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doi:10.2204/iodp.pr.314.2008

Site summaries (continued)

Site C0002

Site C0002 is located at the Kumano forearc basin off the Kii Peninsula (Fig. F10). The goal of drilling at this site was to log the ~1000 m thick Kumano forearc basin section and several hundred meters of the underlying formations, interpreted as older rocks of the accretionary prism and/or early slope basin sediments deposited prior to the development of the megasplay fault. This site also is slated for deep riser drilling across the entire plate boundary system to >5500 mbsf during NanTroSEIZE Stage 3. We were able to drill and log the entire section from 0 to 1401 mbsf with complete success. An excellent suite of logs and seismic VSP data reveals the structure of the forearc basin, gas hydrate–bearing zone, and the underlying deformed rocks of the inner accretionary prism.

Operations

Because of the strong Kuroshio Current, the ship was moved upcurrent, then drifted to the location of Site C0002 while running the LWD BHA. After tagging the seafloor with the LWD/MWD assembly at 1964.5 m drillers depth below rig floor(DRF) (1936 meters below sea level [mbsl]), Hole C0002A was jetted to 70.5 m LSF, then rotary drilling commenced and proceeded smoothly. Drilling continued to TD of 1401.5 m LSF (3366 m DRF) at 0600 h on 17 October 2007 without any significant difficulties. The hole was displaced with kill mud and two joints of pipe were removed to create an 18 m offset in the position of pipe connections for filling in the check shot data during the pipe trip out. All memory data were successfully downloaded. The seismicVISION data had to be sent to land base for preprocessing because of battery failure before retrieval of the tool, but data were uploaded to the ship within a few days. In spite of several major washouts (standoff > 2 inch) in sand-rich intervals, logging data quality is generally good, and image logs in particular are of high quality. These logs provide a very rich data set for interpretation of the geology and stress conditions at this site.

Log characterization and lithologic interpretation

Four logging units were defined based on the trends and character in the full suite of LWD log responses (Fig. F11). Each of the logging units are bounded by dip discontinuities interpreted as angular unconformities. There is also an angular unconformity within logging Unit III but not a sufficient change in log character to require division of the logging unit. Overall logging Unit I is interpreted to be slope basin deposits. Logging Units II and III are interpreted as thick basin fill dominated by repeating turbidite deposits, as seen clearly on the log responses, particularly the gamma ray log. Within logging Unit II are two zones of particular interest defined by changes in the log responses. Zone A (218.1–400.4 m LSF) is interpreted as a gas hydrate–bearing zone based on the resistivity profiles, with the gas hydrates concentrated in the sandy horizons of the turbidite deposits. Zone B (481.6–547.1 m LSF) is interpreted as a potential gas-bearing interval, again within the sandy horizons of the turbidite deposits. The presence of gas is based on the sonic log response across the zone. Logging Unit III is a homogeneous clay-rich interval of mudstone, immediately overlying the top of the older accretionary prism section that forms basement to the basin at 936 m LSF. Logging Unit IV, from 936 to 1401 m, corresponds with the accretionary prism imaged on the seismic profiles, with a distinct change to much more variable responses in virtually all of the logs.

Physical properties

Physical properties in Hole C0002A show very different behaviors for different lithologies, which are classified based not only on the main logging units (slope sediment units and underlying prism) but also on different stratigraphic zones identified. Almost all physical properties are relatively well determined in the slope sediments, whereas they exhibit wide ranges of scatter in the underlying prism section, probably a result of severe borehole instabilities. However, properties not sensitive to hole condition (e.g., bit and deep resistivity) suggest that the accretionary prism is denser and more compacted compared to the forearc basin (Fig. F11). The lower part of the forearc basin below 400 m LSF at the BSR is characterized by physical properties that suggest a relative undercompaction: slightly decreasing or nearly constant resistivity with depth while velocity increase is insignificant, and changes in density and porosity are relatively limited. This implies a possibility of some level of overpressure in this zone. Above the BSR (Zone A), a signature of gas hydrate is indicated by a strongly increasing overall resistivity trend with depth, with numerous local spikes of high resistivity, while changes in neutron porosity and density are minor. The gas hydrate–bearing formations are characterized by a unique velocity and resistivity relation because of a significant increase in resistivity but a muted increase in velocity with depth.

Structural geology and geomechanics

Structure is distinct between the forearc basin and underlying accretionary prism at Site C0002 (Fig. F11). However, the orientations of bedding planes and borehole breakouts are consistent (northeast–southwest and northwest–southeast, respectively) throughout the entire borehole. Bedding dips are shallow (<10°–15°) in the forearc basin with some variation in dip direction with depth, which agrees well with seismic reflection data. Dips increase toward and within logging Unit IV (~30°–60°), the older accretionary prism. Natural fractures are rarer than at Site C0001 and have scattered orientations within the basin but have a more consistent northeast–southwest trend within the prism. All fractures dip between 30° and 85° with no significant difference between basin and prism. Basin fractures have three dominant trends: northeast–southwest, east–west, and northwest–southeast. All three orientations can be identified in seismic data. A few basin fractures offset bedding in a normal sense, consistent with the seismic-scale normal faulting. Resistivity of the fractures varies but includes several wide aperture (10–30 cm) and highly resistive (mineralized or cemented) fractures in the prism. Logging Unit III, at the basin–prism transition, is characterized by increasing bedding dips and a fractured base. Borehole breakouts indicate a northeast–southwest oriented S_Hmax, perpendicular to that at Site C0001. Breakout azimuth gradually rotates clockwise with depth and breakout width increases in the prism relative to the basin. Stress magnitude determined from breakout width is consistent with normal faulting in the basin, as observed, but more ambiguous, in the prism. Fracture and fault orientations, breakouts, and state of stress collectively support margin-normal extension of this part of the forearc. We suggest this is driven by uplift of the megasplay/outer-arc high causing gravitationally driven extension of the inner wedge. This contrasts with Site C0001 in the active prism (outer wedge), where convergence-related compression dominates.

Log-seismic correlation

The portion of the Kumano Basin drilled at Site C0002 includes a series of unconformity-bounded sequences. The shallowest is seismic Unit 2/3/4 undifferentiated, which correlates closely with logging Unit I (Figs. F10, F11). The increase in velocity across this boundary is consistent with a transition from higher porosity hemipelagic sediments to older more compacted Kumano Basin stratigraphy. Seismic Units 5 through uppermost seismic Unit 11 correspond with logging Unit II.

Within logging Unit II there are two zones of interest. Zone A lies above the BSR (~404 m LSF) and shows elevated resistivity and slightly increased velocities. Velocity and density decrease at the BSR, suggesting that the reflection may be generated by the presence of free gas below, rather than by gas hydrate above. At the top of Zone B, a strong, dipping negative polarity reflection within the seismic data correlates with a second region of low velocities. The regional observation that some reflectors brighten in amplitude as they approach the BSR from below, coupled with interpreted sandy turbidite layers based on gamma ray logs of Zone B, supports the suggestion that this reflection may represent sands bearing at least small amounts of free gas.

The top of the accretionary prism is a high-amplitude reflection and a clear boundary in all of the logs. A sequence of low gamma ray values just below the top of the prism suggest that a sandier interval may contribute to this bright reflection.

A complete check shot profile of 72 shots defined a clear velocity profile for the Kumano Basin stratigraphy and drilled portion of the accretionary prism. This curve generally agrees well with the sonic log, with the exception of slightly lower velocities from 550 to 675 m LSF and agrees very well with the PSDM velocities.

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