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doi:10.2204/iodp.proc.338.107.2014

Logging while drilling

LWD and MWD data were collected in Hole C0022A to a total depth of 420.5 mbsf, allowing continuous downhole measurements of in situ properties and downhole drilling parameters. Surface drilling parameters were also recorded. Schlumberger’s TeleScope downhole MWD tool and geoVISION and arcVISION LWD tools were deployed in Hole C0022A (see Fig. F3 in the “Methods” chapter [Strasser et al., 2014a]). Both real-time data and memory data were collected, including annular pressure, gamma ray, resistivity, and azimuthal resistivity images. Overall, data quality was good, with the exception of poor quality of the upper interval (0–49.0 mbsf) of the resistivity images because of wash down. The gamma ray and resistivity logs and resistivity images were interpreted for lithologic and structural features. Porosity and bulk density were calculated using resistivity data, and in situ stress orientations were determined from compressional borehole breakout orientations.

Data quality and processing

Data quality was checked at three points: during data acquisition by monitoring the real-time data, during data processing by validation, and through an inspection of the final, processed data. The Logging Staff Scientist and logging scientists assessed the real-time drilling parameters, rate of penetration, rotations per minute, surface weight on bit, equivalent circulating density, stick-slip, downhole weight on bit, and downhole torque using logging watchdog sheets (see the “Methods” chapter [Strasser et al., 2014a]). The overall quality of the processed logging data was determined to be fair, although somewhat affected by weather conditions during acquisition. No useful downhole temperature data were collected. Because of no rotation during wash down and drilling with low rotations per minute, poor quality resistivity images were recorded above 49 mbsf (2753 m DRF). Sharp horizontal lines, artifacts from ship heave and pipe vibration, were observed throughout the processed resistivity images and make identification of low-angle bedding and features problematic. Missing data because of high stick-slip (>300 cycles/min) were also observed.

The seafloor was confirmed at 2704.0 m DRF (2675.5 m mud depth below sea level [MSL]), based on the natural gamma radiation and resistivity curves extracted from the memory data.

Logging units and lithostratigraphy

Site C0022 is located near the tip of the megasplay fault drilled at Site C0004 and is upslope from Site C0008 (Expedition 314 Scientists, 2009b; Expedition 316 Scientists, 2009a, 2009b). Hole C0022A penetrated slope basin sediment into the very tip of the megasplay. For consistency with previous work (e.g., Expedition 333 Scientists, 2012c; Kimura et al., 2011; Strasser et al., 2011), only one unit composed entirely of slope basin sediment is defined as Unit I (0–420.5 mbsf) in Hole C0022A. The gamma ray log supports this classification, as its character does not change significantly through the entire drilled section and maintains a constant baseline of ~75 gAPI. However, three subunits were identified based on changes in the character of the resistivity logs (Fig. F3).

Subunit IA (0–74.3 mbsf)

Subunit IA extends from the seafloor to 74.3 mbsf, including the 49 mbsf wash down interval. In the upper 7 m, the gamma ray log increases to its baseline of ~75 gAPI. Through the rest of Subunit IA, gamma ray values remain constant, with minor fluctuations (±10 gAPI) around this baseline. Four small (<2 m thick) low gamma ray spikes are observed at 16.2, 40.7, 48.6, and 61.1 mbsf; in all four instances, gamma ray values drop to 59–60 gAPI (Fig. F4). These intervals are interpreted to be thin sand or ash horizons. From the seafloor to ~7 mbsf, resistivity increases to ~0.9 Ωm and exhibits a gradual increase to ~1.1 Ωm through the rest of Subunit IA, with frequent ±0.1 Ωm fluctuations. The base of Subunit IA is placed at 74.3 mbsf, where the character of the resistivity logs changes significantly.

Subunit IB (74.3–212.9 mbsf)

Subunit IB exhibits the most variability in the resistivity logs and contains a highly fractured zone (86.6–105.2 mbsf) identified from the resistivity images and discussed in more detail below (Fig. F3). Throughout Subunit IB, the gamma ray log exhibits repeated decimeter-scale coarsening- and fining-upward cycles (Fig. F4). This subunit also exhibits several strong localized spikes in resistivity and, to a lesser extent, in gamma ray.

Subunit IC (212.9–420.5 mbsf)

The Subunit IB/IC boundary is placed immediately below a prominent low-resistivity spike, marking the end of the higher variability of Subunit IB. Subunit IC extends from 212.9 mbsf to the base of the hole (420.5 mbsf) and is characterized by low variability in resistivity (Fig. F4). Throughout Subunit IC, the gamma ray log exhibits small-scale variations around the baseline and resistivity maintains minor fluctuations (±0.2 Ωm) around a constant baseline (~1.2 Ωm), with the exception of a few prominent spikes. At 254.7 mbsf, resistivity drops to ~0.83 Ωm, with no corresponding change in the gamma ray log. Another resistivity low (~0.7 Ωm) at 266.5 mbsf corresponds to a very low spike (~38.9 gAPI) in the gamma ray log, tentatively interpreted as a sand bed. Between 295 and 299 mbsf, a series of high (>1.6 Ωm) and low (0.7 and 0.4 Ωm) resistivity spikes occur, with no corresponding variations in the gamma ray log. Another low resistivity of 0.69 Ωm occurs at 333.8 mbsf, coincident with a gamma ray low of ~32 gAPI, indicating another possible sand horizon. Between ~300 and 387 mbsf, resistivity exhibits decimeter-scale cycles of gradual increases and decreases around a 1.2 Ωm baseline. From ~387 mbsf to the base of the hole (420.5 mbsf), resistivity returns to a fairly constant value (1.2 Ωm), with only minor fluctuations (±0.1 Ωm), and the gamma ray log shows a gradual increase from ~60 to ≥75 gAPI.

Structural image analysis

Shallow, medium, and deep button resistivity data were used to generate both static and dynamic processed images (see the “Methods” chapter [Strasser et al., 2014a]). Bedding, fractures, faults, and breakouts were recorded. In the absence of a caliper measurement, the bit diameter was used as the borehole diameter and assumed to be constant.

Bedding and fractures

Despite poor quality images because of heave-induced horizontal lines, some bedding and structural features could be identified from the resistivity images. Throughout the entire section, bedding dips are consistently low angle (0°–30°) but exhibit subtle variations with depth (Fig. F3). Bedding dip changes between Subunits IB and IC. Within Subunit IB, the dominant dip direction is to the south-southeast and dip angles predominantly range between 15° and 30°. Below 212.9 mbsf (within Subunit IC), however, the dominant dip direction switches to northwest and dip angles trend shallower (<20°) (Fig. F5).

Fracture orientations are variable throughout the section but are generally high angle (>40°) and primarily dip toward the northwest (Fig. F5), particularly between 86.6 and 105.2 mbsf. This is identified as a highly fractured zone (Fig. F3) and interpreted as the region of the megasplay tip, although no clear fault can be identified. An interval between 170 and 270 mbsf corresponds to a section of particularly low quality resistivity images; therefore, the absence of fractures in this section could be related to image quality rather than to an absence of structure. Below ~320 mbsf, many high-angle resistive fractures can be identified.

Borehole breakouts

In Hole C0022A, few breakouts are observed in the upper 130 m section, with the exception of those breakouts appearing immediately following a pipe connection. Breakouts could be associated with pipe stand connections because of lower annular pressure during connections or because of some time-dependent evolution of breakouts. Breakout occurrence increases with depth, and breakouts are most prevalent in the lower section of logging Subunit IC (Fig. F3). Analyses indicate a mean breakout azimuth of 49.5° and, therefore, an azimuth of 139.5° for the maximum horizontal stress (S_HMAX). This northwest–southeast trend is roughly parallel to the convergent direction of the Philippine Sea plate and to the dominant S_HMAX determined at previous IODP Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) Sites C0001, C0004, C0006, and C0009 (Chang et al., 2010; Lin et al., 2010; Byrne et al., 2009; Kinoshita et al., 2008) and Ocean Drilling Program Site 808 (Ienaga et al., 2006; McNeill et al., 2004).

Physical properties

Estimation of porosity and bulk density from resistivity

Porosity was estimated in Hole C0022A using Archie’s law (Archie, 1947), which describes an empirical relationship between measured resistivity at the bit and porosity (see the “Methods” chapter [Strasser et al., 2014a]). Because no temperature measurements were taken at Site C0022, seawater electrical resistivity was calculated using the temperature profile that was estimated for Hole C0008A during Expedition 316 (Expedition 316 Scientists, 2009b). Site C0008 is located ~600 m southeast of Site C0022 and penetrates lithologically similar sediment, so it should provide a reasonable estimate for temperature. The temperature at the seafloor is estimated at 2.2°C, with an average thermal gradient of 51°C/km (Expedition 316 Scientists, 2009b). Uncertainties in the quality of resistivity measurements taken on cores collected in Hole C0022B meant that data from these cores were not used to estimate the Archie parameters for Hole C0022A (see “Physical properties”). Insufficient data were available from nearby Sites C0004 and C0008 to make reasonable estimates of the Archie parameters. Additionally, estimates of resistivity-derived porosity at IODP Site C0010, which is also closer to Site C0022 than Site C0002, were carried out using parameters nearly identical (a = 1 and m = 2.3) to those estimated at Site C0002 (Expedition 319 Scientists, 2010). Thus, Archie parameters a = 1 and m = 2.4 estimated for Kumano Basin sediment at IODP Site C0002 during Expedition 314 were applied to Hole C0022A for consistency with previous estimations (Expedition 314 Scientists, 2009a). Bulk density was calculated from the resistivity-derived porosity using a grain density (ρ_g) value of 2.70 g/cm³. This grain density value is an average, with a standard deviation of 0.05 g/cm³, of all the moisture and density (MAD)–derived grain densities measured from Hole C0022B (see “Physical properties”).

The resistivity-derived porosity and bulk density depth trends are shown in Figure F3. Porosity is very high in the upper 10 m of the hole, decreasing from ~91% at the seafloor to 63% at 10 mbsf; these large values are caused by bit resistivity measurements that are very close (less than a factor of 2 larger) to the estimated values of seawater resistivity. From 10.0 to 100 mbsf, resistivity-derived porosity decreases in a generally linear fashion to 55%. Porosity remains relatively constant to ~200 mbsf, with two prominent increases in porosity at 121.2 and 187 mbsf, where porosity increases rapidly to 58% and 59%, respectively. From 202.1 mbsf to the base of Hole C0022A (420.3 mbsf), porosity decreases in an approximately linear trend to 48%.

Trends in resistivity-derived bulk density tend to mirror those described above for resistivity-derived porosity (Fig. F3). Generally, bulk density increases from ~1.1 g/cm³ at the seafloor to 1.82 g/cm³ at 121.2 mbsf. Bulk density decreases rapidly to 1.68 g/cm³ at 125.7 mbsf before increasing again to 1.74 g/cm³ at 154 mbsf. From 154 to 202.1 mbsf, bulk density decreases to 1.68 g/cm³. Finally, resistivity-derived bulk density increases to 1.88 g/cm³ at the base of the hole (420.5 mbsf).

Figure F6 shows the resistivity-derived porosity and bulk density logs plotted for comparison along with MAD-derived measurements taken on cores from Hole C0022B. Resistivity-derived porosity and bulk density are generally within the scatter of the MAD-derived values throughout the depth of the hole. However, at ~335 mbsf, the porosity offset (Δϕ = ϕ_MAD – ϕ_resistivity) becomes skewed toward negative values. That is, on average the resistivity-derived values are slightly higher than the MAD-derived values near the base of the hole, although the absolute value of the offset is never greater than |Δϕ| = 0.1, excluding a few outliers.

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