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doi:10.2204/iodp.proc.317.104.2011

Downhole logging

Operations

Two holes were logged at Site U1352. Hole U1352B was logged after reaching refusal with the XCB coring system, and Hole U1352C was logged after coring deep enough to be able to log and fully characterize the Marshall Paraconformity.

Hole U1352B

After an extended period of low core recovery, slow rate of penetration, and hot conditions at the bit, APC/XCB coring operations were terminated, with the last core on deck at ~1900 h on 4 December 2009 (all times are ship local time, UTC + 13 h). Logging operations began by conditioning the hole immediately after the last core was recovered from a total depth of 1185.5 m DRF (831 m DSF). Because of the tool recovery operations in Hole U1351C (see "Downhole logging" in the "Site U1351" chapter), mud supplies on the ship were limited for the remaining sites; however, the drilling process and lithology indicated that this hole could be displaced with seawater rather than logging gel. After the hole was swept, circulated with 50 bbl of high-viscosity mud, and displaced with seawater, the bit was raised to the logging depth of 436.6 m DRF (82 m DSF). Rig up of the triple combo tool string (natural gamma ray, bulk density, electrical resistivity, and porosity) began at 2315 h and was complete at 0025 h on 5 December, and the tool string was RIH at 0045 h. While the tool string was being lowered at a speed of 2500–3000 ft/h, gamma ray and resistivity data were recorded from the seafloor to 842 m wireline log depth below rig floor (WRF), below which it proved impossible to lower the tool string. After multiple unsuccessful attempts to pass the blockage at ~842 m WRF, we decided to log up from this depth, more than 300 m shallower than the total hole depth. A first repeat pass was completed at 0255 h (470 m WRF), and the tool was run back down to 842 m WRF for the full pass, which began at 0305 h at a speed of 900 ft/h. At 0420 h (477 m WRF), the caliper was closed for reentry into the pipe, and the pass was completed at 0500 h when the seafloor was identified by a drop in the gamma ray log at 355.5 m WRF. The tool string was back on deck at 0520 h and rigged down at 0610 h.

Overall, the caliper of the density sonde showed a very irregular borehole, with a hole diameter >20 inches over many intervals. We decided that, even if the quality of FMS images was likely to be poor in some intervals, there was no risk to the tools and the deployment of the FMS-sonic tool string would provide worthwhile velocity and image data. The FMS-sonic tool string was rigged up by 0700 h and RIH at a speed of 2000 ft/h to record sonic velocities on the way down. At 0800 h, a shallower obstruction was met at 795 m WRF and could not be passed. At 0812 h, we decided to begin logging up with both the FMS and sonic tools from the deepest depth reached (797 m WRF) at a speed of 1200 ft/h. The first pass was completed at 0900 h at 480 m WRF, before the top of the tool string reached the bottom of the pipe. The FMS calipers were closed, and the tool was sent down for a second pass. At 0920 h, the same obstruction was reached at 796 m WRF, and the second pass started at a speed of 1500 ft/h. The FMS calipers were closed after the second pass was completed at 1005 h (480 m WRF), before the top of the tool string was pulled into the pipe. Data acquisition was concluded at 1020 h when the sonic log identified the bottom of the drill string. The tool string was back at the surface at 1050 h and was rigged down completely at 1130 h. The initial logging plan included a VSP; however, this was postponed until the next hole because of hole conditions, and the rig floor was cleared to resume drilling operations at 1145 h.

Hole U1352C

The last core was recovered from Hole U1352C at 1740 h on 19 December from a total depth of 1927.5 m DSF. After sweeping the bottom of the hole with 50 bbl of high-viscosity sepiolite/attapulgite mud and circulating two times the annular capacity of the hole with seawater, the RCB bit was dropped at ~2300 h. In order to reserve mud supplies for the remaining sites and in consideration of the stable nature of the formation in the deepest section of the hole, a 450 m interval was targeted between 900 and 450 m DSF to be displaced with logging mud. The pipe was pulled to 900 m DSF, and 400 bbl of logging mud was pumped into the borehole to stabilize this interval, which previous logging and coring operations had shown to be unstable. Shortly thereafter, the drill string indicated significant drag from the formation, which required reconnection of the top drive to restore rotation. The continued upward progress of the pipe was slow between 880 and ~190 m DSF, suggesting a series of ledges rather than a full hole collapse but also indicating that lowering the logging tool could be hazardous. Considering the potential benefits of logging this unique hole, we decided to run the triple combo tool string without radioactive sources.

At 1400 h on 20 December, the pipe was set at a logging depth of ~100 m DSF (354.6 m DRF), and rig-up of the triple combo began at 1520 h. 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 1600 h and RIH at 1620 h at a speed of 4000 ft/h. A downlog was started at 340 m WRF at a speed of 3500 ft/h, and the seafloor and base of pipe were identified at 355 and 457 m WRF, respectively. At 1700 h, cable tension indicated signs of drag from the formation at ~550 m WRF, similar to the shallowest trouble spot identified by the driller as the drill string was pulled out of the hole. After several attempts to pass the obstruction, reaching a maximum depth of 562 m WRF, further downhole progress was deemed hazardous. At 1720 h, the caliper was opened, recording a hole diameter of 6 inches, significantly narrower than the 9 inch bit size, and an uplog was started at a speed of 900 ft/h. The caliper was closed at 490 m WRF before the top of the tool string reached the bottom of the drill string, and the uplog was completed at 1800 h when the gamma ray log identified the seafloor at 355 m WRF. The tool string was at the surface at 1825 h and completely rigged down by 1851 h. The rig floor was cleared to start the hole abandonment protocol at 1904 h.

Data quality

Figures F68, F69, and F70 show a summary of the main logging data recorded in Hole U1352B. These data were converted from 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. Hole size measured by the HLDS caliper during the triple combo run and by the FMS arms are shown in Figures F68 and F69, respectively. Although both runs indicate an enlarged and irregular hole, the readings of the two orthogonal FMS calipers (Fig. F69) suggest that the borehole cross section is not circular and is probably elliptical below ~270 m WSF. One caliper read close to 12 inches over most of the lower half of the logged interval, whereas the other caliper read close to ~14 inches, near the limit of its range. The fact that the curves display variability over most of the hole suggests that both sets of arms made some kind of contact with the formation, possibly with only one pad in some places. The larger HLDS caliper readings above 270 m WMSF indicate that the stronger and narrower single-arm caliper was likely pushing into the formation. Both sets of calipers show that the hole diameter is almost uniformly larger than 16 inches (>46 cm) above 270 m WMSF, suggesting that the tools were not able to make good contact with the formation above this depth. Porosity and density data, in particular, should be used with caution, most clearly between 200 and 270 m WSF, where log data deviate significantly from the same measurements made on cores (see below). Below this depth, the hole is apparently less enlarged, but it remains very irregular, and many anomalously low density readings below this depth are likely indicative of multiple narrow washouts that significantly affect the quality of the density readings.

Logs recorded in Hole U1352C show similar trends to those in Hole U1352B, reflecting the same response to lithology (Fig. F71). Lower values in gamma ray and medium resistivity in Hole U1352C can be explained by an extremely enlarged borehole, which is supported by HLDS caliper readings of >19.5 inches throughout the logged interval. Low gamma ray and resistivity above 115 m WMSF and between 185 and 190 m WMSF, in particular, may reflect large borehole washouts.

The quality of the logs can also be assessed by comparing log data with core measurements from the same hole. Figure F68 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 U1352B (see "Physical properties"). The gamma ray measurements generally agree well, even in the upper half of the hole, but the density log is seriously affected by hole conditions below ~200 m WMSF, with highly variable and anomalously low readings.

The high coherence in sonic waveforms indicated by distinct red areas in the V_P and V_S tracks in Figure F69 suggests that, despite the enlarged hole, the Dipole Sonic Imager (DSI) was able to generate strong compressional and flexural waves and should provide reliable compressional and shear velocity values. However, all of the velocity profiles display a high variability that is likely not representative of formation properties and will require additional postcruise processing to be fully characterized. In addition, V_S values above ~150 m WMSF are systematically on the fastest edge of the high-coherence domain, suggesting a processing drift that should also be corrected postcruise.

Porosity and density estimation from the resistivity log

In order to provide a measure of porosity and density from the logs despite the poor hole conditions, 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 more reliable 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 30 ppt (or 3%; see "Geochemistry and microbiology"), and temperature was assumed to follow a linear gradient of 42°C/km, as established by in situ temperature measurements (see "Heat flow"). 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 F68, where it compares very 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 data and does not display any of the anomalous variability of the original density log.

Logging stratigraphy

The combined analysis of gamma ray, resistivity, density, and velocity logs allows for the identification of logging units defined by characteristic trends. Because of the uniformity of the sediments in the logged interval (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 two logging units.

Logging Unit 1 (82–250 m WMSF) is characterized by relatively low amplitude variations in gamma ray, resistivity, and acoustic velocities. A distinct increasing-upward trend in gamma ray occurs at 250 m WMSF, and a high gamma ray interval occurs between ~160 and 170 m WMSF, which corresponds to an interval of homogeneous mud with clay beds (Core 317-U1352B-19H). Gamma ray spectroscopy shows a significant uranium contribution to the total radioactivity throughout this unit (Fig. F70). 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. Gamma ray decreases above 160 m WMSF, which may reflect generally decreasing clay content in this depth interval. The increasing-upward and then decreasing-upward pattern in gamma ray is consistent with gamma ray logs from an equivalent depth at Site 1119 (Shipboard Scientific Party, 1999b). Resistivity decreases gradually with depth, whereas velocity increases with depth. Caliper measurements that were consistently >19.5 inches (the maximum reach of the HLDS caliper) indicate an enlarged borehole.

Logging Unit 2 (250–487 m WMSF) is defined by a change to higher amplitude variations in gamma ray, resistivity, and acoustic velocities. Gamma ray and velocity increase with depth. Sharp peaks in uranium (Fig. F70) are associated with high resistivity values and can be correlated with green calcareous sandy intervals observed in the core recovered in this unit (see "Core-log correlation"). Another distinct feature of this unit is a 15 m thick fining-upward sequence between 435 and 450 m WMSF, characterized by an upward increase in gamma ray that is mirrored by an upward decrease in resistivity. The borehole diameter in this unit is smaller but highly irregular, ranging from 6 to 19.5 inches, which may reflect the presence of more cohesive marls in the formation.

Core-log correlation

Some of the most remarkable features in the logs recorded in Hole U1352B are the sharp peaks in uranium below ~270 m WMSF, all of which are associated with more modest peaks in resistivity and very distinct peaks in velocity, indicating fine indurated layers (Figs. F68, F69, F70). The higher resistivity of these layers, although less pronounced than the changes in velocity or uranium content, is enough to make them appear as bright features in the FMS electrical images, making them easy to correlate with core images. However, some of these peaks coincide with intervals of incomplete recovery (e.g., Cores 317-U1352B-37X, 42X, and 49X), likely because of the hardened, highly resistive layers, and the only means of correlation are missing intervals or traces in the core catcher (e.g., the calcareous nodule in Section 317-U1352B-49X-CC).

Some of these layers are shown in Figure F72. The sections of core that seem to match the high-uranium resistive layers are alternately described as dark greenish gray calcareous sands (Core 317-U1352B-32H), dark greenish gray sandy calcareous mud (Core 317-U1352B-35H), olive calcareous sandy marl (Core 317-U1352B-36H), olive calcareous muddy sand and sandy mud (Core 317-U1352B-37X), or olive sandy mud with calcareous concretions (Core 317-U1352B-42X). These sections of core have in common a calcareous sandy component and the occurrence of nodules or some level of cementation.

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, to features in the seismic stratigraphy, recorded in the time domain. A synthetic seismogram was constructed for the logged interval in Hole U1352B (82–487 m WSF) using sonic data and the density curve calculated from the resistivity log using Archie's relationship. Several packages of strong seismic reflections were reproduced in the synthetic seismogram shown in Figure F73, particularly below ~250 m WMSF (0.77 s two-way traveltime). In the uppermost 250 m of the borehole, sonic velocity values measured in Hole U1352B are slightly lower than, but in general agreement with, sonic velocities recorded during Leg 181 at Site 1119, located in the slope environment ~35 km from Site U1352 (Fig. F74) (Shipboard Scientific Party, 1999b). Although the general trends in velocity between 250 and 390 m WMSF are consistent, several discrete intervals (e.g., 250–270 and 300–320 m WMSF) have higher velocities at Site 1119. Below 390 m WMSF, sonic velocity at Site 1119 appears significantly higher than that in Hole U1352B. The velocity logs recorded at these two sites are also consistently slower than the velocity model used to determine the depth of the reflectors during the Expedition 317 transect (Fig. F74; see "Traveltime/depth conversion" in the "Expedition 317 summary" chapter). This velocity model was derived from data recorded in Hole 1119C and in the Clipper-1 well and predicted higher velocity values, which resulted in an offset of ~40 m at the bottom of Hole U1352B between the depths of the reflectors predicted by the velocity model and those predicted by the Hole U1352B synthetic seismogram (Figs. F73, F74). Because the velocity measured on core samples recovered from Hole U1352C suggests significantly higher velocity at depth (see "Physical properties"), it is possible that the time–depth relationships could have been reconciled at depth if logging of the deep interval in Hole U1352C had been successful.

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