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doi:10.2204/iodp.proc.314315316.114.2009

Discussion and synthesis

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. We were able to drill and log the entire section from 0 to 1401 mbsf with 100% success. An excellent suite of logs and seismic VSP data reveal the structure of the forearc basin, gas hydrate–bearing zone, and the underlying deformed rocks of the inner accretionary prism.

Site C0002 is slated for riser drilling across the entire plate boundary system to >5500 mbsf during later stages of the NanTroSEIZE project. Similar to Site C0001, which is planned for riser drilling to 3500 mbsf, it is important for the installation of the blow-out preventer on the seafloor and a few hundred meters of 20 inch casing pipe to obtain geotechnical properties in the upper sections. In addition, P-wave velocity structure in the prism, which is essential for a precise estimate of the depth to the seismogenic fault zone, has been determined using the 3-D seismic survey (Moore et al., 2007) but still needs to be calibrated using in situ data. Using continuous drilling and logging to 1401 mbsf, we obtained data that can serve to prepare for future riser drilling.

The hole was tagged at 1936 mbsl and was jetted-in to 70.5 m LSF. Approximately 3 days of drilling time (excluding ~0.5 days of wiper trip and other down time) was required to reach TD (1401 m LSF) without any significant difficulties. The average rate of penetration was ~30 m/h. All LWD tool memory data were successfully downloaded. In spite of several major washouts (stand-off > 2 inches) in sand-rich intervals, log data quality is generally good, and image logs in particular are of high quality. These logs provide a rich data set for interpretation of the geology and stress conditions at this site.

Four logging units bounded by dip discontinuities interpreted as angular unconformities were identified. Logging Unit I (0–135.5 m LSF) is the youngest of the slope basin deposits at this site. Logging Unit II (135.5–830.4 m LSF) is interpreted as thick basin fill marked by numerous turbidite deposits, as seen clearly on the log responses, particularly the gamma ray log. Logging Unit III (830.4–935.6 m LSF) is a homogeneous clay-rich interval of mudstone immediately overlying the top of the older accretionary prism section that forms basement to the forearc basin at 936 m LSF. Logging Unit IV (936–1401 m LSF) corresponds to the accretionary prism imaged on the seismic profiles, with a distinct change to much more variable responses in virtually all of the logs.

Kumano forearc basin features

The Kumano forearc basin is composed of logging Units I–III, or 13 seismic units (Kumano 2–13 and Lower Sediments). Bedding dips are shallow (<15°) in the forearc basin and increase downhole toward the base of the section. All fractures dip between 30° and 85° with no significant difference between basin and prism. Basin fractures have three dominant trends: (1) northeast–southwest, (2) east–west, and (3) northwest–southeast. All three orientations can be identified in faults imaged in seismic data. A few basin fractures offset bedding in a normal sense, consistent with the seismic-scale normal faulting.

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 hydrates concentrated in the sandy bases of the turbidite deposits. Velocity and density decrease at the BSR, suggesting that the reflection may be generated by the presence of small amounts of free gas below.

Zone B (481.6–547.1 m LSF) is interpreted as an interval in which the sandy horizons of the turbidite deposits are potentially gas-bearing. At the top of Zone B, a strong, dipping, negative polarity reflection in the seismic data correlates with a low-velocity region in the sonic log. 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 lower part of the forearc basin below 400 m LSF at the BSR is characterized by physical properties that suggest changing lithology, relative undercompaction, or other effects: slightly decreasing or nearly constant resistivity with depth, whereas velocity increase is insignificant and changes in density and porosity are relatively limited.

Logging Unit III, at the basin–prism transition, is a homogeneous clay-rich interval of mudstone and is characterized by increasing bedding dips and a fractured base.

Old accretionary prism features

Logging Unit IV from 936 to 1401 m LSF corresponds with the chaotic reflectivity imaged on the seismic profiles and interpreted as the inner older accretionary prism, with a high-amplitude reflection and a distinct change to much more variable responses in virtually 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.

Physical properties not sensitive to hole condition (e.g., bit and deep resistivity) suggest that the accretionary prism is denser and more compacted than the forearc basin.

Bedding dips increase within logging Unit IV (~30°–60°). Clearly imaged natural fractures are rarer than at Site C0001 and have scattered orientations within the basin but a more consistent northeast–southwest trend within the prism. All fractures dip between 30° and 85° with no significant difference between basin and prism. Resistivity of the fractures is varied, but includes several wide aperture (10–30 cm) and highly resistive (mineralized or cemented) fractures in the prism.

Stress field and tectonics in the Kumano Basin

Whereas the structural styles of the forearc basin and underlying accretionary prism at Site C0002 are different, the orientations of bedding planes and borehole breakouts are consistent, northeast–southwest and northwest–southeast, respectively, throughout the entire borehole.

Borehole breakouts indicate a northeast–southwest oriented SHmax, nearly 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 observed dominance of normal faulting in the basin but is 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 prism. This contrasts with Site C0001 in the active outer prism where convergence-related compression dominates.