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doi:10.2204/iodp.proc.338.105.2014

Logging while drilling

LWD and MWD data were collected in Hole C0018B through slope-basin sediment to a total depth of 350.0 mbsf. Schlumberger’s MWD TeleScope tool provided drilling parameters, and the LWD geoVISION tool recorded gamma ray, resistivity, and azimuthal resistivity images (see the “Methods” chapter [Strasser et al., 2014a]). Both real-time and memory data were collected. Overall, data quality was found to be good, with the exception of the upper interval (0–47 mbsf) of the resistivity images, which is missing because of wash down. The gamma ray, resistivity, and resistivity images were interpreted for lithologic and structural features, with two clear subunits identified in the slope sediment and evidence for two MTDs. Porosity and bulk density were calculated from resistivity data, and in situ stress orientations were determined from breakout orientations.

Data processing

The seafloor was confirmed at 3113.0 m DRF, based on the NGR and resistivity curves extracted from the memory data.

Data quality

The overall quality of the processed logging data was found to be good. Because of no rotation during wash down and ROP exceeding 40 m/h, resistivity images were not produced above 3160 m DRF (47 mbsf). Sharp horizontal lines, artifacts from ship heave and pipe vibration, were observed throughout the processed resistivity images. Missing data caused by high stick-slip (>300 cycles/min) were also observed. Breakouts were regularly observed at depths corresponding to pipe connections.

Logging units and lithostratigraphy

Site C0018 is located in a region of large MTDs and was cored during Expedition 333 (Expedition 333 Scientists, 2012b). Data from the downhole tools were assessed in terms of realistic values for the known lithology of the recovered core. Hole C0018B penetrated slope-basin sediment, which was defined as Unit I during previous expeditions. To maintain consistency, only one logging unit is defined encompassing the entire hole (0–350.0 mbsf). The gamma ray log supports this classification, as its character does not change significantly through the entire drilled section; however, two subunits were identified based on changes in the character of the resistivity logs (Fig. F4; Table T1). In Hole C0018A, six MTDs were identified between 0 and 190 mbsf (Expedition 333 Scientists, 2012b); although these MTDs are unidentifiable from the gamma ray and resistivity logs alone, evidence is seen in the resistivity images (see “Resistivity image analysis”).

Subunit IA (0–179.8 mbsf)

The gamma ray log exhibits only minor variations through Subunit IA, fluctuating around a constant baseline of ~75 gAPI, and resistivity also exhibits overall low variability through this subunit (Fig. F5). From 0.0 to 14.8 mbsf, gamma ray values gradually increase from 29 to 75 gAPI, and resistivity also exhibits a gradual increase (0.3–0.9 Ωm). Below 14.8 mbsf, gamma ray values fluctuate between ~45 and ~105 gAPI around the baseline of ~75 gAPI. From 14.8 to 179.8 mbsf, the gamma ray log exhibits repeated 10–20 m scale increasing and decreasing gamma ray cycles interpreted as coarsening- and fining-upward cycles. Between 101.3 and 179.8 mbsf, the interpreted coarsening- and fining-upward cycles are thinner, occurring over a few meters. The resistivity logs exhibit a similar character. From 14.8 to 23.5 mbsf, resistivity is constant at ~0.9 Ωm, with only minor fluctuations; a sharp spike to 0.75 Ωm at 23.5 mbsf corresponds to low gamma ray values (~53 gAPI). Another spike to low resistivity (~0.6 Ωm) occurs at 29.1 mbsf, but no observed change in gamma ray values was found at this horizon. Between 29.1 and 39.7 mbsf, resistivity is constant at ~1.0 Ωm with minor fluctuations. From 39.7 to 44.7 mbsf, resistivity gradually increases to ~1.2 Ωm before gradually decreasing back to 1.0 Ωm at 54.0 mbsf. Resistivity remains constant (~1.0 Ωm) to 90.25 mbsf, where it increases to 1.2 Ωm over a 3 m interval before returning gradually to ~1.0 Ωm by 95.4 mbsf. From 95.4 mbsf to the base of Subunit IA (179.8 mbsf), resistivity exhibits repeated increasing and decreasing profiles (ranging from 1.20 to 0.92 Ωm), reflecting the interpreted coarsening- and fining-upward sequences in the gamma ray log, and an area of high-angle and variable dip (Fig. F4). The base of Subunit IA is placed where resistivity sharply drops to 0.2 Ωm.

Subunit IB (179.8–350.0 mbsf)

Within Subunit IB, the gamma ray log exhibits similar characteristics to Subunit IA (Fig. F6) but fluctuates around a slightly higher gamma ray baseline of 85 gAPI. Interpreted coarsening- and fining-upward packages on the 5–10 m scale are observed throughout this subunit. Two prominent gamma ray lows of ~35 and ~55 gAPI occur at 205.4–206.5 and 208.3–210.0 mbsf, respectively. A corresponding low in resistivity (0.9 Ωm) is observed between 205.4 and 206.5 mbsf, suggesting the presence of a thick, permeable sand horizon.

Through Subunit IB resistivity exhibits a very different character to Subunit IA, with continuous fluctuations between high and low values (between >3.0 and 0.8 Ωm) over narrow (<1–2 m) depth intervals, possibly indicative of thin, interbedded sand/ash and muddy sediment. Within Subunit IB, three intervals are identified based on changes in resistivity character (Table T1; Fig. F6). Subunit IBi (179.8–230.9 mbsf) is directly below the base of Subunit IA. The subunit exhibits three prominent resistivity lows (~0.3, 0.54, and 0.6 Ωm at 179.9, 181.6, and 182.8 mbsf, respectively) and then fluctuates at ~1.4 Ωm (±0.2 Ωm) downward to another prominent low (~0.5 Ωm) at 191.7 mbsf. Between 191.7 and 194.78 mbsf, resistivity decreases from 1.45 to 1.18 Ωm with very little fluctuation. From 194.78 to 217.02 mbsf, resistivity exhibits large fluctuations between <0.8 and >1.7 Ωm. From 217.02 mbsf, resistivity fluctuations decrease in amplitude (±0.3 Ωm) and the general trend decreases. Between 225.9 and 230.9 mbsf, resistivity maintains a constant value of ~1.3 Ωm before a sharp spike at 230.9 mbsf (~3.2 Ωm), which corresponds to a sharp drop in gamma ray values (from ~107 to ~68 gAPI); this marks the Subunit IBi/IBii boundary.

Subunit IBii (230.9–292.9 mbsf) is characterized by continuous fluctuations around a resistivity baseline of ~1.5 Ωm, reaching highs of >3.0 Ωm and lows of <0.8 Ωm. Toward the base of Subunit IBii (276.4–292.9 mbsf) there is a shift to more resistivity spikes with fewer low-value spikes, which corresponds to an increase in bedding dip (Fig. F4). The base of Subunit IBii (292.9 mbsf) is below a large resistivity spike (~2 Ωm), where resistivity drops to ~1.3 Ωm.

Through Subunit IBiii (292.9–350.0 mbsf) resistivity exhibits a lower baseline (~1.3 Ωm) and less frequent fluctuations than the overlying intervals (Fig. F6). Between 293 and 329 mbsf, resistivity gradually increases from 1.3 to 1.6 Ωm. At 330.6 mbsf, resistivity increases sharply to ~2.2 Ωm, maintaining higher values to 334.98 mbsf, where it drops sharply back to the ~1.3 Ωm baseline. From 334.98 mbsf to the base of the hole (350 mbsf), resistivity remains ~1.3 Ωm, with local spikes to higher values (~2.3 Ωm).

Resistivity image analysis

Shallow, medium, and deep button resistivities were used to generate both statically and dynamically processed images (see “Logging while drilling” in the “Methods” chapter [Strasser et al., 2014a]). Bedding, fractures, faults, and breakouts were interpreted. In the absence of a caliper measurement, the bit diameter was used as the borehole diameter and assumed to be constant (see “Logging while drilling” in the “Methods” chapter [Strasser et al., 2014a]).

Bedding and fractures

Overall, bedding dips were fairly constant throughout the drilled section, exhibiting a consistent shallow (0°–25°) dip angle (Fig. F7), with the exception of two intervals (81–83 and 127–168 mbsf) (Fig. F4) that correlate with MTDs observed in core from Hole C0018A (Expedition 333 Scientists, 2012b). Bedding dip direction ranges from west-northwest to south-southeast but does not exhibit a consistent change with depth. The observed shallow dip angle is in good agreement with core observations made in Hole C0018A (Expedition 333 Scientists, 2012b), although the dip direction is more variable in the images.

Abundant fractures and faults primarily occur in the upper section (Subunit IA), with only local moderate–high angle (50°–60°) faults observed below 179.8 mbsf (Fig. F4). There is little consistency in the dip angle or direction of the identified faults and fractures (Fig. F7). The highest abundance of faults and fractures occurs in two intervals (61.5–83 and 127–176 mbsf) and are concentrated in the area of high-angle bedding (Fig. F4).

The two identified intervals of irregularly oriented high-angle bedding and abundant fractures/faults exhibit a chaotic nature in the resistivity images (Fig. F8) and are interpreted to be chaotic deposits, probably associated with MTDs. The intervals over which the images exhibit this chaotic nature are consistent with the depths of chaotic bedding and slump folds observed in core from Hole C0018A (Expedition 333 Scientists, 2012b). In Hole C0018A, six MTDs (1–6) were identified between the seafloor and 190 mbsf (Expedition 333 Scientists, 2012b). In Hole C0018B, there is clear evidence for two MTDs: MTD A at 61.5–83.0 mbsf and MTD B at 127.0–176.0 mbsf. The top and base of MTDs A and B are based primarily on the change from high-angle, randomly oriented bedding back to consistently low-angle bedding. MTD B most closely corresponds to MTD 6 defined in core from Hole C0018A (Expedition 333 Scientists, 2012b), but the base cannot be easily distinguished in the resistivity images. It is possible that a region of very conductive horizons (Fig. F9) corresponds to the pyrite-rich zones observed at the base of MTD 6 in core from Hole C0018A (Expedition 333 Scientists, 2012b). The depth range of MTD A encompasses core-defined MTDs 3, 4, and 5 from Hole C0018A (Expedition 333 Scientists, 2012b); however, given the low contrast between the material in the MTDs and the background hemipelagic sediment, the resistivity images do not allow separating individual MTDs within this interval. Other core-defined MTDs (1 and 2) from Hole C0018A are at shallower depths than covered by the resistivity images, but a possible basal surface of a disturbed interval observed at 48.0 mbsf, just below the start of the images, suggests the presence of another MTD.

Borehole breakouts

Borehole breakout analysis was performed to assess the orientation of the maximum horizontal stress direction within the borehole. Breakouts appear as two vertical conductive zones, with 180° of separation between them. Breakouts typically form perpendicular to the direction of maximum horizontal stress (S_HMAX) in the borehole. Breakout width can be related to the horizontal differential stress (e.g., Chang et al., 2010). In Hole C0018B, the depth intervals immediately following a pipe connection, indicated by time-after-bit measurements, show significantly wider breakouts than the rest of the formation, particularly in the upper section (Fig. F4). This could be due to time-dependent evolution of pore pressure (e.g., Moore et al., 2011) or changes in annular pressure during pipe connections. These ideas will be explored further in postcruise research.

In Hole C0018B, few breakouts are observed in the upper section, with the exception of those breakouts immediately following pipe connections. Breakout occurrence increases with depth, and breakouts are most prevalent in the lower section of logging Subunit IB (Fig. F4). Analyses indicate a mean azimuth of 51° and, therefore, an azimuth of 141° for S_HMAX. This northwest–southeast trend is roughly parallel to the convergent direction of the Philippine Sea plate and to the dominant S_HMAX determined at previous 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]) and calculated seawater electrical resistivity using the temperature profile that was estimated for Site C0018 during Expedition 333. The temperature at the seafloor is estimated at 1.48°C, with an average thermal gradient of 63°C/km (Expedition 333 Scientists, 2012b). Archie’s law parameters were found by fitting the Archie equation to resistivity measurements (conducted at 2 kHz) and moisture and density (MAD)-derived porosity measurements taken during Expedition 333 on sediment recovered from Hole C0018A (Expedition 333 Scientists, 2012b). The parameters that best fit the data are a = 1.72 and m = 1.68, with a coefficient of determination (R²) of 0.50. Archie’s law with the parameters estimated based on density and resistivity logs at IODP Site C0002 during Expedition 314, a = 1 and m = 2.4 (Expedition 314 Scientists, 2009; see the “Site C0002” chapter [Strasser et al., 2014b]), is shown for comparison in Figure F10. Note that R² = 0.29 for the Site C0002 parameters. The parameters estimated from the Hole C0018A data are applied to the entire drilled section of Hole C0018B; thus, the values of a and m do not account for lithologic variations with depth and may be affected by the presence of the MTDs. Additionally, a value of a ≠ 1 may indicate that a clayey formation rock matrix is conducting electricity (Glover et al., 2000). In this case, Archie’s law would not be appropriate to describe the electrical resistivity and MAD-derived porosity measurements taken during Expedition 333. Bulk density was estimated from the resistivity-derived porosity using a grain density (ρ_g) value of 2.66 g/cm³. This grain density value is an average, with a standard deviation of 0.10 g/cm³, of all of the MAD-derived grain densities measured during Expedition 333 (Expedition 333 Scientists, 2012b).

The resistivity-derived porosity and bulk density depth trends are shown in Figure F4. Porosity values are very high in the upper 20 m of the hole, decreasing from >95% at the seafloor to 73% at 20 mbsf; these large values reflect bit resistivity measurements that are very close (less than a factor of 2 larger) to the estimated values of seawater resistivity. Below 20 mbsf, there is some scatter in the resistivity-derived porosity values, but porosity decreases in a generally linear fashion to 51% at the base of Subunit IA (0–179.8 mbsf). There are several prominent spikes in the porosity values within Subunit IBi (179.8–230.9 mbsf). Within the upper 3 m of Subunit IBi, porosity spikes to 93% (179.8 mbsf), 72% (181.8 mbsf), and 63% (182.7 mbsf), decreasing to ~50%. There are spikes to 66% (191.6 mbsf) and 63% (206.2 mbsf), below which the porosity decreases to 43% at the base of Subunit IBi (230.9 mbsf). Porosity increases to 48% near the top of Subunit IBii (230.9–292.9 mbsf). Within Subunit IBii, porosity values are scattered but generally decrease to 40% at the base of the unit (292.9 mbsf). Within Subunit IBiii, porosity increases to 47% (293.4 mbsf) before decreasing to 37% at 334.7 mbsf. Resistivity-derived porosity increases to 44% at 337.3 mbsf and remains relatively constant through to the base of Hole C0018B (350.0 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. F4). Generally, bulk density increases from ~1 g/cm³ at the seafloor to 1.8 g/cm³ at the base of Subunit IA (179.8 mbsf). Bulk density values decrease to 1.1 g/cm³ at the top of Subunit IBi before generally increasing to 1.9 g/cm³ at the base of Subunit IBi (230.9 mbsf). Several large decreases in the estimated bulk density values within Subunit IBi are coincident with the porosity spikes noted above. Bulk density generally increases to 2.0 g/cm³ at the base of Subunit IBii (292.2 mbsf) then drops to 1.9 g/cm³ near the top of Subunit IBiii before increasing to 2.1 g/cm³ at 334.7 mbsf, decreasing to 1.95 g/cm³ at 337.3 mbsf, and remaining generally constant to the bottom of the hole (350.0 mbsf).

Figure F11 shows the resistivity-derived porosity and bulk density logs plotted for comparison along with MAD-derived measurements taken during Expedition 333 (Expedition 333 Scientists, 2012b). The resistivity-derived porosity values are noticeably higher than the MAD-derived values in the upper 80 m of Site C0018. The offset Δϕ = ϕ_MAD – ϕ_resistivity is greatest at the seafloor (Δϕ = –0.4) but decreases steadily, becoming ~0 by 80 mbsf. The resistivity- and MAD-derived values maintain a good agreement from 80 mbsf to the bottom of Hole C0018B. For further comparison, a resistivity-derived porosity log using the Archie parameters from Site C0002 is also shown. This porosity log maintains a good agreement with the MAD-derived porosity measurements throughout the hole, although the Archie parameters from Site C0002 produce a poor fit to the relationship between MAD porosity and resistivity that were measured on cores during Expedition 333 in Hole C0018A (Fig. F10 and discussion above).

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