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doi:10.2204/iodp.pr.314.2008

Site summaries (continued)

Site C0006

Site C0006 is located at the frontal thrust of the Nankai accretionary prism near the trench axis, and drilling targeted the main frontal thrust at ~ 700 mbsf, as well as subsidiary faults and deformed sediments above that zone and a footwall zone of strong reflectors likely caused by coarse turbiditic trench fill sediments (Fig. F17). The overall objectives of drilling this site with LWD/MWD instruments were to characterize the lithology, deformation, stress state, and physical properties of the wall rocks and the frontal thrust fault zone. Pilot Hole C0006A was drilled with MWD-APWD and gamma ray tool string to TD of 885.5 m LSF, and Hole C0006B was drilled to the same TD with the full LWD tool string, but without any radioactive source in the adnVISION tool. Because the water depth was beyond ROV limit of 3000 mbsl, both holes were drilled without ROV monitoring. Drilling was smooth; however, the real-time MWD communication was lost at a depth of 274 mbsf, as well as the power from the MWD turbine. The LWD tools recorded data in memory mode on battery power; however, the sonic source transducers were negatively affected and sonic data were of very poor to unusable quality from 274 m LSF to TD (885.5 m LSF).

Operations

The Chikyu moved to proposed Site NT1-03B and set beacons with the ROV at 0415 h on 8 November 2008. Deploying five beacons took ~17 h and finished at 2300 h, followed by a few hours to calibrate their positions. The Chikyu moved upcurrent by 5 nmi at 0000 h on 9 November and began running into the hole with the MWD assembly while drifting from 0400 h. The running of the drill string continued from 0600 h while every connection was checked for loosening caused by the strong vibration during drilling of the previous hole. The drill string began running in at a ROP of 100 m/h and pump rate of 85 rpm. With no ROV monitoring, Hole C0006A was spudded at 0015 h on 10 November (3903.5 m DRF). The hole was drilled to 4210 m DRF and swept with Hi-Vis gel and spotted with heavy mud at bottom at 1400 h on 10 November. A short wiper trip was made from 4210 to 4060 m DRF between 1415 and 1530 h. Drilling was continued to 4371 m DRF and hole was again swept with Hi-Vis gel and spotted with heavy mud before making another short trip at 0500 h on 11 November. Drilling was continued after a short trip between 4503 and 4358 m DRF, then a lube-oil gear was tightened which had shaken loose as a result of heavy vibration. The target depth of 885.5 mbsf was finally reached at 2045 h on 11 November. After sweeping Hi-Vis gel and spotting kill mud, the MWD BHA was pulled out the hole from 2045 h. Rig floor instruments were checked and tightened again because of the heavy current-induced vibration.

After pulling out the MWD tool string, the ship was moved 2 nmi upcurrent. An LWD tool assembly was made up as at the previous sites (full string without the radioactive source for the adnVISION) at 1115 h on 12 November, and running into the hole began while the ship was drifting back on site. The top drive system was connected at 3854 m DRF and run in with circulation. At 0045 h on 13 November, seabed was confirmed at 3900 m DRF, and Hole C0006B was spudded and jetted down to 40 m, then rotary drilling began. Drilling continued to 4174 m DRF when a pressure drop was noted, apparently caused by the failure of the mud turbine vanes in the PowerPulse MWD tool. This was accompanied by loss of communication from the MWD tool at 1400 h on 13 November. After several attempts to recover data transmission failed, drilling continued to 4502 m DRF without real-time data transmission. Prior to a wiper trip between 4502 and 4350 m DRF, the hole was swept with Hi-Vis gel and spotted with heavy mud at 0315 h on 14 November. Drilling finally reached the 885.5 m LSF TD at 1645 h. The drill string was pulled out of the hole to 3836 m DRF, the top drive made up, and the string was displaced with seawater.

All memory data were successfully downloaded immediately after the LWD tools were pulled out of the hole, and the data quality is good for the whole section except for the sonic data, which is good to 155 m LSF only. The presumed cause of this loss of data quality is that the sonicVISION was running on battery power only, which could not provide enough energy for the transmitters to operate at full strength. Hence the sonicVISION did not record good quality data from the formation for the depth range from 155 mbsf to the TD.

Log characterization and lithologic interpretation

Four logging units were defined based on the different trends and character of the LWD log responses (Fig. F18). Logging Unit I (0–197.8 m LSF) is characterized by variable gamma radiation and high-amplitude fluctuation of resistivity and is interpreted as sandy and muddy deposits. Logging Unit II (197.8–428.3 m LSF) is characterized by a gradually increasing trend in the gamma radiation with occasional thick (5 m) low-gamma layers. This unit is interpreted as mudstone with thick sandstone beds. Possible repeated stratigraphic sections are recognized in this logging unit. Logging Unit III (428.3–711.5 m LSF) was defined based on inferred alternating beds of mudstone and sandstone and divided into two logging subunits. Logging Subunit IIIA is characterized by high gamma radiation with thin (1 m) low gamma radiation layers. Logging Subunit IIIB is characterized by high gamma radiation with a large number of thin (1 m) low gamma radiation layers and repeated increasing trends in resistivity. The base of logging Unit III is interpreted as a fault zone as well as a distinct lithologic boundary. Logging Unit IV (711.5 m LSF to TD) is characterized by low gamma radiation and resistivity and interpreted as abundant sandstones.

Physical properties

Porosity and density were estimated using five different resistivity logs (ring; bit; and deep, medium, and shallow button) and the sonic velocity obtained from the DTCO measurements (Fig. F18). No radioactive source was used in Site C0006, and accordingly neither the TNPH nor RHOB logs were available. The deep and medium button resistivity logs show very good agreement. The shallow button resistivity is significantly lower than the other two button logs in logging Units I and IV. Porosity, calculated from bit and ring resistivity using the estimated temperature profile, shows a slowly decreasing trend with depth from 0 to ~650 m LSF and shows increasing trend with depth below 650 m LSF. Because of the malfunction of the sonic tool, P-wave velocity data were measured only to ~160 m LSF. Velocity appears to be nearly constant or slightly increasing over this shallow interval. Velocity and resistivity are generally in good agreement, and high velocity zones correspond to high resistivity zones.

Structural geology and geomechanics

Good quality borehole resistivity images provide information on orientation of bedding, fractures, and breakouts at Site C0006 (Fig. F18). Bedding dips are shallow to moderate with most dips <30°. The mean bedding dip is west–northwest with a strong clustering of easterly dips above 200 m LSF and more northerly dips near the base of the hole. Overall, the deeper bedding dip orientations are consistent with north-northwesterly directed shortening. Fractures at Site C0006 are notable in not being as focused or concentrated into zones as at other Expedition 314 sites. Fractures show diverse orientations, striking mostly northwest–southeast in the upper two lithologic units (above 428 m LSF). Fractures in the deeper two lithologic units (429–853 m LSF) strike dominantly northeast and southwest, dipping both northwest and southeast. Overall fracture orientation at depths below 429 m LSF is consistent with northwesterly directed shortening.

Borehole breakouts occur from 188 to 729 m LSF but are in general much more weakly developed than at other sites drilled during this expedition. Breakouts are not readily discernible at greater depths. This lack of obvious breakouts (below 729 m LSF) occurs just below the transition to lithologic Unit IV, which is dominated by sand or sandstone. Breakouts show a mean S_Hmax orientation of 330°, consistent with that observed at Sites C0001 and C0004 but divergent from the S_Hmax expected from the far-field convergence direction of the Philippine Sea plate and Japan.

Log characterization and lithologic interpretation

The logging units do not correlate well with the seismic reflection data, probably because the section is strongly faulted. Several faults that intersect the well location are visible the seismic data (Fig. F18). Several features in the LWD logs do, however, correlate with features on the seismic data. For example, the sandy layers (low gamma radiation and resistivity) at ~220, 240, 300, and 335 m LSF correlate with strong reflections and appear to be parts of the same unit that have been repeated as a result of thrusting (Fig. F18). Several seismically defined thrust faults also correlate with features in the resistivity images, such as the conductive fractures at 360 and 381 m LSF. The basal décollement correlates with a fracture at 657 m LSF.

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