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doi:10.2204/iodp.proc.343343T.103.2013

Logging while drilling

Data and log quality

Hole C0019B

Available data

Hole C0019B was drilled with LWD/MWD tools. Unfortunately, the proVISION tool was not deployed as planned because of a mechanical issue; therefore, nuclear magnetic resonance measurements were not obtained. MWD data were transmitted in real time with a limited set of LWD data (see Table T2 in the “Methods” chapter [Expedition 343/343T Scientists, 2013]) through the drilling fluid telemetry system. When the LWD tools were recovered on the rig floor, memory data were successfully downloaded and processed according to the data flow described in “Shipboard data flow and quality check” and Figure F5, both in the “Methods” chapter (Expedition 343/343T Scientists, 2013). A list of available LWD data is given in Table T2 in the “Methods” chapter (Expedition 343/343T Scientists, 2013).

Depth shift

The mudline (seafloor) was identified from the midpoint of the first break in the gamma ray and resistivity logs (Fig. F2). A mudline was picked at 6918 meters below rig floor (mbrf) in Hole C0019B. All LWD data were converted to LWD depth below seafloor (LSF). In this section, LSF is referred to as meters below seafloor, based on the mudline depth.

Logging data quality

Figure F3 shows the overview of the log quality control (QC) logs. The logs indicate that the borehole was drilled smoothly and no significant vertical tool shocks were detected by the sensors in the tools. Minimal variations in downhole rotations per minute (rpm) indicate that the bit rotation was generally stable. Initial target ROP was <55, <45, and ~30 m/h for 0–200, 200–500, and 800 mbsf to TD, respectively. The actual ROP fluctuated throughout the hole and frequently exceeded the target ROP. Short depth intervals of missing resistivity-at-the-bit (RAB) electrical images resulted when the ROP exceeded 50 m/h. Although the overall quality of the image data is good, image data are missing in a total of 15.2% of the logged section; of this, 8.8% is in the uppermost 237 m of the borehole. Time-after-bit (TAB) logs for deep and shallow button resistivity measurements are shown in Figure F3. Most of the intervals were measured immediately after the borehole was drilled. However, owing to periods of long pipe connection operations, TAB values are exceptionally high over certain intervals: 416–419, 564–567, and 717–720 mbsf, where borehole breakouts are widened. Apparent abrupt temperature deviations also occur at two of the high TAB intervals. The geoVISION tool measured borehole azimuth and inclination. Hole deviation is ~2° to the north-northwest to ~400 mbsf, and then hole inclination increases steadily to 8° (toward the north). The annular pressure profile shows no indication of inflow from the formation into the borehole or obstruction of the borehole because of borehole wall collapse, which would have resulted in an increase in pressure. Also, there is no indication of pressure decreases associated with loss of circulation because of permeable formations or faults. Repeat measurements were taken in three areas of interest while pulling out of the hole. These intervals were between 335 and 377, 682 and 727, and 807 and 847 mbsf. The LWD resistivity data showed some offset compared to the original downpass. This indicates some degradation and enlarging of the borehole over time. Inclination of the borehole also affected the quality of the repeat RAB electrical images, which exhibit an electrical artifact likely due to the tool being pressed against the borehole wall.

Hole C0019C

Available data

Hole C0019C was drilled with LWD/MWD tools. Unfortunately, because of an engineering problem the BHA, including LWD/MWD tools (geoVISION/arcVISION/TeleScope), was lost at ~120 mbsf. Therefore, only real-time data above 120 mbsf were obtained. MWD data and a limited set of LWD data (see Table T2 in the “Methods” chapter [Expedition 343/343T Scientists, 2013]) were transmitted in real time through the drilling fluid telemetry system right up until the moment of BHA disconnection. A list of available real-time data is given in Table T2 in the “Methods” chapter (Expedition 343/343T Scientists, 2013).

Depth shift

The mudline (seafloor) was determined from the drillers depth at 6928.5 mbrf in Hole C0019C. All data were converted to LWD depth below seafloor (LSF) (also referred to as mbsf) based on the mudline depth.

Logging data quality

Figure F4 shows the overview of the QC logs. The logs indicate that the borehole was drilled smoothly. Variations in downhole rotations per minute were small, indicating that the bit rotation was quite stable. The annular pressure profile shows no indication of inflow from the formation into the borehole or obstruction of the borehole because of borehole wall collapse. Also, there is no indication of pressure decreases associated with loss of circulation because of permeable formations or faults. Drilling parameters and MWD data show that the disconnection of the BHA was not a result of any abnormal conditions of the borehole or the response of the formations being drilled.

Log characterization and lithologic interpretation

Hole C0019B

Hole C0019B log units were characterized from visual inspection of gamma ray (primarily) and resistivity log responses (Fig. F5). Resistivity images were used to identify finer scale characteristics within the units. Four primary log units were defined from the variability of log responses (Figs. F5, F6, F7). Overall in the upper 820 m of the borehole, both gamma ray intensity and resistivity increase with depth. In the gamma ray log, this trend is broken by significant lows in the data (~20 gAPI lower) that occur at 168 and 535 mbsf. Resistivity in the upper 820 m of Hole C0019B ranges from 0.4 to 2.7 Ωm, and gamma ray values range from 17 to 71 gAPI.

Log Unit I (0–194.1 mbsf) is characterized by relatively low gamma ray values ranging from 17.2 to 59.6 gAPI (mean = 40.6 gAPI). This unit is also defined by a significant separation in the shallow and deep resistivity curves, with average values of 0.93 and 1.14 Ωm, respectively, which suggests that either the formation is more permeable and unconsolidated than the units beneath or the borehole is more washed out in this section. RAB images collected in this section are of lower quality because of high ROP. Therefore, because of the limited information, it is very difficult to assign an exact lithology to this unit. However, on the basis of previously drilled sites in the Japan Trench (Sacks, Suyehiro, Acton, et al., 2000), we suggest that this could relate to a less compacted siliceous (diatomaceous) mudstone (compared to the unit beneath).

Log Subunit IIa (194.1–537.2 mbsf) is defined as an interval of relatively consistent gamma ray intensity with values ranging from 31.5 to 66.5 gAPI (average = 48.7 gAPI). Resistivity values range from 0.40 to 2.32 Ωm (average = 1.36 Ωm). There appear to be two main intervals where resistivity increases with depth (an interval between the top of Subunit IIa and 363 mbsf, and an interval from ~370 to 443 mbsf), separated by a step function. Additionally, there are two negative excursions at ~340 and 370 mbsf that may relate to structural features. RAB images also show the main trends noted above as zones transitioning from darker, more conductive layers to lighter, more resistive bands. Borehole breakouts visible on RAB images disappear toward the base of this subunit (at ~470 mbsf) (see “Structural geology and geomechanics”).

Log Subunit IIb (537.2–820.6 mbsf) is defined at its top by an increase in gamma ray values and the reappearance of borehole breakouts in RAB images. Gamma ray values range from 21.9 to 71.1 gAPI (average = 51.6 gAPI). Overall, gamma ray intensity increases with depth in this subunit. Additionally, from 670 mbsf downhole, gamma ray values become much more varied and fluctuating (10–30 m scale fluctuations), stabilizing again at ~790 mbsf. In line with this is a steadily increasing trend in resistivity, with values ranging from 0.41 to 2.78 Ωm. Within this subunit there is further evidence of cyclicity in the resistivity log response. Additionally, similar to Subunit IIa the occurrence of borehole breakouts becomes significantly less common near the base of this subunit (from 730 mbsf). There is also a significant conductive feature at ~720 mbsf, shown as a large negative excursion in the standard logs and a dark band in RAB images. A highly resistive band (2 m thick) at the base of this unit may correspond to another zone of interest with regard to structure and faulting (see “Structural geology and geomechanics”).

Comparing the gamma ray and resistivity log responses from Hole C0019B log Unit II to neighboring Ocean Drilling Program (ODP) Leg 186 Sites 1150 and 1151 ~300 km away (Sacks, Suyehiro, Acton, et al., 2000), similar values are seen in diatomaceous muds that were recovered and logged at these sites (and others including Deep Sea Drilling Project [DSDP] Leg 56 Sites 436 and 434; Shipboard Scientific Party, 1980); therefore, we may have similar lithologies in Hole C0019E over this interval. The overall pattern of increasing resistivity is in keeping with a conventional compaction trend; however, much smaller scale cycles are present in the resistivity data (see “Physical properties and hydrogeology”).

Log Unit III (820.6–835.9 mbsf) is defined by a significant increase in the gamma ray intensity (to ~90 gAPI). Gamma ray values remain high for 16 m before dropping sharply to ~20 gAPI. The high gamma ray values, ranging from 47.5 to 103.3 gAPI, define log Unit III (820–836 mbsf). Resistivity exhibits a decreasing trend over this unit. Such elevated gamma ray values would normally indicate a clay-rich unit (potentially a similar lithologic unit to the clay recovered from Site 436; Shipboard Scientific Party, 1980) at these depths in the borehole. RAB images also indicate a more conductive area compared to log Unit II.

Log Unit IV (835.9–850 mbsf) is characterized by a sharp decrease in gamma ray values and increase in resistivity log responses. Overall, this log unit is defined by low gamma ray values and high resistivity. Gamma ray values range from 14.6 to 45.3 gAPI (average = 25.8 gAPI) and resistivity ranges from 0.89 to 8.61 Ωm. RAB images primarily show a region of very high resistivity in layers and patches throughout this unit. A number of sharp peaks in resistivity reach a maximum of 8.61 Ωm below which resistivity decreases. With the exception of the very upper 10 m of this unit, gamma ray values are the lowest measured in the borehole. Such values (low gamma ray and high resistivity) are consistent with lithologies such as chert. The varying resistivity values shown in the log curves and RAB images may indicate some interbedding of chert and a more conductive material. Cherts were also observed below a brown clay at Site 436 (Shipboard Scientific Party, 1980).

In a cross-plot of gamma ray values and resistivity (Fig. F6) log Units I and II can be identified as the main grouping of points with low resistivity and low to intermediate gamma ray values. Very little difference between the two units can be seen in this plot. Log Units III and IV are easily distinguished from this main grouping. The high gamma ray values and low resistivity of log Unit III is visible on the right-hand side of the plot. Unit IV centers around 20 gAPI as resistivity increases. The log units are more easily differentiated by plotting gamma ray and resistivity data ranges on a box and whisker plot (Fig. F7). As previously stated, these units were primarily defined using the gamma ray data.

Hole C0019C

As noted above in “Data and log quality,” only real-time data were acquired in Hole C0019C. These data include total gamma radiation, annular pressure, annular temperature, ring resistivity, bit resistivity, and average shallow and deep button resistivity and were measured between ~51 and 120 mbsf. Gamma ray values range between 35 and 60 gAPI. The values show no particular trend over this short interval (Fig. F8); average gamma ray values slightly decrease from 70 mbsf to the base of the section. Resistivity values range from 0.4 to 1.5 Ωm with the exception of bit resistivity, which has values up to 22 Ωm; however, this exception relates to an anomalous peak in the data. Overall, the resistivity curves show very little significant variation downhole. However, two negative excursions occur at ~66 and ~77 mbsf (indicating more conductive zones), with the sharpest and largest being the latter (between 73 and 81 mbsf). Finally, annular temperature increases from 3° to 4°C at ~62.5 mbsf.

The data ranges for both gamma ray and resistivity are very similar to those encountered within log Unit I of Hole C0019B in the same depth range. However, the conductive signal observed at ~77 mbsf is not present at a similar shallow level in the resistivity data collected in the previously logged hole.

Physical properties and hydrogeology

Resistivity logs

Figure F9 shows a comparison of the gamma ray log, five different resistivity measurements (ring; bit; and shallow, medium, and deep button resistivity), and the difference between the shallow and deep button resistivity. Superposition of the deep, medium, and shallow button resistivity shows generally good agreement among the logs. The logs show overall increasing resistivity values from the seafloor to the base of log Unit II; smaller scale deviations from this trend will be discussed in this text. A sharp increase in resistivity at the base of log Unit II is followed by a gradual decrease through log Unit III. There is another sharp increase in resistivity at the top of log Unit IV, which then follows a series of resistivity peaks and decreasing resistivity trends to the bottom of the hole.

Systematic comparisons of the resistivity logs were made using cross-correlations. Figure F10 shows cross-plots of shallow and deep button resistivity as well as bit and ring resistivity. Ring resistivity is generally greater than bit resistivity, and this is most evident in log Unit IV. The difference between the two measurements is ~0–5.6 Ωm for all resistivity values. Comparison of shallow and deep button measurements indicates that deep resistivity is generally higher than shallow resistivity. Zones where deep resistivity is significantly higher than shallow resistivity tend to correlate with zones of lower gamma ray values.

In log Unit I, the overall resistivity values increase from ~0.6 to 1.3 Ωm with noticeable ~100 m cycles of an increasing trend followed by a decreasing trend (arrows in Fig. F9). The increasing trends probably correspond to the increased compaction of sediments, whereas the decreasing trends may correspond to gradual changes in lithology. In log Unit I, the deep resistivity button values are significantly higher than those for the shallow button, possibly indicating washouts or invasion of the formation by drilling fluid. The latter case could indicate more permeable sediments. The difference between shallow and deep resistivity decreases at the log Unit I/II boundary. In log Unit I, the average difference between the two measurements is 0.22 Ωm, whereas the average difference in log Unit II is 0.16 Ωm.

In log Unit II, resistivity continues to gradually increase from ~1.3 to 1.6 Ωm, but with the presence of ~10 to ~100 m cycles of increasing trends, which are generally followed by more rapid decreasing trends (stepped/sawtooth trends). Most of the increasing trends are bounded by conductive features, commonly accompanied by upward increasing resistive intervals several meters above them. The tops of these packages, represented by a conductive peak and higher resistivity interval, are found at the top of the unit at 194–210, 260–270, 362–371, 443–457, and 523–532 mbsf. Between 640 and 720 mbsf resistivity decreases from ~1.6 to ~1.3 Ωm. Between 688 and 701 mbsf a series of lower resistivity peaks are observed. At 720 mbsf a low resistivity peak of 0.88 Ωm is observed, followed by an increasing trend of resistivity to the base of the unit. Smaller scale features can be found at other depths within log Unit II. The patterns described above could imply sediment packages of different lithologies, an overall homogeneous sediment unit cut by thrust faults (producing repeated sediment packages), or a combination of these. At its base, log Unit II includes an interval of higher resistivity (~2.14 Ωm) from 818 to 820 mbsf.

At the top of log Unit III is a 1 m wide interval of low resistivity before a higher resistivity value interval from ~821 to ~827 mbsf. Gamma radiation increases sharply at the top of this interval, followed by a gradual decrease in resistivity from 2.4 to 1.1 Ωm over the ~827 to ~836 mbsf interval (Figs. F9, F11).

The top of log Unit IV is marked by a sharp increase in resistivity to 4.8 Ωm, and then a decrease to 0.9 Ωm (Figs. F9, F11). Resistivity jumps to 7.9 Ωm at 840.3 mbsf before following a gradual decreasing trend to the base of the hole. Resistivity peaks that are a few meters thick probably correspond to chert layers (see “Log characterization and lithologic interpretation”). The shallow, medium, and deep button resistivities become very noisy through log Units III and IV, and the deep and shallow signals are significantly different.

Estimation of porosity from ring resistivity measurements

We calculated seawater electrical resistivity using an estimated temperature profile and used it to evaluate the formation factor from the ring resistivity (see “Logging while drilling” in the “Methods” chapter [Expedition 343/343T Scientists, 2013]). We estimated a temperature profile in the formation using a surface heat flow of 45 mW/m² and constant thermal conductivity. We assume a 1.3°C bottom water temperature and a steady-state conductive temperature profile. The estimated temperature at the bottom of the hole is 36°C (Fig. F12). Formation factors were then converted to estimated porosity values using Archie’s law (Archie, 1942). Archie’s law parameters (a = 1; m = 2.7) were constrained after core was recovered by cross-plotting discrete resistivity and porosity measurements.

Estimated porosity decreases gradually from ~70% at the top of the hole to ~43% at the top of log Unit II (Fig. F12). Log Unit II is marked by cyclic trends of decreasing and increasing porosity. If we assume that porosity decreases exponentially with depth because of vertical compaction (e.g., Athy, 1930), the observed cyclic trends imply the appearance of different compaction trends. These different patterns may reflect packages of sediments with different chemical composition; alternatively, they may reflect true porosity differences related to variations in stress history, local changes in lithology, and/or the presence of fractures or faults.

At the top of log Unit III (820–823 mbsf), estimated porosity stays around 41% whereas the gamma ray values increase, possibly indicating higher clay content. Porosity sharply increases to 44% at 827 mbsf and then gradually increases to 47% at the base of the unit. The estimated porosity of the resistive layers in log Unit IV is ~20%–30%. At the base of this unit porosity is ~36%.

The estimated density log mirrors the porosity log, with values ranging from 1.55 to 1.98 g/cm³ in the layers (probably chert) of Unit IV (Fig. F12).

Structural geology and geomechanics

Structural geology

Orientations of bedding and fractures

We made a major effort to interpret bedding (Fig. F13) and fractures accurately from borehole resistivity images. We utilized three simultaneous observers. Each pick was critically examined by this team, with classification as bed or fracture by consensus.

In the upper 275 mbsf, bedding orientations of the Hole C0019B resistivity images tend to be shallow, averaging 27° (Fig. F14). In the section from 275 to 820 mbsf, bedding dips average 57°. Finally, the section below 820 mbsf showed an average dip of 22°. Fractures are rare in the upper 300 mbsf and common from 300 mbsf to deeper than 800 mbsf. A zone of enhanced fracturing occurs at ~720 mbsf.

Three structural domains are defined on the basis of bedding orientation (dip) distributions: Domain 1 (upper frontal wedge, 0–275 mbsf), Domain 2 (frontal wedge core, 276–820 mbsf), and Domain 3 (frontal wedge base, 820 mbsf to base of hole). Note that Domain 1 shows sparse fractures/faults, whereas fractures/faults are more numerous in Domains 2 and 3 (Fig. F14). Domains 1 and 2 have logging signatures similar to the siliceous mudstone that is widespread along the Japan Trench slope (see “Log characterization and lithologic interpretation”). Conversely, the sediments of Domain 3 show large variations in gamma ray values and resistivity and could be interpreted as a clay-chert sequence, going downsection.

On a stereographic projection, the poles to bedding define a girdle that trends 300° (Fig. F15A). This girdle of bedding suggests cylindrical folding with a mean axial trend of 030°, which is ~20° northeast of a line parallel to the regional trench slope. This mean axial direction is nearly normal to the convergence direction of 292° at 93 mm/y (Argus et al., 2011). The stereographic projection of the poles to fractures shows a broader girdle; however, the trend is still to the northwest (Fig. F15B).

Probable major faults

Because the principal goal of Expedition 343 was to instrument the fault that caused the 2011 Tohoku-oki earthquake, there was considerable interest in finding this fault in the drilled section. Numerous fractures (and also some potential faults) were observed in the logged section. However, two zones appear to be the most probable major fault zones:

• Probable fault at 720 mbsf: at 720 mbsf, a high-conductivity zone ~1 m thick is associated with a dip transition from shallow to more steeply dipping beds and an interval of concentrated fracturing (Figs. F16, F17A).

• Probable fault at 820 mbsf: at 820 mbsf, a transition from steep bedding dips to dominantly shallow dips occurs just above the top of a zone of high gamma ray values (Figs. F14, F18). High gamma ray values below 820 mbsf are interpreted as a clay layer characteristic of the incoming oceanic plate of the North Pacific Ocean (Shipboard Scientific Party, 1980). The structural transition resembles that of a décollement and lies in the zone of layered reflectors above the basaltic oceanic crust (Fig. F19). The high gamma ray values decrease downsection, transitioning to a section interpreted as chert (see “Log characterization and lithologic interpretation”).

Geomechanics: borehole breakout analysis

Breakouts occurred over a wide depth range in Hole C0019B (e.g., Figs. F20, F21) and were observed in log Units I and II but were not found in log Units III (clay) or IV (chert). We counted a total of 221 breakouts. The cumulative length of all breakouts reached ~96 m, corresponding to ~11% of penetrated borehole total depth. No drilling-induced tensile fractures were observed in this borehole.

Interestingly, the breakout azimuth distribution appears different in the shallow portion of the hole compared to the deeper part (corresponding to approximately 50–537 and 537–820 mbsf, respectively) (Fig. F21). In the shallower portion, the breakout azimuths systematically change orientation from 060° at 70 mbsf to 180° at 140 mbsf and then change back to 135° from 140 to 200 mbsf. From 200 to 530 mbsf, breakouts are rather sporadic and their azimuths are variable. In the deeper portions of the borehole, breakouts display a strong preferred orientation of 045°.

The mean azimuth of breakouts in the deeper portion of the hole is 49° (or 229°) and the standard deviation is 23°. The mean azimuth was not calculated for the shallow portion of the hole because of the apparent progressive change in orientation. Because the borehole breakouts occur at the same azimuth as the minimum horizontal stress (S_hmin) but perpendicular with the maximum horizontal stress (S_Hmax) (see the “Methods” chapter [Expedition 343/343T Scientists, 2013]), the S_Hmax orientation can be interpreted as 139° ± 23° (or 319° ± 23°) in the deeper part of Hole C0019B. The stress orientation in the shallower portion is not constant. We observe a progressive change in the breakout azimuth orientation to 140 mbsf, here, where this trend terminates there is likely a discontinuity, maybe a fault. This is supported by a conductive peak in the resistivity logs. Changes in bedding dips are also consistent with the presence of a discontinuity at this depth. Breakout width ranged between ~30° and 130° over the entire hole with a mean value of ~67°.

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