Leg 190 was the first of a two-leg program designed to sample a transect of sites (Figures 1 and 2) across the Nankai Trough accretionary prism (SW Japan) within a three-dimensional (3-D) seismic survey. One additional site was drilled to the west of the main transect to compare along-strike variations in accretionary processes. The main logging effort of this program will take place on Leg 196 in 2001 using logging-while-drilling (LWD) technology to collect further in situ physical properties data at most of the same sites. Therefore, wireline logging on Leg 190 was only performed at Site 1173, the Eastern (Muroto) Transect reference site. The figures that follow are from the Leg 190 Preliminary Report: Deformation and fluid flow processes in the Nankai Trough accretionary prism, August 2000.
Primary Leg 190/196 objectives include documentation of:
Spatial distribution and temporal progression of deformation in the accreting and underthrusting sediments
Structural and hydrologic evolution of the décollement zone
Chemical gradients and fluid flow paths across the toe of the margin
Contrasting stratigraphic and deformational framework along strike of the margin
Figure 1: Location map for the Leg 190 drilling region, SW Japan.
Figure 2. Shikoku Island, Japan, coastline, bathymetry, and locations of sites drilled on Leg 190. Wireline logs were collected only at Site 1173.
We drilled Site 1173 in the trench outer margin (Figure 3) in order to provide a reference for the predeformation status of geological and geochemical characteristics of the incoming sedimentary section. The 734 m thick sedimentary section at this site consists of an outer trench wedge facies of turbidites and hemipelagic muds overlying ash-bearing hemipelagic muds and siliceous claystones. It encompasses presumed lateral equivalents to the interval in which the décollement forms at the deformation front, and the entire underthrusting section delivered to the margin.
Figure 3. 2-D time section extracted from 3-D migrated survey (gray polygon on figure 2), showing site location on the eastern (Muroto) transect. Site 1173 (far right) is the reference site documenting the nature of the incoming sedimentary section entering the margin system.
This report presents the preliminary results of the logging operations that were carried out at the site. Detailed descriptions of the preliminary results and conclusions of the leg as a whole are available in the Leg 190 Preliminary Report.
Even though LWD is planned for Site 1173 on Leg 196, wireline logging was considered important because there is at present no LWD capability for sonic velocity measurement in slow, poorly consolidated formations, nor for FMS imaging. Velocity in particular is a key desirable parameter because it is required to convert the 3-D seismic data to accurate depths, as well as to interpret porosity and other physical properties from seismic data. As expected, logging Site 1173 was technically challenging; several logging passes were achieved from 0 to 440 mbsf with difficulty, but no deeper logs were obtained. A highlight of Site 1173 logging was the acquisition of a high quality shear travel time sonic log with the new DSI-2 low-frequency source, despite the very low formation shear velocities (300-700 m/s). Having both shear and compressional velocity logs permits calculation of Vp/Vs or Poisson's ratio, useful for interpreting important petrophysical properties.
Downhole logging operations in Hole 1173A consisted of runs with both the triple combo logging string (HNGS, DIT, APS, HLDT, TAP) and the FMS-sonic tools (DSI, FMS, NGT) deployed (Table 1). Downhole conditions prevented the complete 734 m drilled interval from being logged. The triple combo was run in two stages due to hole bridging; even so, logging was only achieved to 440 mbsf. Despite a subsequent wiper trip and barite mud fill, the FMS-sonic logging string could not be lowered beyond 380 mbsf. The interval 65-373 mbsf was logged twice, and high quality FMS imaging and compressional and shear velocity data were acquired. During the second pass, a new low-frequency (<1 kHz) dipole sonic energy source was used for the first time in an ODP hole, producing excellent shear and compressional waveforms despite very low formation velocity.
|Site 1173 Logging Summary|
|1||HNGS Natural Gamma||97-338 mbsf|
|2||HNGS Gamma Ray||358-440 mbsf|
|3||DSI Sonic Velocity||65-373 mbsf|
|NGT Natural Gamma|
Table 1. Summary of logging runs performed in Hole 1173A.
Gamma, Resistivity, Density, Porosity
A summary of the logging results is presented in Figure 4. The caliper log shows that the hole was in generally fair to good condition down to 332 mbsf. Below 350 mbsf, the hole is wide and highly rugose, considerably degrading much of the data. Gamma ray data are consistent with the homogeneous silty clay lithology in cores from the logged interval, staying in a narrow range of approximately 50-80 API units throughout. No strong lithologic variations are detected.
Figure 4. Summary of logging results at Site 1173, with selected core physical properties for comparison.
Resistivity is low overall, ranging only between about 0.4 and 0.7 ohmm. The interval from 70 to 336 mbsf has an overall trend of decreasing resistivity; we attribute this reverse trend to the strong downhole increase in porosity over the same interval. In the deeper logged interval, resistivity is higher, again consistent with the lower core porosity. Numerous narrow high resistivity spikes, especially well-exhibited by the high-resolution spherically-focused (SFLU) induction log, appear to correlate well with ash layers identified in the cores and in the FMS micro-resistivity log.
Density data indicate an unusually low-density zone from 93 to 336 mbsf, with a trend of slightly decreasing density with depth from 93 to about 130 mbsf, then nearly constant density of ~1.63-1.66 g/cm3 to 320 mbsf. Higher-density intervals at several depths do not clearly correlate with specific features in the cores. Log densities are significantly higher in the 358-440 mbsf interval than above, although the scatter is much greater due to poor hole conditions.
Neutron porosity logging resulted in very high values of 70 to 90% porosity through most of the logged interval. This log is uncorrected for the effect of the clay mineral hydrogen content, which has a significant effect on neutron absorption (Schlumberger Corp., 1989). The very clay-rich lithologies at Site 1173 suggest that this log should be considered unreliable without correction. Porosity calculated from the density log using core-measured grain densities are considered more reliable.
Both compressional and shear slownesses (inverse of velocity) were measured with the new low-frequency, high-power dipole source transducer in the DSI. The travel times were picked through automated semblance coherence analysis of the arrivals at 16 receivers, and converted to P and S wave velocity. Both logs appear to be of good quality over the logged interval (90 to 360 mbsf), which is especially remarkable given the very low shear velocities of 300-600 m/s. P-wave velocities range from 1575 to 1675 m/s and show no downward trend down to 220-230 mbsf, where there is a well-defined change to an increasing trend with depth (Figures 4 and 5). Velocities appear to decrease back to values around 1700 m/s in the short logged interval below 345 mbsf, but return to near 1800 m/s at the very bottom of the log. The overall trend is closely followed by the core-based measurements of P-wave velocity (except for the sharp decrease below 340 mbsf), although core values are 50 to 100 m/s lower, attributed to unloading of the cores from in situ stress.
P-wave velocities are compared to the two-ship split spread seismic experiment interval velocity values near this site of Stoffa et al. (1992) in Figure 5. They report a velocity of 1965 m/s between 133 and 388 mbsf. There is an unexplained discrepancy of 100 to more than 300 m/s between this value and the log data through this interval.
Figure 5. Compressional wave velocities (red line) measured with the DSI tool, core-based ultrasonic velocity (blue dots), and interval velocity values obtained by Stoffa et al. (1992; green line).
Shear velocity follows a similar trend to that of the P-wave log, varying from just over 300 m/s to more than 650 m/s. There is a change in slope of the shear velocity curve at the same ~220-230 mbsf depth as that exhibited by the P-wave. Both P and S velocities exhibit a sharp local peak value at ~265 mbsf, coinciding with a resistivity and density peak as well. This zone corresponds to an interval with a high concentration of ash layers in the cores and FMS logs.
Formation MicroScanner Imaging
The volcanic ash beds and other features observed in the cores were well imaged by both FMS passes. Much of the logged interval exhibits horizontal or near-horizontal bands of high resistivity, which we interpret as individual ash layers identified in the cores (Figure 6). These bands typically have sharp bases above conductive intervals, then grade upward back to the background resistivity values. In the intervening hemipelagic silty clay between the ash bands, subtle features are imaged in the FMS micro-resistivity, including narrow bands interpreted as Zoophycos trace fossils and mottled intervals interpreted as bioturbation and diagenetic sulfide mineralization, again based on correlation with core observations (Figure 7). The Unit II/III boundary at ~343 mbsf was logged with FMS, and a change in resistivity character can be seen across this interval (Figure 8). Above the boundary, bedding and ash layers are more apparent, while below it the sediment has a more uniformly bioturbated appearance.
Figure 6. Example of FMS imaging of ash layers commonly observed in the logged interval above 345 mbsf.
Figure 7. FMS image of bioturbated interval with an ash layer in Unit II. The "mottled" zone with concentric rings of variable resistivity is interpreted as an interval with authigenic alteration and sulfide mineralization.
Figure 8. FMS image of the boundary interval between Units II and III. Although this boundary is somewhat gradational, it is placed at the occurrence of the deepest significant ash layer, imaged here. The differing character of the intervals above and below is evident, with more prominent ash layers above and homogeneous bioturbated mudstone below. FMS data has been processed with dynamic normalization.
Overall, the logging data expand upon core-based observations and provide in situ data at Site 1173. The resistivity and density profiles document an anomalous maintenance of high porosity in lithostratigraphic Unit II, as well as an abrupt shift to lower porosity at ~345 mbsf. The break in slope in the two velocity logs at about 225 mbsf is unusual, given the lack of a similar break in density. The increased formation velocity over this interval, coupled with the near-constant density (or porosity) may be an indicator of cementation effects.
Schlumberger Corporation, 1989, Log Interpretation Principles/Applications, Schlumberger Educational Services, Sugarland, TX.
Stoffa, P., Wood, W, Shipley, T., Moore, G., Nishyama, E., Bothelo, M., Taira, A., Tokuyama, H., and Suyehiro, K., 1992. Deepwater high-resolution expanding spread and split spread seismic profiles in the Nankai Trough, Jour. Geophys. Res., v. 97, p. 1687-1713.
Harold Tobin, Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro NM 87801, USA