).

The reliability of the logs can also be assessed by comparing logging data with core measurements from the same site. Figure F47 shows a comparison of the gamma ray log with NGR track data measured on cores recovered from Hole U1353B, indicating good agreement between the two data sets, particularly in the identification of sand-rich intervals with lower gamma ray readings.

The clear arrivals in the acoustic logging waveforms and the high coherence indicated by distinct red areas in the V_P and V_S tracks in Figure F48 show that the Dipole Sonic Imager (DSI) was able to measure reliable V_P and V_S values. Additional postcruise processing will refine these profiles and characterize some of the high-coherence events that were not labeled automatically at the time of acquisition.

Porosity and density estimation from the resistivity log

In order to provide a measure of porosity and density from the logs obtained without nuclear sources, we used Archie's (1942) relationship to calculate porosity from the phasor deep induction log (IDPH), which is the log least affected by borehole conditions (Schlumberger, 1989), and combined it with MAD grain density data to derive a density profile. Archie (1942) established an empirical relationship between porosity (ϕ), formation resistivity (R), and pore water resistivity (R_w) in sandy formations:

where m and a are two empirical parameters that are often called cementation and tortuosity (or Archie) coefficients, respectively. The resistivity of seawater (R_w) was calculated as a function of temperature and salinity, as described by Fofonoff (1985). Pore water salinity was assumed to be 34 ppt (or 3.4%; see "Geochemistry and microbiology"), and temperature was assumed to follow a local linear gradient of 40°C/km, as suggested by in situ measurements at Site U1352 (see "Heat flow" in the "Site U1352" chapter) and in the Clipper-1 well (Shell BP Todd, 1984). The most realistic value for the cementation coefficient is a = 1 because this gives a resistivity equal to formation water resistivity when porosity is 100%. A value of m = 1.9 was chosen iteratively to provide the best baseline match with MAD porosity data. Although Archie's relationship was originally defined for sand-rich formations, Jarrard et al. (1989) showed that the effect of clay minerals is moderate, and the relationship is commonly used to estimate porosity in clay-rich formations with poor borehole conditions (Collett, 1998; Jarrard et al., 1989). The resulting porosity log is shown in Figure F47, where it compares well with MAD porosity data. Using MAD grain density, we used this resistivity-derived porosity to calculate a new density curve, which is in good agreement with core measurements (Fig. F47).

Logging stratigraphy

The combined analysis of gamma ray spectroscopy, resistivity, and velocity logs allows for the identification of logging units defined by characteristic trends. Because the FMS-sonic tool string could not record data deeper than 248 m WMSF, identification of the units is based mainly on gamma ray and resistivity data from the first logging run. The combined analysis of these data allows two logging units to be identified.

Logging Unit 1 (105–260 m WMSF) is characterized by an increasing trend in gamma ray from the top of the unit to ~180 m WSF, followed by a mostly decreasing trend to the base of the unit. The similarity between the total gamma ray and the potassium and thorium curves suggests that the increase in total gamma ray is related to variations in mineralogy (Fig. F49). These trends are interrupted by intervals of low gamma ray and high resistivity and velocity that are interpreted as sandy intervals, some of which coincide with actual recovery of sand or even gravel from the same depths in Hole U1353B. Figure F48 shows that some of these intervals are several meters thick (particularly 178–185 and 202–208 m WMSF) with very high velocity values and should be associated with significant seismic reflectors (see "Log-seismic correlation"). The trends and variability in this unit are very similar to those observed at an equivalent depth range in the logs recorded at Site U1351, seaward of this site.

Logging Unit 2 (260–528 m WMSF) is characterized by generally decreasing trends with depth in gamma ray and resistivity, with only limited variability. The top of this unit is also the approximate depth at which core recovery became very low and the FMS-sonic tool string was prevented from going deeper, indicating a change in the general fabric of the formation.

Core-log correlation

Because Hole U1353C was drilled without coring as a dedicated logging hole, a direct comparison between FMS images recorded in Hole U1353C and core images from Hole U1353B is not possible. However, the proximity of the two holes and the assumption of a flat stratigraphy allow for identification in the electrical images of several features from the cores and from the intervals not recovered.

Figure F50 shows some examples of the diversity of structures at Site U1353. The bottom of lithologic Unit I at ~150 m CSF-A was chosen at a burrowed unconformity at 151.36 m CSF-A (see "Lithostratigraphy") that coincides in Figure F50A with the bottom of a folded or dipping structure in the FMS images. The speckled nature of the FMS images in Figure F50B suggests that at least some of the gravel recovered in Core 317-U1353B-36H was in situ. The fine layering displayed in Figure F50C shows a series of thin beds (a few centimeters thick) that were not recovered below one of the thick sandy intervals that characterize logging Unit 1.

Log-seismic correlation

A depth–traveltime relationship can be determined from the sonic logs and used to correlate features in the logs, recorded in the depth domain, with features in the seismic stratigraphy, recorded in the time domain. A synthetic seismogram was constructed for the interval logged with the FMS-sonic tool string in Hole U1353C (105–249 m WMSF) from the sonic log and the density curve calculated from the resistivity log using Archie's relationship. Figure F51 shows good correspondence between the synthetic waveform and reflections in the seismic line closest to Site U1353. In particular, seismic sequence boundaries U10–U13, interpreted from seismic data (see "Seismic stratigraphy" in the "Expedition 317 summary" chapter), are well resolved in the synthetic seismogram and correspond to distinct features in the sonic, calculated density, and gamma ray logs. U12, U11, and U10 all have similar log characteristics, falling at relatively abrupt transitions from high gamma ray, lower density, and low velocity below to low gamma ray, higher density, and very high velocity above. Based on the time–depth relationship from the synthetic seismogram, these boundaries correspond to lithologic changes from dominantly muddy sediments below to sand-, shell-, and gravel-dominated sediments above (see "Lithostratigraphy"). U13 shows the opposite polarity and has a different expression in the logs, located at a density low with a relatively low velocity value. This suggests that U13 may have been formed by a different process than U12–U10. Additional postcruise research will refine these correlations by reprocessing the sonic logs and providing a more detailed synthesis of core-log correlations at this site.

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