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Cuttings-core-log-seismic integration

Site C0002 encompasses 11 holes situated within ~150 m of each other (Fig. F4) and includes riser Hole C0002F, a focal point of the NanTroSEIZE project. Both LWD data and cores have been collected at this site during previous expeditions (Expedition 314 Scientists, 2009; Expedition 315 Scientists, 2009b; Expedition 332 Scientists, 2011).

Figure F115 presents a summary of core, log, and seismic data at Site C0002 along with a summary of the units defined from each of those data sets. The seismic reflection profile shows the major regional features, such as the northwest-dipping Kumano Basin strata (seafloor to ~850 mbsf at Site C0002) that are cross-cut by a prominent BSR (~400 mbsf), a transition zone (~850–920 mbsf), and the seismically chaotic accretionary prism (deeper than ~920 mbsf).

A seismic, lithology, and logging correlation of the gas hydrate zone (~200–550 mbsf) (Expedition 314 Scientists, 2009; Expedition 315 Scientists, 2009b) is revisited here using new data from cores collected between 200 and 500 mbsf (Holes C0002K and C0002L), as this interval was not targeted by previous expeditions. The lack of coherent reflections in the seismic data below ~900 mbsf (i.e., within the highly deformed prism) prevents further seismic correlation with deeper logging data. The logging Unit III/IV boundary, placed at 935.6 mbsf in Hole C0002A (Expedition 314 Scientists, 2009) and at 931 mbsf in Hole C0002G (Expedition 332 Scientists, 2011) is revisited using new core data from Hole C0002J cores and LWD data from riser LWD in Hole C0002F. Integration of core, cuttings, and LWD data within the accretionary prism is also discussed.

The Kumano 3-D prestack depth migration (PSDM) seismic volume (Moore et al., 2009) ties to Hole C0002F at the intersection of In-line 2532 and Cross-line 6228. A number of prominent reflections were identified by Gulick et al. (2010) and compared to the logging unit boundaries identified in Hole C0002A (Expedition 314 Scientists, 2009). Many of the reflections do not tie with large lithologic or physical properties contrasts. The strongest correlations were made between Zones A and B (as defined using LWD data from Expedition 314 Scientists [2009]) within logging Unit II (Fig. F115).

New cores in the gas hydrate zone

The base of Zone A matches the BSR in the seismic data at ~400 mbsf (Fig. F115). This zone was suggested to be the gas hydrate zone because of its high resistivity compared to the rest of Unit II. Additionally, the drop in P-wave velocity at the base of Zone A was postulated to represent the presence of trace amounts of free gas (Expedition 314 Scientists, 2009) that leads to a prominent reversed polarity seismic reflection of the BSR.

The interval from 200 and 505 mbsf was cored continuously in two holes (Hole C0002K: 200–286.5 mbsf; Hole C0002L: 277–505 mbsf). Because of a high concentration of trapped gas, the resulting recovery rate was highly variable (<13% to >100%), and one core liner (486.0–495.5 mbsf) failed explosively. Peaks of methane gas concentration in core headspace and void gas samples from 200 to 400 mbsf (see “Geochemistry”) correspond to high-resistivity spikes at 270, 295, 370, and 390 mbsf (Fig. F115). Higher concentrations of propane were also observed between 200 and 380 mbsf, within Zone A, and a small peak in ethane concentrations was observed at 390–400 mbsf (Fig. F74), which correlates with an interval of high resistivity (>4.5 Ωm) and the depth of the BSR observed in the seismic data. A spike to extremely low salinity/chlorinity values at ~400 mbsf in Hole C0002L (Fig. F71) matches the location of the BSR (Fig. F115). Overall, these observations appear to support the interpretations of the Expedition 314 Scientists (2009).

The upper surface of Zone B corresponds to a high-amplitude, negative polarity reflection (~480 mbsf) (Fig. F115). This zone has low P-wave velocity and highly variable resistivity and was suggested to represent a package of coarser sediment containing small amounts of free gas that is migrating updip (Expedition 314 Scientists, 2009).

Peaks of hydrocarbon gas concentration were observed at 460–470 mbsf (see “Geochemistry”), preceding an exploded core liner from the interval of 486.0–495.5 mbsf, suggesting that the upper boundary of Zone B, a gas-rich zone correlating with a negative polarity reflection (Fig. F115), had been reached. A second negative polarity reflection is observed at this depth to the southeast of Hole C0002F. There is also increased variation in resistivity within this zone, which may represent another pocket of gas trapped beneath the dipping reflection.

Logging curves below ~900 mbsf, hole enlargement riserless) versus mud cake (riser)

It was noted that in Hole C0002A, the gamma ray curve matches the borehole shape below ~935 mbsf with low gamma ray values occurring where the borehole diameter increased (Fig. F116) (Expedition 314 Scientists, 2009). A correlation was made between borehole enlargement, the presence of cohesionless sediment (i.e., sandy), and low gamma ray values. A departure to low gamma ray values also occurs in Hole C0002G at ~931 mbsf (Expedition 332 Scientists, 2011). In the same depth interval in Hole C0002F, on the other hand, an increase in gamma ray values from ~70 gAPI at 920 mbsf to ~80 gAPI occurs at 940 mbsf. A similar increase was observed in both the MSCL-W NGR results from Hole C0002J core samples and Hole C0002F cuttings (see also “Physical properties”).

Although no direct caliper measurement was made during Expedition 338, Hole C0002F was drilled in riser mode, and as a result, the hole is generally expected to be more stable than previous riserless operations at this site. A comparison of resistivity images in the region of overlap in both Holes C0002F and C0002A confirms that the borehole wall is generally in better condition in Hole C0002F (Expedition 314 Scientists, 2009) (see “Logging while drilling”). The attenuation distance for gamma radiation (1 MeV energy) through water is ~10 cm (4 inches) (Hubbell and Seltzer, 1996). Therefore, the low gamma ray values obtained below ~920 mbsf in Holes C0002A and C0002G are probably due to a reduction in natural gamma ray emissions reaching the detector. For example, the 6¾ inch diameter geoVISION tool used during Expedition 314 has an operational borehole size range of 8.00–9.88 inches (e.g., Schlumberger, 2011), and as the borehole diameter increases, especially beyond the operational maximum, the gamma ray intensities recorded will be damped with respect to measurements made directly adjacent to the formation.

For this reason, the composite LWD data curves in Figure F115 comprise Hole C0002A data from 0 to 900 mbsf and Hole C0002F data below 900 mbsf. Note that neither the location of the Unit III/IV boundary nor the increase in the abundance of sandy material below the boundary are disputed; rather the interpretation of riserless gamma ray logs is challenged, especially where borehole diameter is unknown.

Cuttings and log integration in the accretionary prism

The Unit III/IV boundary determined based on both core lithology (Holes C0002B and C0002J) and LWD (Holes C0002F and C0002G) data is located between 918.5 and 935.6 mbsf. The mismatches between the depths of this boundary interpreted in different holes reflect the irregular nature of this unconformable boundary between the Kumano Basin sediment and underlying, older accretionary prism. Detailed mapping of this surface in the 3-D seismic data shows that the boundary is highly variable in three dimensions (G. Moore, unpubl. data).

Unit and subunit boundaries identified from cuttings analyses (Hole C0002F) are consistently deeper (by ~100 m) than those identified from the logging data (Fig. F115). The population of cuttings sampled at each depth reflect the mixing experienced both during transport in the return mud flow up the riser pipe and on the shale shaker, and for this reason lithologic cuttings boundaries should be defined by the first downhole occurrence of marker features from the underlying unit (see “Lithology”).