Downhole Logging Summary
The Nankai subduction zone off of southwest Japan forms an "end-member" sediment-dominated accretionary prism. Here, a sedimentary section ~1 km thick (Figs. 1, 2) is accreted to or underthrust beneath the margin in the style of a fold and thrust belt (Moore et al., 2001). The Philippine Sea plate underthrusts the margin at a rate of ~4 cm/yr along an azimuth of 310o-315o (Seno et al., 1993) down an interface dipping 3o-7o (Kodaira et al., 2000), causing repeated great earthquakes (magnitudes >8) with an average recurrence interval of ~180 yr. (Ando, 1975). Currently the margin is locked with little convergence between the Muroto Peninsula and the Philippine Sea plate (Mazzotti et al., 2000). The convergent margin of southwest Japan has a geologic record of accretion of deep-sea deposits extending to at least the Cretaceous (Taira et al., 1988). However, rocks cored during Leg 190 (Fig. 2) and even those subducted to seismogenic depths entered the subduction zone no earlier than the Pliocene (Moore, Taira, Klaus, et al., 2001).
In the area of Leg 190/196 drilling, the Muroto Transect (Fig. 1), the basin to margin transition can be divided into the undeformed Shikoku Basin and overlying trench fill, the protothrust zone, the imbricate thrust zone, the frontal out-of-sequence thrust zone, the large thrust slice zone, and the landward-dipping reflector zone (Fig. 2). A condensed summary of these tectonic provinces from Moore et al. (2001) and Moore, Taira, Klaus, et al. (2001) follows.
Figure 2. Generalized depth section showing site locations and major structural features and provinces
During Leg 196, four Anadrill LWD and measurement-while-drilling (MWD) tools were deployed. LWD operations were planned for three sites near the toe of the Nankai accretionary prism. Wireline logs have been difficult to obtain during previous ODP legs in the region (Legs 131 and 190) due to unstable hole conditions. Because coring cannot be conducted while using LWD tools, the coring results from previous legs were correlated with the LWD/MWD data collected during Leg 196.
LWD and MWD tools measure different parameters. LWD tools measure in situ formation properties with instruments that are located in the drill collars immediately above the drill bit. The LWD and MWD tools used during Leg 196 include the resistivity-at-the-bit (RAB) tool, the power pulse MWD tool, the Anadrill Integrated Drilling Evaluation and Logging (IDEAL) sonic-while-drilling (ISONIC) velocity tool, and the azimuthal density neutron (ADN) tool. This was the first time the ISONIC tool was used during an ODP leg. Figure 3 shows the configuration of the LWD/MWD bottom hole assembly (BHA).
We drilled Holes 1173B and 1173C to obtain logging-while-drilling (LWD) data at a reference site on the seaward flank of the Nankai Trough. These holes complement Hole 1173A, which was cored from the surface to basement during Leg 190. This site provides a basis for comparison of physical and chemical properties between the incoming undeformed sediments and rocks of the Shikoku Basin with deformed materials of the accretionary prism and underthrust sediments cored at sites to the northwest.
At Site 1173, the LWD tools measured resistivity at the bit (RAB), sonic velocity, density, porosity, natural gamma ray production, and photoelectric effect from the seafloor to basaltic basement (Fig. 4). Additionally, the tools provided estimates of hole size and borehole resistivity images. A measurement-while-drilling (MWD) system provided information on weight on bit, torque, heave, resistivity, density, and sonic velocity that was communicated to the surface and displayed instantaneously during drilling.
The overall quality of the LWD logs recorded in Holes 1173B and 1173C is excellent. The LWD logs generally agree with the more limited Hole 1173A wireline logs. In Holes 1173B and 1173C, the drilling rate was maintained between 35 and 60 m/hr throughout the section and all measurements were made within 1 hr. of bit penetration. At least two depth points were measured in each 0.3-m interval. The caliper shows that the gap between the bit radius and the hole is <1 in throughout both holes, with the exception of the uppermost 75 m of Hole 1173C, where soft sediment washed out a gap up to 2 in long. Therefore, the density log over this shallowest 75-m interval is unreliable. This was the first ever use of the ISONIC velocity tool in such fine-grained, unlithified sediment and the first use ever by the ODP. Although the tool worked well, the processing of the waveforms was not straightforward and will have to be improved post-cruise to yield reliable sonic velocity data.
Both visual and multivariate statistical analyses of the logs define five log units that account for the lithologic variations observed in the cores. Summary of log and lithologic data is shown in Fig. 5.
A high variability in the differential caliper log and a large number of caliper values >1 in reflect bad borehole conditions during drilling of the upper 75 m of log Unit 1 (0-122 mbsf). This log unit shows high neutron porosity and low density values with a high standard deviation. These variations might be in part real and reflect the presence of silt and sand turbidites of the outer trench-wedge facies. A significant decrease in resistivity, density, and gamma ray and an increase in neutron porosity with depth show an abnormal compaction trend and define log Unit 2 (122-340 mbsf). This log unit correlates with lithologic Unit II (102-344 mbsf), which consists of hemipelagic mud with abundant interbeds of volcanic ash. The low density could be related to a cementation effect caused by the formation of cristobalite. The log Unit 2/3 boundary correlates with the diagenetic phase transition between cristobalite and quartz. High gamma ray, density, and photoelectric effect log values that increase continuously with depth characterize log Unit 3 (340-698 mbsf). Resistivity and, less obviously, gamma ray logs show a cyclicity of values (480-700 mbsf) that reflects changes in lithology, which may in turn reflect an interbedding of coarser and finer grained sediments. Log Unit 4 (698-731 mbsf) is defined by broad variations in photoelectric effect, resistivity, neutron porosity, and gamma ray logs that correlate well with the presence of the volcaniclastic facies of lithologic Unit IV (688-724 mbsf). Log Unit 5 (731-735 mbsf) shows an abrupt increase in resistivity and a decrease in gamma ray log values, which characterize the basaltic oceanic basement.
Structural data determined from RAB images of medium-focused resistivity (penetration depth 7.6 cm beyond standard borehole radius) indicate sparse deformation and predominantly subhorizontal bedding dips. Increases in bedding dips (5o-35o) at 50-200 mbsf and below ~370 mbsf are in agreement with core data from Leg 190 Hole 1173A. Fractures are oriented at high angles (40o-80o), show normal displacement where measurable, and have variable strike orientation. Resistive fractures dominate and might reflect nonconductive clay gouge, mineralization, or porosity collapse due to compaction. An increase in fracture intensity occurs at 380-520 mbsf, correlating with an increase in bedding dip. The upper limit of this zone corresponds to the projected stratigraphic equivalent of the décollement zone. At ~500 mbsf, bands of heterogeneous (mottled) high resistivity are thought to represent zones of intense deformation or brecciation. In general, the deformation observed in Holes 1173B and 1173C is consistent with extensional faulting probably related to basinal compaction and burial and not to propagating compressional deformation from an accretionary wedge. Fracture dip and strike data from RAB image are shown in Fig. 6. Examples from RAB image data of fractures are shown in Fig. 7.
Figure 7. RAB images of fractures (Hole 1173B) cutting subhorizontal stratigraphy
LWD density data in Holes 1173B and 1173C closely match core physical properties data from Hole 1173A, except for the uppermost 60 m, where differential caliper values exceeded 1 in (Fig. 8). LWD densities are nearly constant in log Subunit 1b (55-122 mbsf) and Unit 2 (122-340 mbsf), with the notable exceptions of two high-amplitude variations near the transition from lithologic Unit II (upper Shikoku Basin facies) to III (lower Shikoku Basin facies). Log Unit 3 (340-698 mbsf) is characterized by a steady increase in density consistent with normal compaction. The LWD resistivity logs clearly respond to the lithologic boundaries identified in Hole 1173A. Within log Unit 2 resistivity decreases with depth while density is constant, whereas in log Unit 3, resistivity is about constant with depth and density increases. All LWD resistivity logs show a similar overall trend, in good agreement with wireline logs, where available. Shallow button resistivities that are consistently higher than medium and deep resistivities is an unusual and unexplained feature of Site 1173 LWD data.
Figure 8. LWD density (RHOB) profile from Holes 1173B and 1173C compared to bulk density values from Hole 1173A cores.
The velocities from the core and wireline data and densities from the core and LWD data from 0 to 350 mbsf were used to generate a synthetic seismogram in good agreement with the seismic reflection data (Fig. 9). Good correlations exist between the synthetic seismogram and the seismic reflection data at ~80-100 (trench-basin transition facies), ~175, ~265-270, and ~300-350 mbsf (associated with the upper to lower Shikoku Basin unit boundary and the log Unit 2/3 boundary). A change in physical properties associated with the phase transition from cristobalite to quartz may be, in part, responsible for this reflection. High reflectivity in the synthetic seismogram beneath ~350 mbsf does not match with the low reflectivity in the seismic data of the lower Shikoku Basin unit (log Unit 3) and may be caused by a sampling bias in the core velocity measurements.
We drilled Hole 808I to obtain LWD data through the frontal thrust and décollement zones at the deformation front of the Nankai (Fig. 10). This hole complements Site 808 cores, which were recovered during ODP Leg 131. Coring, logging, and monitoring here are intended to document the physical and chemical state of the Nankai accretionary prism and underthrust sediments through the frontal thrust zone, the décollement zone, and into oceanic basement.
The overall quality of the LWD logs recorded in Hole 808I is variable. We recorded at least one sample per 15 cm over 99% of the total section. Sections of enlarged borehole indicated by differential caliper measurements yield unreliable density and associated porosity data, which is confirmed by a comparison to core data. Unreliable data are primarily associated with the depth intervals at 725-776 and 967-1057 mbsf, where there was a long gap between drilling and recording of the logs due to wiper trips or poor hole conditions. Density and density-derived porosity should be used cautiously until more complete corrections and editing is completed postcruise. Although the ISONIC velocity tool worked well, the processing of the waveforms was not straightforward and post-cruise processing is required to yield reliable sonic data.
Four log units and six log subunits were defined through a combination of visual interpretation and multivariate statistical analysis. Log Unit 1 (156-268 mbsf) is characterized by the overall lowest mean values of gamma ray, density, and photoelectric effect and overall highest mean values of resistivity and neutron porosity. These values coincide with very fine grained sandstones, siltstones, and clayey siltstone/silty claystones observed in the cores. Log Unit 2 (268-530 mbsf) has constant values of gamma ray and neutron porosity and a decreasing resistivity log. A high variability in the differential caliper log and a large number of values >1 in reflect bad borehole conditions. Log Unit 3 (530-620 mbsf) is marked by a significant increase in mean values of gamma ray, density, and photoelectric effect. Log Unit 4 (620-1035 mbsf) is characterized by the overall highest mean values of gamma ray, photoelectric effect, and density and the lowest mean values of resistivity and neutron porosity. Generally, a positive correlation between gamma ray and photoelectric effect is observed. A continuous increase in gamma ray and photoelectric effect is observed from log Unit 1 to Unit 4, which reflects an increase of clay and carbonate content. Log Units 3 and 4 are characterized by a positive correlation between resistivity and density. Summary of log and lithologic data is shown in Fig. 11.
RAB tools imaged fracture populations and borehole breakouts throughout much of the borehole. We identified both resistive and conductive fractures, respectively interpreted as compactively deformed fractures (leading to porosity collapse) and open fractures. Fractures are concentrated in discrete deformation zones that correlate with those seen in Site 808 cores during Leg 131: the frontal thrust zone (389-414 mbsf), a fractured interval (559-574 mbsf), and the décollement zone (~940-960 mbsf). Only relatively sparse deformation occurs between these zones. The major deformation zones are dominated by conductive fractures and overall high resistivity with resistive fractures between the zones. Fractures are steeply dipping (majority >30°) and strike predominantly east-northeast-west-southwest, close to perpendicular to the convergence vector (~310°-315°; Seno et al., 1993). Bedding dips are predominantly low angle (<50°) but are difficult to identify in the highly deformed zones, biasing this result. Bedding strike is more random than fracture orientation, but where a preferred orientation is recorded, beds strike subparallel to fractures and approximately perpendicular to the convergence vector. Fracture dip and strike data from RAB image are shown in Fig. 12.
Figure 12. Fracture (A) dip and (B) strike from resistivity-at-the-bit image interpretation, Leg 196 Hole 808I.
The frontal thrust zone (389-414 mbsf) represents the most highly deformed zone at Hole 808I and contains predominantly south-dipping (antithetic to the seismically imaged main thrust fault) and a few north-dipping east-northeast-west-southwest striking fractures (Fig. 13). The highly fractured interval at 559-574 mbsf contains similar fracture patterns to the frontal thrust zone. Both deformation zones are characterized by high-conductivity (open?) fractures within a zone of overall high resistivity.
Deformation at the décollement zone is more subdued and is represented by a series of discrete fracture zones. The décollement zone imaged in the RAB images (937-965 mbsf) is defined by a general increase in fracture density and a marked variability in physical properties. RAB image of décollement zone is shown in Fig. 14.
Borehole breakouts are recorded throughout Hole 808I. They are particularly strongly developed within log Unit 2 (270-530 mbsf), suggesting lithologic control on sediment strength and breakout formation (Fig. 15). Breakouts indicate a northeast-southwest orientation for the minimum horizontal compressive stress (2), consistent with a northwest-southeast convergence vector (310°-315°, parallel to 1; Seno et al., 1993), Breakout orientation deviates slightly from the dominant strike of fractures (east-northeast-west-southwest), but this deviation may be within the measurement error.
Figure 15. RAB images of borehole breakouts (dark low-resistivity parallel lines) at 1-in (shallow), 3-in (medium,) and 5-in (deep) penetration from the borehole, 280-325 mbsf.
The Hole 808I LWD density log shows a good fit to the core bulk density, slightly underestimating core values in the upper 550 m and slightly overestimating core values between 550 and 970 mbsf. Below 156 mbsf the LWD density log shows a steady increase from ~1.7 to ~1.95 g/cm3 at 389 mbsf. Between 389 and 415 mbsf density shows large variations corresponding to the frontal thrust zone (Fig. 16). The low density values here are probably spurious, produced by washout of the borehole. Below the frontal thrust zone density decreases sharply to ~1.85 g/cm3, and below 530 mbsf it increases to ~2.1 g/cm3. Between 725 and 776 mbsf density drops sharply to ~1.75 g/cm3, corresponding to a period of borehole wiper trips. Density increases more rapidly from ~1.95 g/cm3 at 776 mbsf to 2.25 g/cm3 at 930 mbsf, decreasing steadily to ~2.15 g/cm3 before stepping down to 1.4 g/cm3 at 965 mbsf. This corresponds to the base of the décollement zone; below, density increases steadily from ~1.7 g/cm3 at 975 mbsf to ~2.0 g/cm3 at 1034.79 mbsf.
Figure 16. LWD density (RHOB) profile from Hole 808I compared to bulk density values from Site 808 cores.
The downhole variations of LWD resistivity measurements show identical trends in all five resistivity logs of Hole 808I. All log unit boundaries are clearly identified. Log Unit 1 has an average resistivity of ~0.8-0.9 m. After a sharp decrease in resistivity from 1.3 to 0.5 m between 156 and 168 mbsf, the signal shows an increasing trend downward to the Unit 1/2 boundary. Unit 2 is characterized by an overall decreasing trend in resistivity from ~0.9 to ~0.6 m. However, this trend is sharply offset at 389-415 mbsf (log Subunit 2b), where resistivity values are ~0.3 m higher. This zone seems to correspond to the frontal thrust; the higher resistivity here may reflect compactive deformation in the frontal thrust zone. Unit 3 is characterized by a higher degree of variability in the resistivity signal. Resistivity again exhibits less variation in Unit 4 where it averages ~0.6 m. At ~925 mbsf resistivity values change from gradually increasing to gradually decreasing. This change in trend occurs near the top of the décollement zone and may represent a tendency toward increasing porosity downward within the décollement zone. As was observed in Hole 1173B, shallow-focused resistivity values are systematically higher than both medium- and deep-focused resistivity values.
Correlations between the synthetic seismogram and seismic reflection data are only broadly consistent beneath the cased section (~150 mbsf). The defined lithologic boundaries or units correlate only at a few depth intervals. The details of amplitude and waveform throughout most of the section do not match the seismic data. Amplitudes of the intervals between ~200 and ~400 mbsf, between ~750 and ~850 mbsf, and below ~925 mbsf are significantly higher in the synthetic seismogram than in the seismic data. The velocity and density logs infer reflections that are not observed in the seismic data, so many of the velocity and density values may not reflect true in situ properties and may be a product of poor hole conditions.
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Saneatsu Saito: Logging Staff Scientist, Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo 164, Japan.
Dave Goldberg: LWD Scientist, Borehole Research Group, Lamont-Doherty Earth Observatory, Palisades, NY 10027, USA.