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doi:10.2204/iodp.proc.348.103.2015

Logging

Log data acquisition

LWD data included NGR, electrical resistivity logs, electrical resistivity images, sonic velocity, and sonic caliper logs. These data were collected together with MWD data from 859.5 to 2329.3 mbsf (2827.0– 4296.8 m BRT) in Hole C0002N (Table T38) and from 2162.5 to 3058.5 mbsf (4130–5026 m BRT) in Hole C0002P (Table T39). Detailed description of the tools and the bottom-hole assembly (BHA) configuration are provided in “Logging” in the “Methods” chapter (Tobin et al., 2015).

In Hole C0002N, LWD data were collected in two runs separated by a WOW event (Table T40; Fig. F111A). Data from the two runs overlapped between 1962.6 and 2008.5 mbsf (between 3930.2 and 3976 m BRT); the data from the earlier run were chosen over the later run when merging the data. The target ROP was set to an average of 40 m/h to optimize the quality of the LWD data collected during drilling. ROP was initially low, <10 m/h, from 872.5 to ~960 mbsf, as the hole drilled through the cemented section of Hole C0002F to kick-off Hole C0002N. Otherwise, ROP was maintained mainly between 30 and 50 m/h to the bottom of logged data in Hole C0002N (2329.3 mbsf; Fig. F111A). Annular pressure measured during drilling shows that the ECD increased from ~1.145 to ~1.18 g/cm³ with depth.

In Hole C0002P, LWD data were collected after coring from 2163 to 2218.5 mbsf (4130.5–4186 m BRT) (Fig. F111B). The cored interval was logged as the borehole was reamed and enlarged from 10 5/8 to 12¼ inches in diameter, and then the formation below was logged as the borehole was newly drilled. The target ROP was 30 and 40 m/h in the cored and newly drilled section, respectively. Throughout Hole C0002P, ROP fluctuated between 5 and 40 m/h but was maintained mostly between 10 and 30 m/h with an average of ~18 m/h. Downhole annular pressure data show that ECD initially increased steadily with depth from ~1.31 to 1.33 g/cm³ to ~2550 mbsf but stabilized within 1.32–1.33 g/cm³ during the rest of the drilling.

Data quality

Overall quality of the data is satisfactory over the logged section of the well. However, several quality issues need to be taken into account in order to properly interpret the processed log data (Table T40). Exposure time is the time it takes for the LWD tools to reach the drilled formation after being penetrated by the drill bit and was provided in the log data relative to the position of the electromagnetic wave resistivity tools. Exposure times were typically between 30 min and 2 h during normal drilling operations but were as high as 50 h when incidents such as WOW or extended mud circulation occurred. Long exposure times cause degradation of the borehole condition (i.e., caving, drilling-mud invasion, etc.) before the logging tools measure the petrophysical properties of the formation; therefore, data quality can be compromised at such depth ranges.

There were several periods when drilling Hole C0002N was halted temporarily due to WOW or borehole condition concerns, which caused some intervals of the LWD well logs to be measured after long exposure times. Also, the absolute magnitudes of the NGR and shallow resistivity data do not properly reflect the formation petrophysical properties in Hole C0002N despite the mud and borehole size corrections, which corrected for the discrepancy between the LWD 12¼ inch calibration and the 17 inch Hole C0002N bore size. In Hole C0002P, resistivity data in the cored interval show possible signs of slight mud filtrate invasion and/or wellbore failure (Fig. F111B).

There were four major occurrences of long exposure times during the drilling of Hole C0002N (Fig. F111; Table T40). Those drilling intervals between 1205–1221 and 2022–2038 mbsf are both related to periods of extended mud circulation for borehole treatments, whereas those occurring at 1662–1678 and 1992–2008 mbsf were caused by temporary termination of drilling activities due to WOW. Resistivity log data at these depth ranges are both noisy and anomalously low (Fig. F111A), which indicates that the borehole conditions were worse compared to the formation logged both above and below these intervals. As a result, these data are not suitable for interpretation. Evidence of compromised borehole condition at these intervals also suggests that the cuttings collected right below the long-exposure time intervals may include more caving material when compared to cuttings collected during normal drilling operations.

Because the borehole diameter (17 inches) in Hole C0002N was larger than the standard borehole size range covered by the available LWD tools used during the drilling, there was more drilling fluid present in the annulus between the LWD tools and the formation. The effect of having more drilling mud present was corrected for by proprietary algorithms, but these may not always work optimally outside of the standard specification range of the tools (i.e., dual gamma ray [DGR] range is 12¼ inches and electromagnetic wave resistivity [EWR]-PHASE4 range is 10½ to 14¾ inches) (Fig. F112). As a result, the final corrected gamma ray data from Hole C0002N were, on average, 10–15 gAPI units lower than those measured from Hole C0002F at the same depth range (Fig. F113). It is also noted that the third-party contractors and logging tools were different between the two holes. These differences in tool calibration standards may also contribute to this discrepancy. Therefore, comparison of NGR data between boreholes is only possible in a qualitative sense. As for the resistivity data, we observe that the shallower measurements (e.g., 9 and 15 inch resistivity) show especially low resistivity values. This is likely due to the fact that the shallow measurements are sampling the highly conductive drilling fluid present in the large gap between the tool and the formation. Therefore, we only considered the two deep measurements (e.g., 27 and 39 inch resistivity) to represent the formation resistivity in our interpretations.

In Hole C0002P, exposure times are consistently low because of the steady drilling operation, which only stopped twice, once for 5.5 h during a scheduled wiper trip operation at 2601.5 mbsf and once during a >66 h interval between coring from 2163 to 2218.5 mbsf and subsequent logging of the same interval with the LWD BHA (Table T40). However the separation between the two resistivity curves is particularly large at the cored interval and largest between 2200 and 2208 mbsf where fault-zone rocks were recovered in Core 348-C0002P-5R, as shown by the ratio of the deepest to shallowest resistivity data (e.g., 48 inch data divided by 16 inch data; Fig. F111B). This indicates possible mud filtrate invasion or wellbore failure in this interval. Considering that the time between the last coring run (Core 6R) and the start of reaming was >66 h, it is possible that the formation was affected by invasion or wellbore failure to a deeper extent than was cut by the reaming operation. Nonetheless, the ratio of deep to shallow resistivity measurements is small compared to that observed in Hole C0002N (i.e., up to 5.5 at long exposure-time intervals), thus data quality was not significantly affected.

The sonic caliper data show that the borehole diameter is generally stable throughout the wellbore, thus borehole collapse does not appear to have been a severe issue for log data acquisition (Fig. F111B). The mean value of the average ellipse diameter is 12.42 inches along the well, indicating a slight increase in borehole diameter. Borehole enlargement is most pronounced at the top section of the borehole between ~2135 and 2210 mbsf, which corresponds to the depth ranges where the borehole was already cut by previous drilling/coring, then reamed to a larger diameter where the LWD data were collected. Borehole diameter is larger in this interval probably because of the longer exposure time (>66 h) of the formation before the interval was imaged by the azimuthal focused resistivity (AFR) tool. The eccentricity of the borehole shape, defined by the ratio of the maximum to minimum ellipse diameter, is also relatively large in this reamed section. If we ignore the occasional spikes in the eccentricity data, eccentricity is 1.04–1.2 in the reamed section, whereas it is generally <1.05 at lower depths.

The quality of the AFR tool resistivity image log was good. In the data collected by the high-resolution sensor, 36.8% of the data were missing due to occasional instantaneous high ROP and rotations per minute during drilling and the likely associated stick-slip movements of the drilling bit. With standard filtering and interpolation procedures, a high-quality smoothed image of the borehole was produced.

Hole C0002N

Logging data characterization and interpretations

The near-seafloor portion of Hole C0002F, and thus Hole C0002N, was drilled during Expedition 326 in 2010; however, logging data were not collected. Log Units I–III were identified during previous Expeditions 314 and 332 for Holes C0002A and C0002G (Expedition 314 Scientists, 2009; Expedition 332 Scientists, 2011). Expedition 338 extended Hole C0002F (Strasser et al., 2014b) and identified the bottom section of Unit III. The lowermost section of Unit III is the first unit drilled in Hole C0002N during Expedition 348. The log units presented below for Hole C0002N correlate with Hole C0002F and allow for the full suite of LWD from Hole C0002F to help interpret the geology in Hole C0002N (Figs. F112, F113). Average values of gamma radiation and resistivity are shown in Table T41.

Unit III (from beginning of LWD acquisition at 872.0–915.0 mbsf)

The log Unit II/III boundary was not drilled and therefore is not recorded on LWD logs for Expedition 348. Additionally, correlation with Holes C0002A and C0002G puts the top of Unit III above the kick-off point of Hole C0002N at 865.5 mbsf (2833 m BRT). Logging through Unit III started with drilling out the cement plug placed during Expedition 338 at 860.5 mbsf and cannot be properly characterized by the log response. This resulted in lower gamma ray values near the top of the hole (between 860.5 and 872.0 mbsf; Fig. F113). After the cement plug was drilled, gamma ray values increased by 11 gAPI at 873.5 mbsf. Fluctuation in the log response occurring in ~2 m intervals is thought to be from alternating silty/sandy layers to clay-rich layers, with gamma ray variations up to 9 gAPI. Resistivity in the deep, medium, and shallow logs exhibits similar trends and shows a decrease of ≤0.3 Ωm, which corresponds to the same fluctuation intervals seen in the gamma ray logs reported above.

Unit III has an average gamma ray value of 61.6 gAPI and shows a general trend of gamma ray increase of ~20 gAPI downhole to the basal boundary at 915.0 mbsf, where a decrease of ~25 gAPI occurs. The decrease is interpreted as a change in lithology from a clay-dominated sediment at the base of Unit III to a sandy hemipelagic sediment at the top of Unit IV. This change defines the boundary between Units III and IV and was correlated between the other LWD data from Holes C0002F, C0002A, and C0002G (Strasser et al., 2014b; Expedition 314 Scientists, 2009; Expedition 332 Scientists, 2011). The lowermost 10 m of Unit III is likely a clay-dominated section based on high gamma ray values averaging 73 gAPI with small sections of silty to sandy hemipelagic sediment. However, overall variations in resistivity are modest.

Unit IV (915.0–1656.3 mbsf)

Gamma ray and resistivity data show the largest variability in log Unit IV, with average gamma ray values of 66.5 gAPI (Fig. F113). We define five subunits based on overall trends in log response and comparison with the subunits defined during Expedition 338 in Hole C0002F on the basis of a complete data set of LWD data including images. The base of Unit IV is marked by a sharp increase in the values of average natural radioactivity.

The top of Unit IV (Subunit IVa) shows a gradual increase in gamma ray values. This subunit starts with average gamma ray values of ~61 gAPI and increases to a maximum of ~83 gAPI. The uppermost 13 m of the section contains alternating higher to lower excursions of gamma radioactivity, which could indicate alternating sandy and silty layers every ~2 m. At 928 mbsf, there is an increase to 65 gAPI that we interpret as an indication of increasing clay content. Fluctuations within the log data values are minor through ~974 mbsf (72 gAPI). Then gamma ray values decrease slightly, with changes of up to ~20 gAPI and four additional sequences of increasing–decreasing radioactivity that we interpret as indication of slight increases and decreases in clay. The log-sequence thicknesses are 5–20 m. The uppermost section of Subunit IVa is interpreted as coarsening upward and the lowermost as coarsening downward if higher gamma ray values respond to increasing clay and lower gamma ray values to silty/sandy sediment, as inferred from the lithology recovered and described from cuttings (see “Lithology”). The lower section of Subunit IVa (1032–1036.5 mbsf) is interpreted to consist of interlayered sandy layers or beds 2–4 m thick.

At 1036.5 mbsf (top of Subunit IVb), the gamma radiation changes with depth from downhole decreasing to increasing values. Between 1044 and 1048 mbsf is the largest change in the subunit, of ~40 gAPI. Less variation is seen throughout the rest of Subunit IVb, compatible with a homogeneous lithology with only slight fluctuations in gamma ray values that could be related to small changes in proportion of clay and silt. Resistivity decreases slightly from the bottom to the top of the subunit, which also defines the log properties for this section.

Between 1099.4 and 1360.5 mbsf (Subunit IVc), the average gamma ray values have a slightly higher range when compared to the average for Unit IV (>65 gAPI), and resistivity values are within a range of 2 Ωm. The resistivity logs show a notable feature between 1150 and 1175 mbsf, where there is a steady increase in resistivity values above a sharp decrease. For this depth interval, the gamma ray log exhibits variations of radioactivity that could be interpreted in terms of changing abundance of sand-silt and clay. The high resistivities seen here are interpreted to be a well-cemented sand-rich interval. The gamma ray log below the high-resistivity section shows alternating higher and lower radioactivity at 1 to 5 m intervals. Variations in the gamma radiation within this subunit can be compared with increased sand described in the lithology section (see “Lithology”). A sharp downhole decrease in gamma ray values and downhole increase in resistivity define the boundary between Subunits IVc and IVd.

The abrupt changes in resistivity and gamma ray values observed at the top of Subunit IVd at 1360.5 mbsf can be interpreted in terms of increased sand content, taking into account the lithology end-members defined on cutting samples (see “Lithology”). We identified two sand-rich sections from 1360.5 to 1377.7 mbsf and from 1426 to 1432 mbsf. Also notable is a section with increased radioactivity from 1385 to 1426 mbsf that is interpreted to be more clay rich. The boundary with Subunit IVe is characterized by a sharp downhole increase in gamma ray values followed by a decreasing downhole trend.

From the upper boundary with Subunit IVd to the lower boundary with log Unit V, Subunit IVe (1514.0–1656.3 mbsf) generally shows a downhole decrease in gamma ray response and increase in resistivity. We observed that sand content increases slightly in this section with depth. The highest resistivity of 5.5 Ωm is found at 1639.4 mbsf within the lowermost 20 m section, which can also be correlated with the decrease in gamma ray values. The cause of this response is unknown, but it could be the result of high sand content and/or low porosity. The Unit IV/V boundary at 1656.3 mbsf corresponds to a gamma ray change of ~20 gAPI and a resistivity spike (Figs. F112, F113). Data quality of this region is poor due to WOW (Fig. F111A).

Unit V (1656.3 mbsf to total depth)

Log Unit V is interpreted to be homogeneous and clay rich overall based on the relatively small fluctuation of log responses and the relatively higher gamma ray values (Fig. F112, F113), which is in agreement with the descriptions of core cuttings lithologies (see “Lithology”).

Three subunits are defined based on variations in log responses (Fig. F113). Gamma ray values average ~87 gAPI throughout this unit, with variations of up to 28 gAPI. The Unit IV/V boundary at 1656.3 mbsf is marked by a shift of ~20 gAPI in the gamma ray data and a local spike in resistivity. Distinctive features of Unit V, when compared with Unit IV, have a rather homogeneous log response, increased radioactivity, and show similar average resistivity values with depth within each of the subunits.

Subunit Va is characterized by downhole-increasing gamma ray values until ~1730 mbsf, followed by near-constant values of 80–90 gAPI. Resistivity values decrease moderately toward the base of Subunit Va.

The boundary between Subunits Va and Vb was placed at a sharp downhole decrease in gamma ray and resistivity values at 1942.5 mbsf. From the top of Subunit Vb to the base, resistivity increases with depth and gamma radiation decreases with depth.

A shift in resistivity values and gamma ray variation at 2191.0 mbsf was interpreted as an abrupt change in rock composition. Subunit Vc is characterized by an increase in gamma radioactivity and decrease in the baseline resistivity values relative to Subunit Vb.

Subunits Va and Vb were defined in Hole C0002F and correlate with Hole C0002N (Fig. F113). Subunit Vc was defined in Hole C0002N and was not reached by drilling in Hole C0002F.

Correlation with previous Site C0002 LWD data

We correlate LWD data from Hole C0002F with data from Hole C0002N (Fig. F113). Differences in data can be due to the use of different tools, causing differences in data quality, resolution, and accuracy among logs. The comparison and correlation are based on the measurements that are common to all the holes of Site C0002, which are natural gamma radioactivity and resistivity (Table T41). Resistivity images with bedding and structural interpretation were only available for Holes C0002A and C0002F. Furthermore, because units are highly deformed and steeply dipping at depth (Units IV and V), some depth variations are expected even given the small distances between the holes.

The Unit III/IV boundary was correlated between holes based on a marked downhole shift to decreased gamma ray values. Depth of the boundary only varies by 3.5 m between Holes C0002F and C0002N but varied as much as ~20 m between Holes C0002G and C0002A (Expedition 332 Scientists, 2011). No LWD data recorded the top of Unit III in Holes C0002F and C0002N; therefore, the actual thickness of this unit is poorly constrained. Measurements of bedding in Holes C0002A and C0002F suggest that the boundary between Units III and IV is an unconformity, as discussed for previous expeditions (Expedition 314 Scientists, 2009; Strasser et al., 2014b).

Because Unit IV had more variability in log response, due to the complex geology and relatively variable lithology (from predominantly claystone to silt and sand), it was difficult to correlate units in detail, but overall trends were consistent in all the data sets. The Subunit IVa/IVb boundary was not reached from the total depth in Hole C0002G. However, gamma radiation does show an increase in the top of the section, which is seen in the other holes as well.

Structural analysis derived from both bedding and fracture measurements made on resistivity images from Hole C0002F (Strasser et al., 2014b) shows rather complex geometry and deformation features, with large changes in the orientations of bedding. Furthermore, the Subunit IVa/IVb boundary were interpreted as a change in bedding dip and increase in gamma ray response. The top of Subunit IVc is similar in all holes, with a slight increase in gamma ray values followed by a decrease. Subunit IVd is marked by a decrease in gamma radiation at the top. An increase in sonic velocity observed in Hole C0002F indicates this transition, and Subunit IVe may have lower porosities. The bottom of Subunit IVd is not reached in Hole C0002A. The Subunit IVd/IVe boundary between Holes C0002N and C0002F is offset by 14 m. The boundary is marked by a sharp increase in gamma ray values in both holes followed by a gradual decrease.

The top of log Unit V is a very sharp boundary for both Holes C0002N and C0002F. Both holes are marked by a ~20 gAPI shift in gamma ray values and changes in resistivity response. The boundary is offset by 18.3 m between the two holes. Image logs reveal a heavily deformed section around this boundary with the offset being potentially structurally related. Subunit Va shows a slight increase in gamma ray values followed by a small but sharp decrease at the Subunit Va/Vb boundary. The bottom of Subunit Vb and all of Subunit Vc are not present in Hole C0002F.

Hole C0002P

Logging data characterization and interpretations

Hole C0002P logging data record variations in log responses, which were characterized on the available measurements by inspection of the gamma ray, compressional acoustic velocity, and phase resistivity log responses (Figs. F114, F115, F116, F117). Subunits were defined based on variations in trend lines and log character (Table T42). Depth intervals (>200 m in all cases) displaying similar log responses were designated as log subunits, following criteria established during previous NanTroSEIZE expeditions (Kinoshita, M., Tobin, H., Ashi, J., Kimura, G., Lallemant, S., Screaton, E.J., Curewitz, D., Masago, H., Moe, K.T., and the Expedition 314/315/316 Scientists, 2009; Strasser, Dugan, Kanagawa, Moore, Toczko, Maeda, and the Expedition 338 Scientists, 2014), consistent with what was described for Holes C0002N and Hole C0002F. Resistivity image data acquired with the AFR tool were also processed and interpreted to complete Hole C0002P characterization. A preliminary interpretation of bedding, structures, and stress indicators has been integrated with shipboard interpretation results.

No major or abrupt change was identified in the logging data acquired in Hole C0002P; therefore, changes in trends of log response and values are interpreted as minor changes and designated as subunits. Subunits Vc′–Ve display distinctive trends.

From top to bottom in the section, LWD tools indicate that gamma radiation increases (from 75 to 95 gAPI) and resistivity decreases, followed by an increasing trend in resistivity toward the bottom. The compressional acoustic slowness/velocity shows constant or decreasing velocity from top to bottom. Within shorter intervals, the overall decrease in velocity is punctuated by different steps to higher–lower velocities that can be observed in Figure F114. The most prominent drops in velocity can be identified toward the bottom of Subunit Ve.

The subunits boundaries we define in Hole C0002P are suggested based on the log character and relative values for the same depth intervals and in the absence of significant compositional changes described from cuttings (see “Lithology”). We interpreted the background lithology as hemipelagic silty claystone with relatively high gamma ray values. Variations in the overall trend shown as excursions and spikes of low gamma ray values were observed and interpreted based on the sonic and resistivity data (Fig. F117). Low gamma radiation, low velocity, and low resistivity were interpreted as permeable sand within Subunit Vc′. Low gamma radiation and high resistivity were interpreted as possible ash within Subunit Vd. Low gamma radiation, high velocity, and high resistivity peaks were interpreted as carbonate/silica veins or cemented sandstone within Subunit Ve.

The proposed description and interpretation of Subunit Vc’ is based on log subunits and comparison with the log response of the subunits defined in Hole C0002N. The depth intervals for subunits are shown on Table T42 together with the average, minimum, and maximum values of gamma radioactivity, resistivity, and acoustic slowness/velocity.

Subunit Vc′ (2163–2365.6 mbsf)

In Hole C0002P, Subunit Vc′ radioactivity shows variations in gamma ray values (from 58 to 94 gAPI) and an average value of 84 gAPI. Acoustic slowness values are from 83 to 110 µs/ft with an average value of 94 µs/ft. Formation resistivity varies between 1.3 and 3.2 Ωm, and the average value is 2.2 Ωm.

The uppermost interval of the logs boundary for Subunit Vc′ is characterized by an increasing trend in gamma ray values and a decrease in sonic velocities and resistivity (Fig. F114). A defining character of Subunit Vc′ is five ≤15 m thick local minima found in gamma radiation, resistivity, and sonic velocity at ~2205, 2223, 2281, 2332, and 2365 mbsf. Variations are up to ~30 gAPI, ~1.4 Ωm, and ~570 m/s in gamma radiation, resistivity, and sonic velocity, respectively. These responses were interpreted as permeable sands, with the low resistivity resulting probably from mud invasion or from high-salinity formation fluids in the sandy layers. Typically, sharp increases in gamma radiation, velocity, and resistivity are observed at the top and bottom of these levels, potentially caused by a change from coarser grained sediment within the minima zones to a background consisting of hemipelagic silty claystone. Between 2210 and 2217 mbsf, a decrease in sonic velocity to <2900 m/s does not correspond to decreases in gamma ray values or resistivity. The uppermost sandy intervals (2221–2230 and 2203–2208 mbsf) show patterns that suggest a gradual transition from clay to sand at the base, whereas the lowermost permeable intervals (2279–2282, 2332.5–2339, and 2360–2365 mbsf) show a pattern possibly reflecting a relatively gradual transition from sand to clay (from lower to higher gamma radiation) at the top. The observations of the log properties and shapes are consistent with the interpretation of turbiditic deposits and interbedding of sandy layers in hemipelagic silty claystone (see “Lithology”). Possible normal grading trends at the base and reverse grading at the top are observed in the log data. Postexpedition interpretation of borehole resistivity images will help define the structures described in the cored section (see “Lithology” and “Structural geology”).

Subunit Vd (2365.6–2753 mbsf)

Below sharp but small downhole increases in gamma radiation and resistivity, which may be interpreted as a change from coarser grained sediment to background hemipelagic silty claystone, this subunit (2365.6–2753 mbsf) exhibits rather constant trends (average values for gamma radiation, resistivity, and acoustic slowness are 87 gAPI, ~2 Ωm, and 93 µs/ft, respectively). The gamma ray values progressively increase from the top, with average values of ~80 gAPI, to the bottom, reaching maximum values of 101.75 gAPI and a minimum of 69 gAPI. The minor fluctuations around this trend line suggest silt–sand alternation within the dominant hemipelagic mud.

The most striking features in this section are the subtle but continuous increasing trend in gamma radioactivity and decreasing trend in resistivity from top to bottom followed by a sharp increase near the bottom (Fig. F114). The compressional acoustic velocity displays a trend of decreasing velocity followed by a slight increase in velocity at 2685 mbsf.

At a finer scale, high-value resistivity spikes (e.g., from 2 to 4.5 Ωm at 2497–2502 mbsf) correspond to slightly lower gamma ray and acoustic velocity values. The comparison of four phase-shift resistivities (RH16PC to RH48PC; Fig. F116) suggests invasion of the layers based on the slightly differing responses of the shallow and deep measurements. A similar log character was identified in Hole C0011A (Expedition 322 Scientists, 2010a). There, the different intervals with a similar thickness from 1 to 4 m based on the resistivity and gamma ray log characteristics and core descriptions, were identified as ash and volcaniclastic layers. The log features observed in Hole C0002P similarly could be related to the possible presence of ash and volcaniclastic layers (Fig. F117). The first such possible layers were identified at 2385 mbsf, and the most prominent occurrence was identified at 2499 mbsf. Other thin intervals with similar features were identified at 2444, 2450, 2527, 2532, and 2535 mbsf. Although no ash layers were identified by cuttings analyses (see “Lithology”), our interpretation is based on correlation of the log response with logs and lithologies observed in Hole C0011A. Detailed analysis of resistivity images may help to interpret these features.

From 2585 to 2640 mbsf and 2664 to 2713.54 mbsf, variation in resistivity readings from different depths of investigation indicates the absence of separation of between curves, a possible indication of impermeable formations. Within the upper section of the subunit and downhole to 2507 mbsf, no distinctive features are recognized on the gamma ray log. In the lower section, there is correspondence in the log response among low gamma radiation, high resistivity, and increased velocity. Sharp and prominent spikes in resistivity and compressional velocity values at 2697 mbsf (Fig. F114) need further examination.

Subunit Ve (2753–3058.5 mbsf)

The top of this interval is characterized by a sharp downhole decrease followed by a gradual increase in gamma radioactivity, decrease of velocity values, and a gradual increase of resistivity values (2–3 Ωm). An increasing trend from the top of this interval to 2882 mbsf is followed by a decreasing trend from to 3041 mbsf in both resistivity and velocity values. Average gamma ray values are 95 gAPI, with minimum and maximum of 81.3 and 104.3 gAPI, respectively. Acoustic compressional slowness average values are 94.2 µs/ft, with minimum and maximum values of 79.7 and 106.2 µs/ft, respectively. The average value for resistivity within this section is 2.6 Ωm, and minimum and maximum values are 1.9 and 4.0 Ωm, respectively.

Prominent local maxima in resistivity values related to lower gamma radiation and higher acoustic velocity were identified at 2765–2768, 2800–2806, 2887–2898, and 2942–2966 mbsf. Based on the log response on the shipboard logs (gamma radioactivity, resistivity, and acoustic velocity), these features were interpreted as cemented layers and/or vein-rich intervals related to possible deformational structures. Postexpedition analysis of resistivity images will help to refine the interpretation of these features.

Also noticeable is the sharp drop in velocity observed between 2896.5 and 2941.2 mbsf associated with relatively low resistivity values. This drop is followed by a sharp increase at the lower boundary of this interval. The gamma ray log does not indicate relevant compositional changes at this level. One likely interpretation is the existence of a low-velocity interval bounded by highly resistive and higher velocity levels associated with tectonic structures. These features might be associated with fault-related structures seen in cuttings (see “Structural geology”). Below this interval, a sharp increase in velocity with a broader (~10 m) decrease is found, followed by a general decrease with small spikes.

Resistivity images interpretation

The AFR images are of good quality throughout, although very noisy and locally affected by distortion resulting from drilling difficulties during acquisition and borehole wall damage. In the uppermost interval (2149.7–2216.8 mbsf), image quality suggests bad hole conditions (Figs. F117, F118). The section from 2149.7 to 2163 mbsf was drilled previous to coring at the depth interval below, and the hole condition is worse than the cored section between 2163 and 2218.5 mbsf.

Standard image processing and smoothing routines improved image quality for interpretation and provided correct orientations and angles of bedding and tectonic structures (fractures and faults). Hole azimuth and deviation values used for image interpretation were obtained from the drilling deviation survey. For image display and interpretation, the high-resolution image (lower transmitter) has been used.

Bedding and tectonic structures

Bedding planes are easily identified on the processed images for most of the logged section. Bedding plane orientation could be measured and characterized nearly continuously with depth (Fig. F118). Gaps of measured bedding dips occur only at zones of bad hole conditions, commonly at strongly deformed zones. The alternation of layers with slight contrasts in texture and composition favor resistivity contrast, and the good definition of bedding planes seen on the resistivity images show orientation that can easily be measured to define the orientation of bedding and planar structures.

The resistivity images predominantly display northwest-dipping steep bedding (varying from 60° to 90°). Locally, south–southeast-dipping beds are also present, especially along the section between 2600 and 2750 mbsf (Figs. F117, F118). Because of the severity of dip changes and the high density of fractures and faults, we interpret this section as strongly tectonically deformed (Fig. F117). Dips decreased from 2860 (very steep; ~90°) to 3040 mbsf (~60°) at the bottom of the logged section (Fig. F118).

Locally highly resistive features following bedding structures were identified within Subunits Vd and Ve and require further investigation and postexpedition analysis. Tentatively, these could be interpreted as cemented layers, but other interpretations (e.g., ash layers, resistive fluids) should be considered as well. Highly conductive layers aligned parallel to bedding surfaces also occur locally, especially in Subunit Vc′. These may possibly indicate hydraulically active structures with higher water content.

Preliminary interpretation of structures, both fractures and faults, is shown in Figure F118. A detailed structural analysis will be carried out postexpedition. Both fractures and faults could be characterized on the resistivity images. Fractures and faults show a wide range of orientations that are generally steeply dipping, with a range of dips between ~30° and 90°. Fracture density varies with depth, with the highest concentration in the lower section of Subunit Vd and uppermost section of Subunit Ve (Figs. F117, F118).

Wellbore failures

Numerous wellbore failures were observed in the AFR image log. The resistivity image was the primary source of information for identifying potential wellbore failures and classification. As the wellbore failures were examined closely, we characterized the position, orientation, vertical extent, and width of the feature and classified whether a feature was a breakout or a drilling-induced tensile fracture. Most features in this data set were identified as breakouts, which appeared as dark, vertically continuous features in pairs with 180° separation in the resistivity image. The sonic caliper borehole cross-section was also used to help decide the classification for features that were not continuous and less clear. When it was still unclear whether a feature was a breakout or a drilling-induced tensile fracture, we recorded the feature as unidentified.

The most prominent features are concentrated in the uppermost 67 m in Hole C0002P, where the borehole was exposed by previous drilling and coring activity (Fig. F119). From 2150 to 2163 mbsf, the borehole is nearly washed out, although the preferential development of the breakout in the northwest–southeast direction can be seen in the resistivity image (Fig. F120). This section was drilled before coring took place and thus had been exposed for 6.5 days before being imaged with resistivity logging tools. Breakout widths average 95° and reach an observed maximum of 140°. In the cored section between 2163 and 2218.5 mbsf, clear continuous breakout features were observed in the same northwest–southeast direction as the depths above but with moderate angular widths (average = 70°). Here, the borehole cross-section derived from sonic caliper data showed enlargement of borehole diameter in the azimuth consistent with the breakouts identified in the image log. However, we note that such correspondence is not persistent, and there are depth ranges where the cross-section does not necessary match the pattern expected by the presence of breakouts in the image log.

Below 2163 mbsf, occurrences of wellbore failures were sparse, and their widths were much smaller (average = 23°) compared to the depth ranges above. With the exception of several features identified above 2600 mbsf, it was not possible to conclusively classify these features as breakouts even with the aid of sonic caliper data, although their azimuthal directions were consistent with those observed at the cored section of the well (Fig. F120). Also, some narrow wellbore failures observed toward the bottom of the well appear different from the breakouts above and may possibly be drilling-induced tensile fractures (Fig. F120). Without conclusive evidence, which is currently unavailable, many of the features below the cored section are classified as unidentified.

If we limit the discussion to the depths above and within the cored section (<2218.5 mbsf), the length-weighted average value of the breakout azimuths is in the N35°W/S35°E direction. This suggests that the direction of the maximum horizontal principal stress in the upper 65 m of the imaged well is in the northeast–southwest direction.

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