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doi:10.2204/iodp.proc.338.103.2014

Physical properties

At Site C0002, physical properties measurements were performed on unconsolidated to slightly consolidated sediment from Holes C0002K and C0002L, consolidated mudstone/sandstone samples from Holes C0002H and C0002J, and cuttings from Hole C0002F. These data provide essential material characterizations for lithologic unit discriminations and their corresponding consolidation states. Determination of physical properties on cores and cuttings also helps calibration as well as correlation with LWD data (see “Logging while drilling” and “Cuttings-core-log-seismic integration”).

MAD measurements were conducted on both cores and cuttings. Thermal conductivity was measured on whole-round cores of soft sediment from Holes C0002K and C0002L using a full-space needle probe, whereas it was measured on the working halves from Holes C0002H and C0002J using a half-space probe. Electrical resistivity was measured on soft-sediment cores from Holes C0002K and C0002L with a four-pin electrode array inserted directly into the working half. Where sediment was too consolidated (Holes C0002H and C0002J), discrete samples were taken from the working half for P-wave and electrical resistivity measurements. Vane shear and penetrometer measurements were made on soft-sediment cores from Holes C0002K and C0002L, and discrete samples were taken for unconfined compressive strength (UCS) measurements for consolidated cores from Holes C0002H and C0002J. Two LOTs were performed at 872.5 mbsf in Hole C0002F.

Whole-round multisensor core logger

Whole-round cores from Holes C0002H and C0002J–C0002L were analyzed using the whole-round multisensor core logger (MSCL-W). The results of gamma ray attenuation (GRA) density, magnetic susceptibility, natural gamma radiation (NGR), and electrical resistivity measurements (see the “Methods” chapter [Strasser et al., 2014a]) on whole-round cores are summarized with Expedition 315 data (Fig. F85) (Expedition 315 Scientists, 2009b). MSCL-W P-wave measurements are not presented because they exhibit an extreme amount of noise because of poor contact between liner and sediment.

MSCL-W data collected during Expeditions 315 and 338 provide a continuous record for the forearc basin sediment. GRA density increases with depth. In lithologic Unit II, magnetic susceptibility and NGR increase with depth above 450 mbsf; below 450 mbsf both decrease with depth. Electrical resistivity also shows a similar but less pronounced trend.

The diameter of the core (called “core thickness” and equal to the liner wall plus the core thickness; see the “Methods” chapter [Strasser et al., 2014a]) is usually measured by a linear variable differential transformer located 42 cm from the zero-distance reference where the P-wave transducer is also located. During the multisensor core logger (MSCL) runs on Cores 338-C0002H-1R and 2R, the core thickness was accidentally measured with an offset of 2 cm compared to the P-wave traveltime measurement. A 40 cm long core liner filled with water was used to estimate errors associated with the offset (Table T38). The errors in thickness are mostly within 1% except at the ends of the core liner. The errors at the ends were probably caused by end caps and vinyl tape wrapping the liner and caps. This offset of 2 cm in core thickness measurements thus has little effect on the measured values of P-wave velocity (V_P), GRA density, and magnetic susceptibility. However, the offset error has been corrected on V_P for Cores 338-C0002H-1R and 2R.

Moisture and density measurements (cores)

A total of 355 discrete samples from Holes C0002H and C0002J–C0002L were measured for MAD. All MAD data from Expedition 338 cores are summarized in Table T39 and Figure F86. Between 200 and 502 mbsf in Holes C0002K and C0002L, bulk density ranges from 1.47 to 2.08 g/cm³, grain density ranges from 2.40 to 2.96 g/cm³, and porosity ranges from 37% to 74%. Both bulk density and porosity show less scatter with an increase in depth. This probably corresponds to a decrease in the number of sand layers with depth (see “Lithology”). Sandy samples yield lower porosity and higher bulk density. Between 902 and 1113 mbsf in Holes C0002H and C0002J, bulk density ranges from 1.83 to 2.15 g/cm³, grain density ranges from 2.54 to 2.84 g/cm³, and porosity ranges from 35% to 52%.

Thermal conductivity (cores)

Thermal conductivity was measured on whole-round cores from Holes C0002K and C0002L using a needle probe sensor and measured on working-half cores from Holes C0002H and C0002J. All data are shown with data from Expedition 315 Holes C0002B–C0002D in Figure F87 (Expedition 315 Scientists, 2009b). Thermal conductivity ranges from 0.74 to 1.40 W/(m·K) in Holes C0002K and C0002L between 200 and 502 mbsf and from 1.11 to 2.19 W/(m·K) in Holes C0002H and C0002J. Thermal conductivity increases linearly with depth to ~550 mbsf, whereas it shifted to values of 1.50 W/(m·K) and slightly increases through lithologic Units III and IV.

Ultrasonic P-wave velocity and electrical resistivity (cores)

A total of 11 discrete cube samples from Holes C0002H and C0002J (2 samples from Hole C0002H and 9 samples from Hole C0002J) were analyzed for electrical resistivity and V_P along three orthogonal directions (x, y, and z). The results of electrical resistivity and V_P are summarized in Tables T40 and T41 and Figure F88.

Electrical resistivity ranges from 1.28 to 3.32 Ωm. All samples except three (Samples 338-C0002H-1R-1, 13 cm, 338-C0002J-1R-1, 45 cm, and 2R-1, 53 cm) record an anisotropy such that electrical resistivity in the vertical z-direction is higher than that in the horizontal x- or y-direction because of the bedding oriented within the x-y plane (Fig. F88). Resistivity is usually the lowest along the bedding plane in sedimentary rocks because of better pore connectivity. Vertical anisotropy is between 6.6% and 48.6% with negative values, except for the three cubes that probably have a bedding aligned within x-z or y-z planes. The measurements with bedding within the x-y plane present a transverse anisotropy (i.e., lineation) with a horizontal anisotropy from 0.2% (quasi-isotropic) up to 50.3% (strong lineation). V_P ranges from 1.923 to 2.307 km/s. The horizontal and vertical anisotropies range from 0.4% to 5.2% in Hole C0002H and from 1.9% to 9.5% in Hole C0002J. The higher vertical anisotropy of electrical resistivity and V_P suggests dominantly gravitational-driven porosity reduction at the base of the Kumano Basin.

Between 200 and 503 mbsf (Holes C0002K and C0002L), a total of 428 electrical resistivity measurements were conducted on working-half cores using the Wenner four-pin array probe for soft sediment. Each measurement was recorded in the dominant lithology types per section to evaluate the general resistivity of the mud, silty mud, and sand as well as their textures such as consolidated or soupy. The results of electrical resistivity from Holes C0002K and C0002L are summarized in Table T42 and Figure F89.

Electrical resistivity ranges from 0.037 to 7.56 Ωm with an average of 0.93 Ωm through lithologic Unit II. Resistivity increases with depth associated with porosity loss (Fig. F89). Sandy samples have higher electrical resistivity (~1.2 Ωm) than muddy sediment (~0.9 Ωm). This observation may reflect the higher porosity of the muddy sediment in combination with a strong electrical clay-bound water effect that leads to lower resistivity and may also be influenced by the high electrical surface conduction along the extended clay surfaces compared to quartz or other lithic minerals. Soupy sand and soupy mud sediments show much lower resistivity of 0.86 and 0.40 Ωm, respectively, and the rare ash layers show a resistivity of 0.95 Ωm. Although the resistivity of ash layers is similar to that of mud, ash layers systematically have a nonnegligible increase of resistivity toward their base, often marked by a much whiter colored thin layer.

The formation factor (F), cementation factor (m), and pore network tortuosity (τ) are calculated based on Archie’s law using resistivity and MAD:

where

• R₀ = resistivity of the formation water–saturated sample;
• R_W = resistivity of the formation pore fluid;
• m = –(log[F]/log[ϕ]), where ϕ is the fractional porosity of the rock; and
• τ = Fϕ also defined as ϕ^{1 – m}.

The MAD porosity is chosen at the closest to the resistivity measurement location. These parameters are essential to calibrate the LWD electrical resistivity logs for water and porosity estimates. The derived Archie’s parameters are summarized in Table T42 and Figure F90.

The formation factor ranges from 1.05 to 5 with some outliers up to 28.21 (Table T42; Fig. F90). The average formation factor is 3.68 and is close to the typical values found during previous expeditions (ODP Legs 131 and 196; Shipboard Scientific Party, 1991; Bourlange et al., 2003). The cementation factor (Archie’s m exponent) is ~1.78 with maximum values up to 5.02. The pore network tortuosity parameters of Archie’s law are 7.53. The values are higher in mud-rich sediment, often defined by higher porosity, and lower in less porous sand-rich sediment.

The relationship between the cementation factor and electrical resistivity is independent of lithology at least at the first order (Fig. F91). As previously observed, lower values of m as well as resistivity are found in soupy units, intermediate values are found in muddy lithologies, and higher values are found in sandy units along this trend.

Similar trends are observed in the MSCL-W resistivity data (Fig. F92). MSCL-W resistivity shows a progressive increase from 200 to 320 mbsf and stays constant below 320 mbsf. Note that MSCL-W resistivity data from Holes C0002K and C0002L are higher than electrical resistivity measurements on working-half cores. The difference is ~0.65 Ωm at shallower depth and decreases with depth to 0.2 Ωm. This difference is possibly because the correct values of standard seawater resistivity were not obtained for the Wenner probe because of the use of a small container when measurements on cores from Sites C0002 and C0022 were conducted. Unexpectedly low-resistivity values obtained for cores at Sites C0002 and C0022 are probably due to overestimation of seawater impedance. This problem was resolved when resistivity was measured on cores from Site C0021 by using a larger container of seawater (see also “Physical properties” in the “Site C0021” chapter [Strasser et al., 2014b]).

Shear strength (working halves)

Shear strength measurements using a vane shear device and a pocket penetrometer (see the “Methods” chapter [Strasser et al., 2014a]) were made on Hole C0002K and C0002L working halves from 200 to 500 mbsf. One measurement for each method was made per core. Penetrometer measurements range from 55 to 255 kPa, and vane shear measurements range from 24 to 158 kPa and are consistently lower than the penetrometer measurements (Fig. F93; Table T43). There is considerable scatter in the data, which increases with depth. One trend that may be observed is an increase in the maximum penetrometer measurements with depth, which generally correspond to the maximum values measured during Expedition 315 (Expedition 315 Scientists, 2009b) (Fig. F93).

Unconfined compressive strength (discrete cores)

Unconfined compression tests (see the “Methods” chapter [Strasser et al., 2014a]) were conducted on four samples (1 cm × 1 cm × 2 cm each) that were subsampled from cubic samples (2 cm × 2 cm × 2 cm) used for V_P and electrical resistivity measurements at 1111 mbsf in Hole C0002H and nine samples that were subsampled from 902 to 912 mbsf in Hole C0002J. A vertical load is applied along the long axis of the sample, which is parallel to the core axis. UCS (maximum force per unit area) ranges between 1.5 and 9.7 MPa with an average of 6.9 MPa (Fig. F93). Two measurements made at 907 mbsf yielded noticeably lower UCS values (1.5 and 2.9 MPa); however, both samples may have been fractured before testing. The variability in UCS data is probably due to sample heterogeneity, sample size, control of loading rate (manual versus computer control), and sensitivity of the load cell (±0.02 kN for the load cell on the Chikyu).

Color spectroscopy (archive halves)

The results of color reflectance measurements using the color spectroscopy logger (MSCL-C) are summarized in Figure F94. L*, a*, and b* values (see the “Methods” chapter [Strasser et al., 2014a]) show no significant difference from those obtained from Holes C0002B–C0002D cores during Expedition 315 (Expedition 315 Scientists, 2009b). L* ranges mainly from 29 to 52, a* ranges mainly from –3.4 to 2.5, and b* ranges mainly from –0.95 to 4.3 for the entire coring intervals. Higher values in both L* and b* observed in Core 338-C0002L-3X (296–299 mbsf) reflect volcanic fine ash.

Moisture and density (cuttings)

MAD measurements were made on 285 cuttings samples from 875.5 to 2004.5 mbsf to provide detailed characterization of grain density, bulk density, and porosity. The sampling interval was 5 m but was changed to 10 m below 1060.5 mbsf. Samples from the 1–4 mm size fraction were measured from 915.5 to 2004.5 mbsf (n = 125) and samples from the >4 mm size fraction (n = 133) were measured below 875.5 mbsf. Shallow samples (<940.5 mbsf) were primarily used to assess the mixing of cuttings when the bit and the hole opener (underreamer) crossed the transition of the cement at the 20 inch casing shoe (860.2 mbsf) and the formation.

Density and porosity

Measured grain density values for both the 1–4 and >4 mm size fractions maintain close agreement throughout Hole C0002F (Fig. F95A). In addition, grain density values generally show less scatter compared to previous NanTroSEIZE expeditions, which reported values of 2.02–3.5 g/cm³ (Expedition 315 Scientists, 2009b; Expedition 316 Scientists, 2009; Expedition 322 Scientists, 2010a). The good data quality obtained during Expedition 338 may be related to the calm sea conditions necessary for riser drilling operations, providing stable conditions during pycnometer measurements. Also, the high sample volume of 20 cm³ used for MAD measurements of cuttings reduces analytical error.

Grain density measurements above 945.5 mbsf were influenced by cement at the 20 inch casing point. Grain density values range from ~1.87 g/cm³ for the cement to an average of 2.61 g/cm³ for the formation (see “Mixing of cuttings across lithologic and structural boundaries”). From 945.5 to 1050.5 mbsf, grain density is characterized by considerable scatter with values as low as 2.52 g/cm³ and as high as 2.75 g/cm³ (Fig. F95A). Low grain density values suggest that contamination with cement cuttings occurs below 945.5 mbsf. Based on a cement grain density of 1.87 g/cm³ and a formation grain density of 2.61 g/cm³, a cement content of up to 12% can explain the low grain densities between 945.5 and 1050.5 mbsf. The low grain density values may also be explained by abundant wood content, discovered during Expedition 319 in lithologic Unit III (Expedition 319 Scientists, 2010), if a mixing interval of up to 90 m for cuttings is assumed. At this depth interval, relative abundant organic material/wood/lignite was also identified in the >63 µm sand-size fraction from cuttings (see “Lithology”) (Fig. F20). Assuming a wood density of 0.9 g/cm³, a wood content of up to 5% can explain the low grain density values in the 945.5–1050.5 mbsf interval. This wood content is in the range of the 5%–10% content reported by Expedition 319 Scientists (2010). From 1050.5 to 1135.5 mbsf, the scatter in grain density diminishes and grain density decreases slightly to ~2.62 g/cm³. This interval correlates to lithologic Subunit IVA. Grain density values from 1135.5 to 1400.5 mbsf remain relatively constant with an average of ~2.61 g/cm³. A transition zone with increasing grain density exists between 1400.5 and 1550.5 mbsf before grain density resumes a constant value of 2.66 g/cm³ downhole. At 1920.5 mbsf, grain density shifts to a higher value of 2.68 g/cm³.

In contrast to grain density values, bulk density for the two size fractions maintains a close agreement only to ~1500 mbsf. Beneath 1500 mbsf, bulk density values for the 1–4 mm size fraction are consistently lower than for the >4 mm size fraction. Bulk density values generally increase from 1.85 g/cm³ at 945.5 mbsf to 1.91 g/cm³ at 1406 mbsf. This corresponds to a decrease in water content with constant grain density (Fig. F95B). An increase in bulk density to an average of 1.96 g/cm³ occurs from 1406 to 1426 mbsf; bulk density remains generally constant at this value to 1500 mbsf. Below 1500 mbsf, the trends for the two size fractions become more scattered and begin to diverge, while maintaining a similar pattern in changes with depth to 1740.5 mbsf. From 1500 to 1740.5 mbsf, there is a large spike at 1620.5 mbsf, where bulk density reaches 2.07 g/cm³ (1–4 mm size fraction) and 2.21 g/cm³ (>4 mm size fraction). Bulk density values for the two size fractions begin to completely diverge below the lithologic Unit IV/V boundary (1740.5 mbsf). Below 1740.5 mbsf, bulk density of the 1–4 mm size fraction decreases gradually to 1.89 g/cm³, whereas bulk density of the >4 mm size fraction follows a constant average of 2.05 g/cm³ although the data show larger scatter. The difference in bulk density between the 1–4 and >4 mm size fractions may relate to DICAs that are formed by the drilling process. DICAs are more prominent in the smaller cuttings size fraction (see “MAD cuttings data quality” and “Structural geology”).

Similar to the bulk density measurements, measured porosity values for the two size fractions maintain a close agreement to 1500 mbsf and follow similar trends to 1740.5 mbsf. From 1536 to 1740.5 mbsf, the >4 mm size fraction produces porosity values that are consistently lower than the 1–4 mm size fraction, although the two trends continue to correspond in terms of where minimum and maximum values occur. Below 1740.5 mbsf, the two trends diverge. Porosity generally decreases downhole from 48% at 945 mbsf to 40% at 1456 mbsf before increasing slightly to 43% at 1500 mbsf (Fig. F95C). This general decreasing trend is in accordance with normal compaction because of increasing overburden. From 1500 to 1740.5 mbsf, porosity is scattered about 42% (1–4 mm) and 39% (>4 mm), with a large negative spike at 1625.5 mbsf to 37% (1–4 mm) and 29% (>4 mm). The separation by cuttings size beneath 1500 mbsf and the low-porosity zone at ~1625.5 mbsf may be attributed to lithology. Sedimentological observations suggest that sandstone is present in this zone (see “Lithology”). Drying of cuttings surfaces (see the “Methods” chapter [Strasser et al., 2014a]) probably leads to removal of additional water from the interior of the permeable sandstone cuttings, and thus, the porosity is underestimated in this zone. Beneath 1740.5 mbsf, porosity shows contrasting trends for the two size fractions. Porosity of the 1–4 mm size fraction gradually increases from 42% at 1740.5 mbsf to 47% at 2005 mbsf. For the >4 mm size fraction, there is a large amount of scatter in porosity values below 1740.5 mbsf with a minimum of 32% at 1886 mbsf and a maximum of 42% at 1956 mbsf. As shown in the bulk density data, separation of porosity by size fraction is probably due to a larger amount of DICAs in the smaller cuttings size fraction (see “MAD cuttings data quality”).

MAD cuttings data quality

Expedition 338 is the third IODP expedition to use cuttings to characterize physical properties (Expedition 319 Scientists, 2010; Inagaki et al., 2012). Previous expeditions focused on analyzing the 1–4 mm size fraction and reported an overestimation of porosity and a relatively large variation of measured values (Expedition 319 Scientists, 2010; Inagaki et al., 2012). Compared to the MAD results on cores, cuttings show higher porosity and lower bulk density. Possible reasons for this difference include (1) incomplete removal of water from the surface of the cuttings, (2) drilling-induced microcracks, (3) swelling of clay minerals and mechanical expansion during washing and soaking (up to 18 h) in seawater, and (4) residue from drilling mud on the surface of cuttings. The procedure for MAD measurements on cuttings during Expedition 338 was therefore changed based on sensitivity tests of cuttings before cuttings from Hole C0002F were collected. MAD measurements were conducted directly after washing to reduce the effect of swelling and mechanical expansion. Care was taken to completely remove water from the surface (see the “Methods” chapter [Strasser et al., 2014a]).

The size fractions show good agreement between porosity and bulk density above 1500 mbsf with ratios of ~1, suggesting that surface effects do not play a role (Fig. F96). However, the different porosity and bulk density trends between the two cuttings sizes below 1500 mbsf suggest other causes may lead to higher water content in cuttings of the smaller size fraction. Visual observation suggests that cuttings below 1500 mbsf include two types with different induration or strength (see “Structural geology”). The first type consists of hard, intact formation cuttings, which are sometimes characterized by sharp edges. The other type appears stiff but is weaker than the first type; these DICAs may be formed during the drilling and recovery process. The good correlation of grain density values for both 1–4 and >4 mm size fractions suggests that both fractions originate from the same depth (Fig. F96A). The prominent difference between the two size fractions in bulk density and porosity values below 1500 mbsf is probably a result of greater induration of the formation in combination with less formation cuttings (or more DICAs) of 1–4 mm size fraction (see “Structural geology”).

Errors in MAD measurements on cuttings were estimated from a reference compaction curve that is determined based on MAD measurements of both handpicked hard formation cuttings and cores recovered during Expeditions 315 and 338 (Expedition 315 Scientists, 2009b). Hand-picked samples, which are considered as representative formation samples, and DICAs were taken from the >4 mm size fraction from 1700.5 to 2000.5 mbsf at 100 m intervals. Four additional samples, recovered from 1975 and 1982.5 mbsf during the opening of the hole in preparation of casing the borehole, were also measured. A representative porosity-depth model (Athy, 1930) can be constructed for the prism sediment by combining porosity data from the hand-picked cuttings and the cores from previous expeditions (Fig. F97A):

where

• z = depth below seafloor,
• α = 5.15 × 10^–4 is an empirical constant, and
• ϕ₀ = 0.64 (a reference porosity).

The computed porosity can also be used to correct the bulk density by (Fig. F97B)

where

• ρ_cb = corrected bulk density,
• ρ_g = measured grain density, and
• ρ_f = density of the pore fluid (1.024 g/cm³).

The difference between the corrected and the original data set of the >4 mm size fraction is shown in Figure F97C. Measured porosity on cuttings is overestimated by 6% at 940.5 mbsf and 17% at 2004.5 mbsf. The errors in bulk density are 0.14 g/cm³ at 940.5 mbsf and 0.28 g/cm³ at 2004.5 mbsf. The average bulk density of DICAs is 1.93 g/cm³, which is equivalent to the bulk density of the 1–4 mm size fraction in that depth interval. The bulk density of these DICAs is higher than the MAD bulk density at shallower depths (e.g., ~1.85 g/cm³ at 1000 mbsf), which suggests an increase in induration and strength of the DICAs. Based on an average value of 1.93 g/cm³ for the interval from 1800 to 2000.5 mbsf, a DICA content of 47%–83% is necessary to explain the bulk density of the bulk cuttings with >4 mm size fraction (Fig. F98). Underreamer depths with a small fraction of DICAs partly correlate with periods when the bit was off bottom. This corroborates the assumption that the formation of DICAs is related to the RWD process. In summary, the differences in porosity and bulk density between cuttings and cores are caused by mixing of the formation material with DICAs. Drilling-induced microcracks, swelling of clay minerals, mechanical expansion, or residue from drilling mud on the surface of cuttings are second-order effects.

Magnetic susceptibility (cuttings)

Magnetic susceptibility (MS) was measured for comparison with MAD data, NGR measurements, and lithologic descriptions. A total of 299 vacuum-dried cuttings samples from both 1–4 and >4 mm size fractions were measured. Because sample weight varied between the two cuttings sizes as a result of cuttings packing in the sample cylinder, we calculated the mass magnetic susceptibility (MMS) from measured raw data MS (bulk susceptibility):

MMS ranges from 1.03 × 10^–7 to 4.40 × 10^–6 m³/kg above 1050.5 mbsf, probably as a result of the cement that extended to 872.5 mbsf and was mixed into cuttings to 1050.5 mbsf (Fig. F99). However, scatter below the interpreted cement contamination (940.5 mbsf) may also reflect a heterogeneity of lithologic Unit III consistent with the observed scatter in MAD results because of wood content (Expedition 319 Scientists, 2010; see “Lithology”) (Fig. F20). Between 1050.5 and 1170.5 mbsf, MMS decreases slightly from ~2.02 × 10^–7 to 1.07 × 10^–7 m³/kg, which correlates broadly with Subunit IVA. MMS then decreases to ~1.00 × 10^–7 m³/kg at 1200 mbsf and remains relatively constant at that value to 1400.5 mbsf. This zone corresponds to lithologic Subunits IVB and IVC (see “Lithology”). MMS values increase again to 1.17 × 10^–7 m³/kg at 1550.5 mbsf, probably associated with the Subunit IVD/IVE boundary. Beneath 1550.5 mbsf, MMS gradually decreases to 1.00 × 10^–7 m³/kg at 2004.5 mbsf. Contrary to MAD data, magnetic susceptibility is independent of cuttings size fraction. This suggests that there is no fractionation of the solid phase during the drilling and recovery process between the two cuttings sizes or between DICAs and formation cuttings.

Natural gamma radiation (cuttings)

Unwashed cuttings were collected in the core cutting area and packed in a 12 cm long core liner. The liner filled with cuttings was scanned with a MSCL-W to determine the NGR of the cuttings mix. To provide a background reference, NGR was measured from a liner, identical to those used for cuttings, filled with distilled water. The recorded value was 34.0 counts per second (cps). The NGR of drilling mud measured on a regular basis for background values has no significant effect on data bias. The MSCL-W NGR data and the downhole gamma ray (GR) logging data from LWD (see “Logging while drilling”) have similar values and trends (Fig. F100). The unit of logging data (gAPI) was converted to counts per second (cps) for comparison with MSCL-W data using the following equation (Mountain, Miller, Blum, et al., 1994):

There is a sharp increase in MSCL-W NGR at ~920 mbsf (877.2 mbsf underreamer depth); however, there is no significant variation in LWD-GR data at this depth. The low values of MSCL-W NGR above 920 mbsf are likely influenced by cement cuttings dominating the cuttings mixture. There is no such variation in LWD-GR because the LWD sensor started below the interface and detected signals mostly from the formation. MSCL-W NGR gradually increases with depth from 920 to 1200 mbsf. There is no noticeable variation in the interval of 1200–1750 mbsf. A remarkable increase from ~35 to ~40 cps at ~1750–1900 mbsf may reflect a change in lithology to a more claystone rich interval (see “Lithology”). MSCL-W NGR is slightly decreased below 1900 mbsf.

Consistent values of LWD-GR to 1500 mbsf in logging depth are followed by a clear increase at 1600–1800 mbsf, which correlates to the increase in MSCL-W NGR. Below 1800 mbsf, no significant variation is found in either MSCL-W NGR or LWD-GR. Other noticeable correlations between MSCL-W NGR and LWD-GR are found at ~1300 and 1600 mbsf and may relate to lithologic changes. Based on those correlations, the cuttings depths are deeper than the logging depths by ~30–70 m. This can be explained by the configuration of the BHA. The cuttings from the drill bit (~⅓ of the total cuttings volume) and those from the underreamer (~⅔ of the total cuttings volume) were mixed while they traveled with drilling mud to the surface (see also the “Methods” chapter [Strasser et al., 2014a]).

Mixing of cuttings across lithologic and structural boundaries

Mixing of cuttings occurs when a lithologic or structural boundary is penetrated with RWD. Grain density and NGR data allow us to constrain the mixing interval when the bit and the underreamer crossed the transition of the cement at the 20 inch casing shoe (860.2 mbsf) and the formation. Grain density has an average value of 1.87 g/cm³ above 895.5 mbsf, which characterizes the cement plug. Density then increases to an average value of 2.61 g/cm³ at 940.5 mbsf, which represents the formation (Fig. F101). Mixing of cuttings is also observed in the MSCL-W NGR data. The transition in MSCL-W NGR from 10 to 30 cps at ~920 mbsf correlates with the mixture of cement cuttings and formation cuttings. One potential explanation is that this transition results from the variation in the velocity of cuttings related to drilling mud velocity as well as particle shape, size, and density. This causes hydrodynamic dispersion of cement cuttings and formation cuttings. Assuming a Gaussian distribution of cuttings velocity, we used dispersion theory to characterize the transition of the cuttings mixture (Todd and Mays, 2005). The average velocity of drilling mud calculated from the average pumping rate (272.4 m³/h) and the average annulus area (0.171 m²) provided by Geoservices Ltd. was 1593 m/h. The average ROP given by LWD data for this interval was 24 m/h. As a model parameter, the coefficient of dispersion used for curve fitting for both data sets is 50 m²/s. Based on this simplified model (the S-shaped curve), most cuttings are from the underreamer in this interval, and the cuttings were mixed in a range of at least ~42.8 m at this depth.

Dielectrics and electrical conductivity (cuttings)

A total of 110 seawater-washed cuttings samples from Hole C0002F were sampled at a 10 m interval. Pastes prepared from each cuttings sample (1–4 mm size fraction) were used to measure electrical properties at high frequency (30 kHz–6 GHz). Salinity index from the extracted water and mass water content from cuttings pastes was compared with the porosity/density data set of the same cuttings using the MAD method to complement the data.

The purpose behind this pilot study experiment is to

• Assess if the dielectrics can detect any change from the formation despite the mixing interval of cuttings,

• Evaluate the cuttings as a proxy for formation evaluation,

• Extract the rough pore water salinity with depth, and

• Test the correlations among cuttings, core, and LWD data sets.

Before preparation of pastes, 1–4 mm size fraction cuttings samples, which were preserved in sealed bags and stored in the refrigerator since recovery, were photographed to evaluate color and general texture (Fig. F102A, F102B). The pictures were taken at the same distance, light, and field of view for comparative analysis. The color was evaluated in a representative area of the cuttings using a circle with an 11 pixel diameter on Adobe Photoshop software. The gray color value mean, minimum, and maximum are measured in 8 bit format from 0 (black) to 256 (white) along with their corresponding gray color histogram. Note that seawater washing was able to remove most of the drilling mud that was coating the cuttings and would have affected the original colors of the cuttings.

The analysis of gray color histograms revealed four main populations of samples (Fig. F103):

• Population P1 records the lowest gray values centered around 50 ± 10 grayscale and occurs in all of the sample collection.

• Population P2 is centered around 75 ± 10 grayscale and represents 46 samples (i.e., 42% of the sample collection). Population P2 particularly integrates Population P1 as a secondary population.

• Population P3 is centered at 95 ± 10 grayscale and represents the dominant population from the collection with 52 samples (47% of the collection).

• Population P4 exists in some specific intervals with the highest grayscale centered at 110 ± 10.

The computation of the gray mean values from each histogram as well as the difference between their minimum and mean gray value with depth seems to reveal a pattern related to some lithologic units (Fig. F104; Table T44). The general gray mean value spans from 40 to 90 with an average of ~63 ± 8. The gray mean with depth can be correlated with lithologic subunits based on silty claystone percentage (see “Lithology”). Data smoothing using a moving average window of five data points from the data set, which computes a difference between the minimum and mean gray values, well correlates with the suggested lithologic or logging units. The trough in mean gray value at ~1027 mbsf correlates with the lithologic Unit III/IV boundary, and the negative peaks of mean gray value and decrease in difference of gray value at ~1610 mbsf correlates with the logging Unit IV/V boundary.

Salinity and porosity from cuttings

During the sample paste preparation from cuttings for dielectrics measurement, the decanted water obtained after centrifuging was measured for salinity index (Table T45). Because the same amount of powder and milli-Q water was used for the whole cuttings collection, relative comparison with depth can be used to check any change in the general salinity of the formation, assuming no invasion by drilling fluid (Fig. F105A). Indeed, despite the dilution effect by the addition of water to the dried cuttings powder, the salinity evolution reveals some useful aspects. The general relative salinity is ~11.5 g/L through the whole formation, but at the base of lithologic Units III and IV, salinity increases up to 31 and 25 g/L, respectively (2.5–3 times the salinity background). Within lithologic Unit IV, salinity slightly decreases with depth from 16 to 10 g/L before becoming relatively constant at 10.5 ± 1 g/L through Unit V. This is consistent with pore water geochemistry analysis of cored prism sediment that documents decreasing chlorinity content (see “Geochemistry”). Not enough points are available in lithologic Unit III to average the salinity with reasonable confidence.

The water mass content from the prepared paste was converted into porosity using the grain density results from the MAD method and plotted against the porosity measured from the MAD method on the same cuttings (Fig. F105B). Two trends can be observed that are related to the lithologic types. The muddy samples are characterized with the following trend of porosity:

The average porosity from paste and MAD in such lithology type is 40% ± 1.6% and 43% ± 2.2%, respectively. In the sandy mud lithology, the porosity between MAD and paste is almost 1 to 1 with a slight positive linear shift of 4.4 for the MAD porosity. The average porosity is 39% ± 1.5% from the paste method and 45% ± 1.8% from the MAD method. Note that the coarse sand sample (338-C0002F-311-SMW) records much lower porosity than the muddy and sandy mud lithology units (29% of paste porosity and 24% of MAD porosity).

Dielectric properties

The dielectrics measurements were acquired from pastes as soon as possible after centrifuging. The dielectric constant (ε′_r) and the dielectric absorption (ε″_r) were measured, and the equivalent electrical conductivity was computed. Table T45 summarizes these results at different frequencies of acquisition. Only the dielectrics data above 3 MHz are reported because the data below 3 MHz include low-frequency parasitic noise probably due to ship heave and drill string vibration.

The dielectric constant (real part) at 3 MHz is ~356 with a maximum of 504 and a minimum of 205. The standard deviation at 3 MHz remains very low (<3). The dielectric constant decreases from 56 to 33 toward the 6 GHz frequency, with a standard deviation lower than 0.3. The dielectric absorption (imaginary part of the dielectric) is very high at low frequency (3 MHz) with an average value of 3958 and maximum and minimum values of 6171 and 1885. This absorption intensity decreases rapidly to 7 at 6 GHz, where no loading electrical charge can occur. The standard deviation is also very low with values of 36 at 3 MHz and <1 at higher frequencies.

The dielectric constant at low frequency is more sensitive to lithologic variations (Fig. F106), as well as the dielectric absorption with depth. Some unit intervals with a specific range of values can be extracted. These sediments have a linear relationship between the dielectric constant and its absorption (Fig. F107): at high frequencies the sediments are purely discharging (dielectric constant very low) because of the dielectrically “lossy” behavior of the clay minerals, which do not load much electrical charge. At lower frequencies of 3–30 MHz, the water–clay interaction is fully charging and polarizable, and the dielectric constant becomes higher, with a material much more conductive through the water film at the clay surface. At higher frequencies between 100 MHz and 1 GHz, on the other hand, two groups are observed along the linear trend. The dominant group corresponds to the mudstone, and the minor group, which is often defined by lower salinity, corresponds to the sandy materials. These two groups are not clear at lower frequencies.

The electrical conductivity was derived from the dielectric results. Because the material is more conductive at low frequencies, the 3 MHz frequency is the most appropriate to evaluate the electrical resistivity (Fig. F106). Electrical resistivity ranges from 1 to 3.36 Ωm with an average of 1.66 Ωm (Fig. F108). These values match the resistivity range observed from the LWD resistivity logs. Further analysis may allow for correlation with LWD data and lithologic boundaries.

Leak-off test

A LOT was performed at 872.5 mbsf, 12.3 m below the 20 inch casing shoe (Fig. F109). This test helped define the maximum mud weight for drilling to 2300 mbsf and the proposed location of the 16 inch casing set point and allowed assessment of the least principal stress. To perform the LOT, the cement was drilled out and the hole was deepened to 875.5 mbsf, providing a 3 m long, 17 inch diameter open borehole for performing the LOT with drilling mud of density of 1100 kg/m³. The LOT was conducted with the outer annulus closed by the BOP, and mud pressure was measured at the cement pumps. The pressure at the bottom of the hole was calculated by the recorded pressure plus the static pressure of the mud column.

Two LOTs were conducted in Hole C0002F. The pressure-time and flow rate–time records of the two pressurization cycles allow estimation of the leak-off pressure (LOP) and instantaneous shut in pressure (ISIP) (Fig. F110A). The LOP can be defined at the point where the pressure-volume curve deviates from linear behavior if we assume the borehole is elastic and impermeable (White et al., 2002; Engelder, 1993). During the first pressurization cycle, a total volume of 446 L of drilling mud was injected at a rate of 31.8 L/min. Borehole pressure peaked at 31.9 MPa. Based on the pressure-time curve, estimated values for the LOP and ISIP were 31.6 MPa and 31.8 MPa, respectively. The LOP was not clearly defined because a large volume of mud (302 L) was lost. A second cycle of pressurization was conducted by injecting 144 L at a rate of 47.7 L/min. The pressure-time curve of the second cycle suggests that leak-off took place at ~31.9 MPa because of the clear deviation of the pressure curve from the linear behavior. A proposed value of ISIP is 32.0 MPa from the maximum curvature on the pressure decay curve after pumping ceased (White et al., 2002). Plotting borehole pressure versus injected mud volume shows S-shaped curves with a linear part in the middle in both pressurization cycles (Fig. F110B). The rapid increase in pressure with a lower volume (e.g., ~144 L in total) of mud injected during the second cycle suggests that mud cake formed around the borehole wall, possibly due to mud flowing into the rock formation during the first cycle.

The deviation points (i.e., LOP) on the pressure-volume curves are not consistent between the two cycles (31.6 MPa for the first cycle and 31.9 MPa for the second cycle). If unsteady radial flow of injected drilling mud occurred (likely for the first cycle and possibly even during the second cycle), the deviation point on the pressure-volume curve is no longer valid for the LOP. A method to account for variable pressure gradients (dP/dV) can be used to estimate LOP as borehole pressure increases during continuous loss of mud (Fig. F110C). The variation in dP/dV can be approximated as linear if the permeability of the formation is constant (Todd and Mays, 2005). The linear approximation of dP/dV as a function of pressure in the entire range at the first cycle suggests flow of mud fluid into the formation with no change in permeability until the maximum stress of test. In the second cycle, the dP/dV curve deviates from linear behavior at 32.0 MPa. A sudden increase in formation permeability because of opening of fractures would explain the deviation of the dP/dV curve for the second pressurization cycle (Song et al., 2001). Based on these observations, it is interpreted that leak-off did not take place during the first cycle, but did during the second cycle at 32.0 MPa; thus, the ISIP (32.0 MPa) found in the second cycle is more reliable. The LOT revealed that the least horizontal principal stress is possibly ~32.0 MPa. A summary of the results is listed in Table T46.

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