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doi:10.2204/iodp.proc.319.103.2010

Physical properties

Cuttings and cores

Whole-round multisensor core logger (cores)

The results of whole-round multisensor core logger (MSCL-W) measurements on cores are summarized in Figure F55. MSCL-W GRA density, MSCL-W MS, and NGR all increase slightly with depth, with values ranging from 1.8 to 2.2 g/cm³, 7 × 10^–3 to 12 × 10^–3 SI, and 44 to 47 counts per second (cps), respectively (Fig. F55). The excursion from ~1579 to ~1582 m CSF in all three data sets corresponds to ash-rich layers (see "Lithology"). The zone of decreased density from ~1546 to ~1556 m CSF in the MSCL-W GRA density data corresponds to measurements made on Core 319-C0009A-5R and is possibly a result of poor calibration. P-wave velocity exhibits significant scatter (values between 1200 and 1800 m/s) (Fig. F55D) and is lower than the velocity measured on discrete samples (see "Discrete P-wave velocity [cores]"). The discrepancy may be related to the gap between the core and core liner, which is commonly filled with air and/or water. Only a few reliable MSCL-W resistivity measurements are available, and the values exhibit no trends (Fig. F55E).

Porosity and density (cuttings and cores)

As described in the "Methods" chapter, porosity is computed from the total water content in the sample and includes interstitial pore water as well as water bound in hydrous minerals, including opal and hydrous clays.

Cuttings

A total of 96 discrete samples from the 1–4 mm size fraction cuttings were analyzed for MAD. Measurements start at 1037 m MSF, where recovered cuttings were sufficiently lithified to differentiate them from the drilling mud and to extract usable chips (Fig. F56). Measured porosity in Subunit IIIB remains nearly constant with depth (average value of 56%) above the Subunit IIIB/Unit IV boundary (1285 m WMSF). Within Unit IV, porosity decreases rapidly (from values >50% to 45%) from 1285 to 1450 m MSF, below which the trend reverses and porosity increases to values of ~56% but exhibits significant scatter. Bulk density derived from MAD measurements increases downhole and mirrors the porosity trend (Fig. F56B). Above the Subunit IIIB/Unit IV boundary (from 1037 to 1285 m MSF) bulk density averages 1.7 g/cm³, and it increases with depth to higher values below the Subunit IIIB/Unit IV boundary. Bulk density values below this boundary exhibit some scatter, with an average value of 1.85 g/cm³. Average grain density within Unit III is 2.54 g/cm³ and exhibits significant scatter (from 2.3 to 2.7 g/cm³) (Fig. F56C). Average grain density in Unit IV is 2.66 g/cm³. Values remain relatively constant throughout Unit IV above ~1500 m MSF, where they decrease to an average value of 2.6 g/cm³ and exhibit a slight trend of decreasing grain density with depth to the base of the hole. The low grain densities in Unit III are correlated with increased TOC (Fig. F57). A preliminary calculation taking into account a dry sample containing 90% mineral grains with an assumed density of 2.65 g/cm³ and 10% wood with a density of 0.9 g/cm³ yields an average grain density of 2.48 g/cm³. Thus, a wood abundance on the order of 5%–10% can potentially explain the anomalously low overall grain densities throughout much of Unit III.

The MAD cuttings data indicate a consistent consolidation trend above the Subunit IIIB/Unit IV boundary, with a sharp decrease in porosity below that depth. However, porosity values are anomalously high throughout the depth range of cuttings analyses when compared with porosities reported for Site C0002, ~20 km to the southeast in the Kumano Basin (Ashi et al., 2008). Considering their depth of burial, these samples exhibit high porosity, low bulk density (and low grain density in Unit III), and relatively large variations in porosity. These could be the result of methodological or geological processes. There are at least four methodological effects that could lead to overestimates of porosity in the recovered cuttings:

• Incomplete removal of water prior to determination of wet mass, resulting in films of water from the soaking and washing process remaining on the 1–4 mm sized cuttings;

• Fracturing or mechanical volume change of the cuttings, possibly related to gas expansion or drilling disturbance;

• Interaction with drilling mud and with water during washing and soaking (Fig. F58), resulting in swelling of clay minerals; and

• Residue from drilling mud and additives remaining on the surface of cuttings even after washing.

Hole C0009A is the first IODP hole in which cuttings were recovered and processed for MAD. The techniques and data described here are preliminary and should be a helpful guide for future riser holes. In particular, a number of the potential issues noted here could be related to the size of the analyzed cuttings fraction. The small size makes the cuttings vulnerable to chemical interactions with the drilling mud. Even a small amount of swelling in these small pieces of material will cause large differences in observed porosity. The size also makes it difficult to remove all of the water and drilling mud, especially in cases where the cuttings were very soft.

Several geological processes could also affect porosity, including

• Sediment composition (mineralogy, grain size distribution, and degree and type of cementation) affecting sediment consolidation behavior, which could result in differences in porosity (and bulk density) trends, and

• Presence of overpressured fluid in sediments of the lower Kumano Basin because of high sedimentation rates and genesis of CH₄ from microbially driven wood decomposition.

However, no evidence of underconsolidation or significant overpressure is observed in the logs or MDT measurements (Fig. F59; see "Downhole measurements").

Cores

Porosity measured on core samples ranges from 28% to 44%, with an average value of 34% (Fig. F60). Porosity exhibits a slight trend to lower values downsection, but with some variability. Observed ash layers at ~1510 and 1580 m CSF exhibit low porosity. Bulk density in the cored interval ranges from 1.84 to 2.2 g/cm³, with an average value of 2.1 g/cm³. Although there is some scatter, a downward trend of increasing bulk density is observed. Grain density ranges from 2.3 to 3.0 g/cm³, with most values >2.6 g/cm³ and an average value of 2.7 g/cm³. Other than a few anomalous values in the 2.3 g/cm³ range, the values are constant with depth. It is also important to note that technical problems led to poor recovery in the upper part of the cored section, and it is difficult to determine if the samples collected are fully representative of the formation.

Core samples exhibit significantly lower porosities (30%–35%) than cuttings samples (40%–55%) (Fig. F61). Porosity values from core samples are consistent with those expected at this depth (>1500 m CSF), whereas values from cuttings are significantly higher (e.g., porosity values at Expedition 315 Site C0002; Ashi et al., 2008). The overestimate of porosity from cuttings compared with core measurements is consistent with the anomalously high values observed in cuttings throughout the hole, as described above. Because cuttings appear to overestimate porosity and underestimate bulk density, we compared core and cuttings MAD data in detail in the cored interval (Fig. F62). The trend between the two data sets is approximately linear and indicates that cuttings porosity values may be useful in identifying trends in formation porosity but that the absolute values are subject to large errors.

Discrete P-wave velocity (cores)

Over the cored interval, P-wave velocity (V_P) is nearly constant (Fig. F63A). The average V_P in the x- and y-directions is ~2050 m/s. V_P in the z-direction is consistently lower than the other directions, with an average of 1800 m/s. The ash-rich layers (~1580 m CSF) exhibit lower velocity values than other lithologies (~1700 m/s). P-wave velocity exhibits an inverse relationship with porosity, although there is considerable scatter (Fig. F64); values from the ash-rich samples do not fit this trend (low porosity, low V_P). In some cases, minor faults and/or fractures are present and may give rise to the anisotropy.

P-wave velocity exhibits anisotropy ranging from ~0% to values as high as 15%–20% (Fig. F63B). Horizontal velocity is higher than vertical velocity in most samples, but the ash-rich samples show the opposite relationship.

Thermal conductivity (cores)

Bulk thermal conductivity measurements were conducted on the working half of cores from 1529 to 1594 m CSF (Cores 319-C0009A-3R to 9R, 1 measurement per core) with a half-space probe but could not be conducted on Cores 319-C0009A-1R and 2R because of their low recovery (3% and 22%, respectively). Values remain in a narrow range of 1.57–1.76 W/(m·K), except at 1581 m CSF (interval 319-C0009A-8R-7, 18–32 cm), where thermal conductivity drops to ~1.1 W/(m·K) (Fig. F65). This section coincides with an ash-rich layer. No clear correlation between thermal conductivity and porosity is observed for these samples.

Natural gamma ray (cuttings)

Natural radioactivity was measured with the multisensor core logger (MSCL) on unwashed cuttings. The natural radioactivity background is 35 cps, which is subtracted from the data shown in Figure F66. The MSCL gamma ray data follow the downhole gamma ray trend from the wireline logging data to first order (see "Lithology") but are offset to slightly higher values. NGR increases sharply at ~700 m MSF, consistent with MWD gamma ray measurements (see "Logging and data quality"), and remains high below this depth.

Magnetic susceptibility (cuttings)

MS measurements were conducted on the cuttings used for MAD measurements. To remove effects of measured sample weight, we calculated mass magnetic susceptibility (MMS) from measured raw data MS (bulk susceptibility) by:

The depth range of MS measurements corresponds to Subunit IIIB and Unit IV; results are presented in Figure F67. MMS is high in Subunit IIIB (above 1292.7 m MSF) and decreases abruptly across the boundary. Average MMS values are 1.14 × 10^–7 m³/kg in Subunit IIIB (48 values, from 1037 to 1292.7 m MSF) and 8.93 × 10^–8 m³/kg in Unit IV (48 values, from 1292.7 m MSF to TD).

Logging

Physical properties from logging data include thermal neutron porosity, density, resistivity, and sonic velocity. Porosity is also computed from density and estimated from resistivity. This porosity is based on the total water content of the formation. In mud and mudstone, this includes both pore water and water bound in hydrous minerals (e.g., clays and opal).

Lithodensity and porosity

Density and neutron porosity values are sensitive to borehole conditions and in particular to borehole diameter. Neutron porosity values exhibit significant scatter (Fig. F68). The lower bound of these data is generally consistent with bulk densities and porosities determined from density logs (Figs. F69, F70) but shifted to slightly higher values.

Neutron porosity

Thermal neutron porosity exhibits substantial scatter, with values ranging from ~0.4 to >0.6 in Units II and III. Scatter increases in Unit IV, with values ranging from ~0.4 to 1.0 (Fig. F68). Where the borehole wall is rugose, the thermal neutron porosity measurement tends to incorporate a contribution from the drilling fluid. Therefore, the lower bound of the neutron porosity log should be considered to best reflect formation porosity and should be used for comparison with other porosity measurements. Despite the scatter, neutron porosity variations between units can be identified. Throughout Unit II, the lower bound of porosity remains approximately constant at ~38%. It increases to 50% in Subunit IIIA, decreases to 40% in Subunit IIIB, and increases to ~50% in Unit IV. The degree of scatter decreases significantly at the Subunit IIIA/IIIB boundary and increases at the Unit IV boundary, where hole conditions change dramatically.

Lithodensity

Throughout Unit II, log density is nearly constant at ~2.2 g/cm³ (Fig. F69). Decreased density at the very top of this unit (~715–725 m WMSF) is linked to bad hole conditions as documented by caliper measurement. At the Unit II/Subunit IIIA boundary, the log density drops to ~2.0 g/cm³ and increases slightly downsection within Unit III to 2.2 g/cm³ at its base. In Unit IV, density drops to 1.95 g/cm³ and fluctuates between 1.7 and 2.0 g/cm³ between ~1300 and ~1430 m WMSF. Below this depth, the log exhibits a trend of increasing density downhole with fluctuations at ~1492 and ~1560 m WMSF. When a filter is applied to eliminate data from borehole sections with caliper values >12.5 inches, the density log exhibits an increasing trend throughout Unit IV, but the scatter in the upper 1300 m remains unchanged.

Density-derived porosity

A porosity profile was computed from the lithodensity log assuming a constant grain density (ρ_g = 2.65g/cm³) and fluid density (ρ_w = 1.024g/cm³) (see "Physical properties" in the "Methods" chapter) (Fig. F70). This calculation of porosity does not take into account the possible effects of free gas; it also overestimates the interstitial porosity because bulk density includes water contained in hydrous minerals. Because porosity data from MAD measurements made on core and cuttings samples also incorporate the total water included in the sample (interstitial and bound in hydrous minerals), the log-derived and MAD porosity values can be directly compared. Porosity in the lowermost section of Unit II hovers around 30%, sharply increasing to 40% at the Unit II/Subunit IIIA boundary, and gradually decreases to 35% at the base of Subunit IIIB, with several small fluctuations (Fig. F70). At the Unit III/IV boundary, porosity exhibits a sharp increase from 38% to 42% and decreases gradually downhole below this depth. The increase is coincident with an increase in clay content at the boundary (see "Lithology"). In general, density-derived porosity is ~5% lower than the lowest bound of the neutron porosity. The discrepancy between the two estimations of porosity tends to decrease in zones containing gas in Subunit IIIB (1043–1047, 1060–1199, and 1258–1280 m WMSF; see "Geochemistry"). MAD porosity measurements on core samples and those derived from the density log are comparable in the cored interval (Fig. F71), whereas cuttings porosity values are considerably higher, as discussed above.

Electrical resistivity

Resistivity logs

Several resistivity measurements were acquired at different depths of penetration. The HRLA provides deep resistivity (see Table T5 in the "Methods" chapter) samples every 0.15 m. The Mirco-Cylindrically Focused Log (MCFL) documents the microresistivity of the formation, and the EMS tool is a direct measurement of mud resistivity. Typically, the greater the depth of investigation, the higher the resistivity measurement, because an invasion of the formation by the very conductive mud (~0.7 Ωm) filtrate affects the shallow penetration measurements the most. Resistivity obtained from the High Resolution Laterolog Array Tool (HRLT) is corrected for this effect and provides an estimate of true formation resistivity ("true resistivity"). True resistivity ranges between 0.9 and 2.65 Ωm (Fig. F72) and generally increases with depth but exhibits large fluctuations. Because the true resistivity log integrates the resistivity value over a large volume, it is generally less sensitive to borehole conditions; however, it still exhibits considerable scatter in Unit IV, which is characterized by the largest caliper and highest borehole rugosity.

Within Unit II, true resistivity increases slightly (from 1.4 to 1.8 Ωm) from ~710 to ~780 m WMSF and decreases below this to ~1.4 Ωm at the Unit II/III boundary. In Subunit IIIA, the resistivity value gradually increases from 1.4 to 1.7 Ωm at ~910 m WMSF, where it abruptly decreases to 1.4 Ωm at ~919 m WMSF. This drop is coincident with decreases in P-wave velocity and gamma ray (Fig. F7) and may correspond to a minor change in lithology. Below this depth, resistivity increases to ~2.4 Ωm at ~1020 m WMSF and then decreases sharply to 1.6 Ωm at ~1027 m WMSF, just above the Subunit IIIA/IIIB boundary. Resistivity decreases just above the Unit III/IV boundary at 1285 m WMSF. Below this boundary, resistivity decreases with depth to ~1316 m WMSF, below which it increases to 1459 m WMSF and then fluctuates around 2.1 Ωm to 1572 m WMSF.

Temperature log and estimation of downhole temperature

Two temperature records were obtained by the EMS tool during wireline logging Runs 1 and 2 (drilling Phases 5 and 6; Table T1) from above the 20 inch casing shoe (~690 m WMSF) to 1590 m WMSF (Fig. F73; see "Downhole temperature"). Because these logging data are strongly affected by drilling disturbance and are far from thermal equilibrium, we compute a synthetic temperature profile in the formation using estimated thermal conductivity values and assuming a heat flow of 39 mW/m² (determined at Site C0002) (Ashi et al., 2008). We assumed a 2°C bottom water temperature and a thermal conductivity profile obtained by combining (1) Site C0002 thermal conductivity data for the upper 800 m (where no reliable porosity are available at Site C0009) and (2) thermal conductivity computed from density-derived porosity (see "Physical properties" in the "Methods" chapter) at Site C0009 from 800 to 1572 m WMSF. Below 1572 m WMSF, we assume a constant thermal conductivity of 1.7 W/(m·K) because the density measurements were not reliable. This synthetic temperature profile exhibits slight changes in gradient near the major unit boundaries, and the estimated temperature at the bottom of the hole is 48°C (Fig. F73).

Estimation of porosity from resistivity

We calculated seawater electrical resistivity using the temperature profile estimated above and used it to evaluate the formation factor from true resistivity ("Physical properties" in the "Methods" chapter). Formation factor, in turn, is related to porosity by Archie's law. The Archie's law parameters that best fit to the caliper-filtered density-derived porosity and MAD core porosity are a = 1 and m = 2.4. The resulting estimates of porosity (resistivity-derived porosity) (Fig. F74) are in good agreement with density-derived porosity except in Unit II, where density-derived porosity is lower, and in Unit III at ~910 and ~1017 m WMSF, where resistivity-derived porosity sharply increases and density-derived porosity decreases. Neither the conductivity of clay minerals nor gas saturation are taken into account in this calculation; both will affect the resulting porosity as well as the choice of the Archie's law parameters.

The resistivity-derived porosity generally decreases with depth (Fig. F74). In Unit II, it decreases to 40% at 774 m WMSF, then increases to 43% at the Unit III boundary. From this boundary, it decreases to 37% at ~910 m WMSF, where it abruptly increases to 42% and then decreases gradually to 33% at ~1285 m WMSF. The estimated porosity increases from 32% to 37% at the Unit III/IV boundary. Within Unit IV, it generally decreases with depth.

Sonic log

P-wave velocity

P-wave velocity increases continuously from 2000 m/s at the top of Unit II to 2300 m/s at its base (Fig. F75), decreases sharply to 2000 m/s just above the Unit II/III boundary, and varies (from ~2000 to 2200 m/s) from ~790 to ~1010 m WMSF. From that depth to ~1285 m WMSF, we identify four zones of low P-wave velocity (~1700 m/s): ~1012 to 1025 m WMSF, ~1043 to ~1047 m WMSF, ~1060 to ~1199 m WMSF, and ~1258 to ~1280 m WMSF. These zones broadly coincide with the increased abundance of wood fragments and gas content in Subunit IIIB (see "Geochemistry" and "Lithology"). We interpret the low V_P to indicate zones with increased gas saturation (Fig. F75). P-wave velocity increases sharply from 1940 to 2400 m/s at the Unit III/IV boundary (1285 m WMSF). Although P-wave velocity in Unit IV is somewhat scattered because of degraded borehole conditions, a gradual trend (~1350 to ~1450 m WMSF) of increasing V_P with depth is evident.

S-wave velocity

In Unit II, shear-wave velocity (V_S) increases gradually from 800 m/s at 715 m WMSF to 1070 m/s at 790 m WMSF. S-wave velocity decreases slightly at the Unit II/III boundary and increases gradually with depth within Units III and IV. S-wave velocity is less variable than P-wave velocity in Subunit IIIB (Fig. F75).

V_P/V_S and Poisson's ratio

V_P/V_S and Poisson's ratio were calculated from P-wave and S-wave velocities (Fig. F76). Both are indicators of gas content because they decrease with increased gas saturation. In Units II and IV, V_P/V_S and Poisson's ratios are nearly constant with depth. V_P/V_S ratio in Units II and IV is 2.7 and 2.3, respectively, and Poisson's ratio is 0.41 and 0.38, respectively. In contrast, these indicators exhibit larger variation in Unit III, especially in the zone from 1037 to 1285 m WMSF (Subunit IIIB), where V_P/V_S ratio decreases from 2.5 to 2.0 and Poisson's ratio decreases from 0.39 to 0.32.

Gas-rich intervals can also be identified from the relationship between resistivity and P-wave velocity (Figs. F75, F77). This is based on the fact that P-wave velocity and resistivity both increase with decreasing porosity, but resistivity increases with gas content whereas P-wave velocity decreases. Subunit IIIB is characterized by a significant decrease in P-wave velocity and increase in resistivity, which differs from the trends in Units II and IV. Comparison of the two downhole profiles confirms the presence of four gas-rich intervals.

Stoneley wave velocity

Stoneley wave velocity was measured by the Sonic Scanner and shows similar patterns to the S-wave velocity (Fig. F78). A Stoneley wave is a type of surface wave that propagates along the wall of the borehole. If fractures exist in the borehole or the borehole wall is highly irregular in shape, the Stoneley wave velocity will decrease. Stoneley wave velocity increases gradually from ~700 to ~1000 m/s at 1565 m WMSF and exhibits significant scatter in Unit IV, possibly related to borehole conditions.

Comparison of P-wave velocity with porosity

Cross-plots between P-wave velocity and caliper filtered density-derived porosity and resistivity-derived porosity are presented in Figure F79. Both cross-plots document an increase in velocity with decreased porosity, except in Subunit IIIB. In this unit, velocity is generally lower than in other units, probably due to the presence of gas. In addition, porosity values in this unit may need to be corrected to higher values because the porosities were computed assuming that all pore fluid is seawater, and gas saturation was not taken into account.

Estimation of gas saturation

Cross-plotting V_P/V_S versus the P-wave slowness (1/V_P) is useful to assess the effect of gas saturation on these quantities (see "Physical properties" in the "Methods" chapter) (Fig. F80). Units II and IV plot similarly, whereas part of Unit III follows a trend to lower V_P/V_S values and higher slowness, as expected from the Reuss average for pore fluid bulk modulus K_f (Brie et al., 1995) (see "Physical properties" in the "Methods" chapter). Results of the calculation (parameters of calculation showed in Table T13) indicate a gas saturation of ~10% and true porosity (when accounting for gas) of ~25% in Subunit IIIB.

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