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doi:10.2204/iodp.pr.348.2014

Logging

Log data acquisition

The LWD data included natural gamma ray, electrical resistivity logs, electrical resistivity images, sonic velocity, and sonic caliper logs (Fig. F18A, F18B). These data were collected together with MWD data from 859.5 to 2329.3 mbsf (2827.0–4296.8 mBRT [meters below rotary table]) in Hole C0002N and from 2162.5 to 3058.5 mbsf (4130–5026 mBRT) in Hole C0002P. Data from the two runs overlapped between 1962.6 and 2008.5 mbsf (between 3930.2 and 3976 mBRT). In Hole C0002P, LWD data were collected after coring was conducted from 2163 to 2218.5 mbsf (4130.5–4186 mBRT). The cored interval was logged as the borehole was reamed and enlarged from 10⅝ to 12¼ inch diameter, and then the formation below was logged as the borehole was newly drilled.

There were four major occurrences of long exposure times during the drilling of Hole C0002N (1205–1221, 1662–1678, 1992–2008, and 2022–2038 mbsf). Resistivity values at these depth ranges are both noisy and anomalously low. In Hole C0002P, resistivity data in the cored interval show possible signs of minor mud filtrate invasion and/or wellbore failure.

Hole C0002N

Log data characterization and interpretations

Log Units I–III were identified during IODP Expeditions 314 and 332 for Hole C0002A and C0002G (Expedition 314 Scientists, 2009; Expedition 332 Scientists, 2011). During Expedition 338, Hole C0002F was drilled (Strasser et al., 2014b), and the bottom of Unit III was identified, which was also the first unit drilled during Expedition 348 in Hole C0002N (Fig. F18A).

Log Unit III (859.5–915.0 mbsf) (Fig. F19) has an average gamma ray value of 61.6 gAPI and shows a general trend of gamma ray increase of ~20 gAPI down to the basal boundary at 915.0 mbsf. There, a drop of ~25 gAPI is interpreted as a change in lithology from clay-dominated sediment at the base of Unit III to sandy hemipelagic sediment at the top of Unit IV.

Gamma ray and resistivity data show the largest variability in log Unit IV, with average gamma ray values of 66.5 gAPI. We defined five subunits based on overall trends in log response and comparison with the subunits defined during Expedition 338 in Hole C0002F. The base of Unit IV shows a sharp change in the average natural radioactivity to higher values.

Log Unit V (1656.3–total depth) is interpreted to be homogeneous and clay rich overall based on the relatively small fluctuation of log responses and the uniformly high gamma ray values, in agreement with the descriptions of core cuttings lithologies (see “Lithology”). Three subunits were defined in Unit V based on variations in log responses that reflect the amount of silt and (occasionally) sand. Gamma ray values average around 86 gAPI throughout Unit V, with variations of up to 28 gAPI. The log Unit IV/V boundary at 1656.3 mbsf is marked by a shift of ~20 gAPI in the gamma ray data and a local spike in resistivity.

Correlation with previous Site C0002 LWD data

We correlated LWD data from Hole C0002F with Hole C0002N, with a vertical offset (depth shift) of ~16 m. Different tools were used, however, leading to potential differences in data quality, resolution, and accuracy among logs. The comparison and correlation is based on natural gamma radioactivity and resistivity. Previously existing resistivity images, including bedding and structural interpretation, were only available for Holes C0002A and C0002F. Comparison with preliminary analysis of Hole C0002P images confirms that log Units IV and V are highly deformed. The log Unit III/IV boundary was correlated between Holes C0002A and C0002F (Fig. F20). Measurements of bedding in both holes suggest that the boundary between Units III and IV is an unconformity, as reported previously (Moore et al., 2013). Because Unit IV has more variability in log response, due to the complex geology and relatively variable lithology (from predominantly claystone to silt and sand), it was difficult to match in detail, but overall trends were consistent in all data sets. The top of logging Unit V is a very sharp boundary for both Holes C0002N and C0002F. Both holes are marked by a ~20 gAPI shift in gamma ray values and changes in resistivity response. The boundary is offset by 18.3 m between the two holes. Image logs reveal a heavily deformed section around this boundary, suggesting that the offset may be structurally related.

Hole C0002P

Log data characterization and interpretation

No major or abrupt change was identified in the log data acquired over the Hole C0002P interval; therefore, changes in trends of log response and values are interpreted as relatively minor in terms of lithology. Only subunits, likely caused by tectonic structure rather than lithologic variability, were defined within this interval of logging Unit V.

From the top to the bottom of the section, LWD tools recorded an increasing trend in gamma radiation (75–95 gAPI), with decreasing resistivity followed by an increasing trend toward the bottom of the borehole. Compressional acoustic slowness/velocity shows constant or decreasing velocity from top to bottom. Within shorter intervals, the overall decrease in velocity is the result of discrete steps in velocity (Fig. F21). The most prominent drops in velocity can be identified toward the bottom of logging Subunit Ve.

The logging subunit boundaries we define in Hole C0002P are based on the log character and relative values for the same depth intervals and in the absence of significant compositional changes described from cuttings (see “Lithology”). We interpreted the background lithology as hemipelagic silty claystone with relatively constant gamma ray values. Variations in the overall trend seen as excursions and spikes in log values were interpreted as permeable sands, cemented sands or veins, and potential ash layers. The depth intervals for logging subunits are shown on Table T3 together with the average, minimum, and maximum values of gamma radioactivity, resistivity, and acoustic slowness/velocity.

Resistivity images interpretation

Bedding and structures

Bedding orientations could be measured and characterized throughout the logged interval, with gaps in coverage only at strongly deformed zones where bedding planes are not recognizable in the image logs. Bedding dips predominantly steeply to the northwest (varying from 60° to 90°) (Fig. F22). Locally, south- to southeast-dipping beds are also noted, especially between 2600 and 2750 mbsf. Due to the severity of dip changes and high density of fractures and faults, we interpret this section as strongly structurally deformed. A decrease in dip angle was observed from 2860 mbsf (very steep, ~90°) to 3040 mbsf (~60°) at the bottom of the logged section.

The overall structure defined by bedding and structure interpretation suggests that the drilled interval intersects several faulted blocks. Fractures and faults are generally steeply dipping with a range of dips between ~30° and 90° and a wide range of orientations. Fracture density varies with depth, with the highest concentration in the lower section of logging Subunit Vd. Numerous faults with minimal throw or microfaults were also observed.

Wellbore failures

The most obvious and prominent wellbore failures were concentrated in the upper 67 m of Hole C0002P, where the borehole experienced a long exposure time from previous drilling and coring (Fig. F23). From 2150 to 2163 mbsf, the borehole is nearly washed out, although resistivity images indicate a preferential development of northwest–southeast breakouts. This section was drilled before coring took place and was opened to a greater width during LWD recording. Breakout widths (defined as the full angular width of one breakout of the pair) average 95° and reach a maximum of 140°. In the cored section between 2163 and 2218.5 mbsf, clear continuous breakouts were observed in the same northwest–southeast direction as higher in the hole but with moderate angular widths (average = 70°). Here, a borehole cross-section from sonic caliper data showed enlargement of borehole diameter in an orientation consistent with the breakouts identified in the image log. For the upper cored section (shallower than 2218.5 mbsf), the length-weighted average value of the breakout azimuths is in the N35°W/S35°E direction. This suggests that the direction of the maximum horizontal principal stress (S_Hmax) in the upper 68.5 m of the imaged well is in the northeast–southwest direction.

Below 2163 mbsf, the occurrences of wellbore failures were sparse, and their widths are much smaller (average = 23°) compared to the depth ranges above. With the exception of several features identified shallower than 2600 mbsf, it was not possible to conclusively classify these features as breakouts even with the aid of sonic caliper data, although their orientations are consistent with those observed at the cored section of the well. Some narrow wellbore failures observed toward the bottom of the well appear different from the breakouts above and may possibly be drilling-induced tensile fractures.

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