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doi:10.2204/iodp.pr.314.2008

Site summaries (continued)

Site C0004

As an alternative to Site C0003, which was unsuccessful in reaching the megasplay primary target, Site C0004 (proposed Site NT2-01I) was drilled at a location where the megasplay is only at ~280 mbsf (Figs. F14, F15). To test drilling conditions prior to risking the LWD/MWD tool string, Hole C0004A was drilled without any of the LWD/MWD tools to the planned TD of 400 mbsf, and then Hole C0004B was drilled with the full LWD tool string except adnVISION (neutron porosity/density). The drilling was smooth and reached TD quickly, and data quality was good, particularly for sonic velocity.

Operations

Hole C0004A (pilot hole) was spudded at 0445 h on 1 November 2007 and jetted down from the seafloor to 75.5 m LSF before starting rotary drilling. Without any drilling problems, the hole reached the 400 m TD at 1845 h. The drill string was made up in preparation for LWD-MWD-APWD drilling in Hole C0004B from 0515 h to 0800 h on 2 November. As with the previous site, the BHA was made up with resistivity imaging, sonic, power pulse–annular pressure, seismic, and Azimuthal Density Neutron (ADN) tools, but this time the ADN tool did not include a radioactive source and thus provided only ultrasonic caliper and not density/porosity measurements (Fig. F16). Drilling progressed with an average rate of penetration (ROP) of ~35 m/h to TD (400 m LSF; 3066 m DRF) at 1230 h on 3 November without any cause for concern. All LWD tools were recovered on the rig floor at 1800 h on 3 November, and all memory data were successfully downloaded. Hole conditions were generally poor but had little impact on overall data quality. Logging data quality is good, even for the sonic data. Because of the high ROP, resistivity image logs show some pixel effects that are easily removed by filtering and postprocessing. No localized depth shifts have been observed for Hole C0004B.

Log characterization and lithologic interpretation

Three logging units were defined based on the different trends and character of the LWD log responses (Fig. F16). Logging Unit I (0–67.9 m LSF) is characterized by variable gamma radiation and low resistivity and is interpreted as slope basin deposits. Logging Unit II (67.9–323.8 m LSF) was defined for the thrust sheet and associated complexes and divided into four subunits. Logging Subunit IIA is characterized by increasing gamma radiation, increasing resistivity baselines, and increasing velocity and is interpreted as a chaotic or deformed sediments, possibly gravitational in origin. Logging Subunit IIB is characterized by continuous high-frequency fluctuation in gamma radiation, constant resistivity baseline, and increasing velocity. Logging Subunit IIC is characterized by cyclic changes in gamma radiation, variable resistivity, and again increasing velocity. Logging Subunit IID is characterized by decreasing gamma radiation and repeated intervals of decreasing resistivity and velocity. The upper part of this subunit is strongly deformed and the lower part is weakly deformed. The base of logging Unit II is interpreted as a weakly localized deformation zone. Logging Unit III (323.8 m LSF to TD) is characterized by decreasing gamma radiation, variable resistivity, and relatively constant sonic velocity and is interpreted as sediments underthrust beneath the splay fault.

Physical properties

Given the lack of a neutron log, porosity and density were estimated using five different resistivity logs (ring; bit; and deep, medium, and shallow button) and the sonic velocity obtained from the traveltime (DTCO) measurements (Fig. F16). No radioactive source was used at Site C0004, and accordingly neither the neutron porosity (TNPH) nor the bulk density (RHOB) logs were available. The analyzed physical properties were compared with identified fracture zones (see "Structural geology and geomechanics"). The shallow button resistivity is significantly lower than the other two button readings, most likely a sign of enlarged hole conditions. Porosity, calculated from bit and ring resistivity corrected with the estimated temperature profile, shows a slowly decreasing trend with depth below the older prism. There appears to be some weak relationship between porosity changes and fracture zones. High ring resistivity porosity seems to coincide more clearly with such intervals, even if the relation is not systematic. The sonic P-wave velocity log is of good quality and its response appears to reflect well the effects of fracture zones and logging units, identifiable as low-velocity zones.

Structural geology and geomechanics

Three structural domains were defined by structural characteristics, including fractures, breakouts, and texture in the borehole images. Structural Domain 1 (0–96 m LSF) is characterized by a lack of fractures, weak breakouts, and little variation in the sediments (Fig. F16). Structural Domain 2 (96–292 m LSF) includes heavily or moderately fractured conductive zones and intensive borehole breakouts. Structural Domain 3 (292–396 m LSF) has few fractures and narrower breakouts relative to the underlying domains. Bedding planes in structural Domain 1 are consistent and mostly strike northeast–southwest and dip 30°–40° to the south. The beds in structural Domain 2 are more scattered both in dip and azimuth but generally trend northeast–southwest and dip at 20°–70° to the north. Structural Domain 3 shows similar bed trend to Domain 2 (northeast–southwest) but the dips are generally gentler to the north. Most fractures we identified are conductive and only found in Domains 2 and 3. Fractures in structural Domain 2 are scattered both in trend and dip but with a dominant trend of northeast–southwest, steeply dipping to the north. Fractures in structural Domain 3 show northeast–southwest trend and gentler dip (10°–20°) to the north. Fractured zones were defined by intense development of fractures and wide conductive breakouts and were classified as "major" or "minor." In structural Domain 2, three major fractured zones and four minor fractured zones are identified, whereas structural Domain 3 includes a minor fractured zone.

Borehole breakouts indicate a consistent northeast–southwest orientated S_Hmax throughout the borehole, which is between the S_Hmax direction at Site C0001 and the convergence direction. Breakout width is narrow in Domain 1 and wide in Domain 2, then slightly reduced again in Domain 3. Stress magnitude was analyzed from breakout widths, but stress regime is unclear because of uncertainty in rock strength. Domain 1 consists of logging Units I and IIA, which could be interpreted as two sedimentation stages of slope deposits because of small changes in physical properties. Deformation characteristics at the boundary of structural Domains 2 and 3 suggest a narrow (few meters) transition zone between the hanging wall and footwall of the main thrust. The structural features identified in the borehole images, bedding, and fractured zones are well correlated with the structural style in the reflection seismic profiles and are compatible with convergence related deformation.

Log-seismic correlation

The base of the slope sediment section corresponds to the logging Unit I/II boundary (Figs. F14, F16). The sonic velocity increases at the boundary from little more than drilling fluid velocity to about 1600 m/s. There are similar increases in the gamma ray and resistivity logs. The transition in each of the logs is gradual rather than abrupt. These gradual changes likely account for the relatively low frequency character of the reflection at the base of the slope basin sediments.

Logging Subunit IID corresponds to a thick zone of roughly parallel northwest-dipping reflections interpreted as the signature of a system of faults along which accretionary prism rocks have been thrust over slope basin sediments. The seismic reflections on both the inline and cross-line adjacent to the hole show considerable variation on a scale of 50–100 m. Thus, it is not reasonable to expect an exact correlation between log values in a single hole and the seismic data that smear the image laterally on a scale of 20–40 m.

The velocity in the upper half of the dipping reflection package decreases from ~1900 to ~1800 m/s over the range from 243 to 291 m LSF. The sonic log begins a significant increase in interval velocity, from ~1900 to 2100 m/s at 291 m LSF. Velocity remains high to ~313 m LSF before dropping back to ~2000 m/s. This high velocity corresponds to the base of the broad trough and the top of the basal peak of the dipping reflection section. There is a thin layer with dramatically lower velocity at ~306 m LSF that is within the basal bright peak of the dipping sequence. The nearly flat horizons below have velocities varying between 2000 and 2100 m/s and form a series of bright peaks and troughs.

Usable check shot data were acquired at 21 depths in Hole C0004B that provided reasonable velocity information. Beyond the general increase of interval velocity from 1500 m/s at the seafloor to 2100 m/s at 400 m, there is not an exact match between the check shot velocity curve and the sonic log values.

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