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doi:10.2204/iodp.proc.338.106.2014

Logging while drilling

LWD and measurement-while-drilling (MWD) data were collected in Hole C0021A to a TD of 294.0 mbsf. Both real-time and memory data from Schlumberger’s MWD TeleScope and LWD geoVISION tool were collected (see the “Methods” chapter [Strasser et al., 2014a]), including drilling parameters, gamma radiation, resistivity, and azimuthal resistivity images. The gamma radiation, resistivity, and resistivity images were interpreted for lithologic and structural features, and three subunits were identified. Porosity and bulk density were calculated from resistivity, and in situ stress orientations were determined from compressional borehole breakout orientations.

Data processing

The seafloor was confirmed at 2969.0 m DRF based on the gamma ray and resistivity data extracted from the memory data.

Data quality

The overall quality of the processed logging data was determined to be good. Because of no rotation during wash down and drilling with low rotations per minute, poor quality resistivity images were recorded above 48.0 mbsf. Sharp horizontal lines, artifacts from ship heave and pipe vibration, were observed throughout the processed resistivity images. Missing data because of high stick-slip (>300 cycles/min) were also observed. Compressional borehole breakouts were only observed after pipe stand connections.

Logging units and lithostratigraphy

Site C0021 is located in a region of large MTDs, upslope from Site C0018. Hole C0021A penetrated slope basin sediment only. To maintain consistency with previous work (e.g., Expedition 333 Scientists, 2012b; Kimura et al., 2011; Strasser et al., 2011), a single logging unit is assigned to the entire drilled interval (0–294 mbsf). The gamma ray data support this classification, as its character does not change significantly throughout; however, three subunits were identified based on changes in resistivity (Fig. F5).

Subunit IA (0–176.8 mbsf)

Subunit IA extends from the seafloor to 176.8 mbsf, exhibits three and a half broad gamma ray cycles, and generally has minor variations in the resistivity log (Fig. F6). From 0 to 8.8 mbsf, the gamma ray log exhibits a steady increase to ~70 gAPI and resistivity increases to a constant ~0.9 Ωm baseline. Between 8.8 and 57.5 mbsf, the gamma ray log exhibits fluctuations of ~10–20 gAPI around the 70 gAPI baseline, with some prominent spikes to low values at 23.2 and 48.4 mbsf (44 and 48.7 gAPI, respectively) and a spike to 87 gAPI at 39 mbsf. A gamma ray value low at 23.2 mbsf corresponds to a low spike (0.5 Ωm) in resistivity. Below this spike, resistivity continues to fluctuate around a 1.0 Ωm baseline, with minor increases and decreases to 69.9 mbsf, where the baseline steps down to ~0.9 Ωm, which it maintains to 144 mbsf. Between 57.5 and 96.6 mbsf, the gamma ray log decreases to ~50 gAPI at ~80 mbsf and then increases again to the baseline of ~75 gAPI. From 96.6 to 143 mbsf, the gamma ray log continues to exhibit 10–20 gAPI fluctuations around a baseline of ~75 gAPI, with small-scale fining and coarsening-upward sequences.

From 144 to 166.5 mbsf, gamma ray values gradually increase to ~92 gAPI. Through this same depth interval, the resistivity log exhibits two sharp increases and gradual decreases, peaking at 154 (1.75 Ωm) and 163 mbsf (1.4 Ωm), but no corresponding change is observed in the gamma ray log (Fig. F6). At 166.5 mbsf, gamma ray values sharply drop to ~70 gAPI.

Subunit IB (176.8–276.7 mbsf)

Resistivity through Subunit IB exhibits an overall gradual increase (from ~1.0 to 1.4 Ωm) but with some large-scale fluctuations (Fig. F6), whereas the gamma ray log continues to exhibit large-scale coarsening- and fining-upward cycles. At 276.7 mbsf, resistivity sharply drops from 1.2 to 1.0 Ωm and gamma ray values sharply drop from 100 to 82 gAPI, which marks the base of Subunit IB.

Subunit IC (276.7–294.0 mbsf)

Through Subunit IC, the gamma ray log exhibits an overall coarsening-upward cycle, most notably increasing from ~80 to ~100 gAPI between 276.7 and 285.3 mbsf. From 285.3 mbsf to the base of the hole (294.0 mbsf), gamma ray values gradually decrease to ~80 gAPI (Fig. F6).

Resistivity image analysis

The statically normalized shallow, medium, and deep button resistivity images were the primary images used for structural and geomechanical analyses. Two different resistivity ranges were selected to normalize the data: 0–5 Ωm was used for overall analysis and 0–10 Ωm was used to clearly identify highly resistive features. In the absence of a caliper measurement, the bit diameter was used as the borehole diameter and assumed to be constant.

Bedding and fractures

The dominant bedding dip direction is south-southeast, matching the expected trend for the slope sediment (Fig. F7). The majority of bedding dips are moderate (15°–40°), but many high-angle (>45°) beds are interspersed throughout (Fig. F5). Two distinct intervals exhibiting consistently high-angle (>50°) bedding planes with mixed dip directions were identified between 95 and 100 mbsf and between 148 and 178 mbsf. High-angle fractures were also observed in areas of chaotic, high-angle bedding, with fewer fractures observed in regions characterized by moderate bedding dips.

The upper interval of chaotic, high-angle bedding displays at least one clear switch in dip direction (from south to west-northwest) between 97 and 100 mbsf (Fig. F8A), and at least one of the bedding surfaces is nonplanar. A rapid change in dip direction and nonplanar bedding surfaces could result from deformation, possibly related to mass transport.

In contrast, the lower interval of chaotic, high-angle bedding does not show repeated switches in bedding dip direction throughout. Instead, the changes are either more gradational or separated by mottled, low-resistivity fractures (Fig. F8B). These observations suggest that deformation within this interval was different to that of the upper interval, perhaps reflecting a change in sediment properties. These intervals are interpreted to represent MTDs, also recovered in Hole C0021B (see “Lithology”). Within Subunit IB, several regions displaying patches of low resistivity are often found to occur either above or below distinct low-angle, low-resistivity beds or fractures (e.g., Fig. F8C). It is possible that these are pyrite-rich zones, as observed at Site C0018, located ~2 km to the southeast (see the “Site C0018” chapter [Strasser et al., 2014c]; Expedition 333 Scientists, 2012b).

Borehole breakouts

Well-developed (wide) borehole breakouts were observed at depths where drilling ahead was stopped during pipe connections (Fig. F8) (e.g., at 101.5 mbsf [3070.5 m DRF], 139 mbsf [3108.0 m DRF], 177 mbsf [3146 m DRF], 215.5 mbsf [3184.5 m DRF], and 253.5 mbsf [3222.5 m DRF]). Breakouts could be associated with pipe stand connections because of lower annular pressure during connections or because of time-dependent evolution of breakouts.

Aside from the regular pipe connection–related breakouts (approximately every 38 m), no other failures were observed in Subunit IA. However, below 225 mbsf in Subunits IB and IC, breakouts were frequently observed, although they were generally much thinner than those in Subunit IA (<20° width). Additionally, the width of pipe connection–related breakouts increased from ~50° to >70° at depths >215 mbsf. The average orientations of paired breakout intervals were 42° and 222°, indicating a northwest–southeast direction of maximum horizontal stress (SHMAX). This direction is comparable to the convergence vector of the Philippine Sea plate and matches the overall trend observed at other IODP Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) sites (C0001, C0004, C0006, and C0009) (Chang et al., 2010; Lin et al., 2010; Byrne et al., 2009; Kinoshita et al., 2008) and Ocean Drilling Program Site 808 (Ienaga et al., 2006; McNeill et al., 2004).

Physical properties

Estimation of porosity and bulk density from resistivity

We estimated porosity using Archie’s law (Archie, 1947) (see the “Methods” chapter [Strasser et al., 2014a]). Cores from Site C0021 were subject to postexpedition analysis; thus, the parameters necessary for use of Archie’s law were taken from a variety of sources. Seawater electrical resistivity was calculated using the temperature profile that was estimated for Hole C0018A during Expedition 333. Hole C0018A is ~2 km from Hole C0021A and should provide a reasonable estimate for temperature at Site C0021. The temperature at the seafloor is estimated at 1.48°C, with an average thermal gradient of 63°C/km (Expedition 333 Scientists, 2012b). Because of uncertainties in the Archie parameters estimated for Site C0018 (see the “Site C0018” chapter [Strasser et al., 2014c]), parameters a = 1 and m = 2.4 estimated for Kumano Basin sediments at IODP Site C0002 during IODP Expedition 314 were applied to Hole C0021A (Expedition 314 Scientists, 2009). Bulk density was estimated from the resistivity-derived porosity using the average grain density (ρg) of 2.66 g/cm3 found in Hole C0018A (see the “Site C0018” chapter [Strasser et al., 2014c]; Expedition 333 Scientists, 2012b).

The resistivity-derived porosity and bulk density depth trends are shown in Figure F5. Within Subunit IA, porosity generally decreases rapidly from ~80% at the seafloor to 61% at 35 mbsf. From 35 to 70 mbsf, porosity is slightly scattered but is generally constant at 61% then increases slightly to 64%. From 70 to 162 mbsf, porosity decreases from 64% to 53%. A large negative spike to 46% is at 152 mbsf. From 162 to 166.5 mbsf, porosity increases from 53% to 57%. From 166.5 to 198 mbsf, porosity decreases to 48%, below which it spikes to 57% at 203 mbsf and decreases rapidly to 52% at 204 mbsf. Porosity then generally decreases to 47% at 231 mbsf before generally increasing to 57% at 246 mbsf. At 246 mbsf, porosity again generally decreases to 46% at 271 mbsf before increasing to 51% at the base of Subunit IB at 276.7 mbsf. Within Subunit IC, porosity decreases to 49% at 284 mbsf, then increases to 53% at 288 mbsf, and finally decreases to 50% near the base of Hole C0021A (294.3 mbsf).

Trends in resistivity-derived bulk density mirror those described above for the resistivity-derived porosity because reported bulk density values are a mathematical manipulation of the derived porosity values (Fig. F5). Generally, bulk density increases from ~1.2 g/cm3 at the seafloor to ~1.9 g/cm3 at the base of Subunit 1A. Three increasing trends correlate to the decreasing trends described for porosity. Within Subunit IB, bulk density decreases from ~1.9 to 1.8 g/cm3. Bulk density through Subunit IC to the base of Hole C0021A is slightly scattered at ~1.8 g/cm3.