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doi:10.2204/iodp.proc.314315316.115.2009

Physical properties

The loss of the BHA in Hole C0003A required the use of only MWD and data acquired in real time for our analyses. Data available for physical property analyses were neutron porosity (TNUC, thermal neutron porosity ultrasonic caliper corrected), bulk density (RHOB), three different resistivity logs (bit, ring, and deep button), and two different caliper logs (ADIA and ECAL). ADIA caliper is part of the adnVISION tool and is based on ultrasonic traveltime; ECAL is a less direct measurement based on the difference between shallow and medium button resistivity readings with an assumption of mud (borehole fluid) resistivity. When both calipers are reliable, time-delayed hole washouts can be detected because the ADIA reads caliper from 40 to 60 min after the ECAL does. DTCO was also available for sonic P-wave velocity analysis; however, the data quality turned out to be so poor that no meaningful analysis was possible (see “Log-seismic correlation” for a brief description of the sonic log). As at previous sites, additional analyses were conducted to produce different porosity derived from density and resistivity. Accordingly, estimations of temperature and formation factor were carried out.

Density

The RHOB log is strongly affected by hole conditions, as indicated by caliper data (Fig. F11). Bulk density values in high-caliper intervals are highly underestimated and should not be considered reliable.

The density log averages ~1.6 g/cm³ in the uppermost sediments and in logging Unit I, except for the two spikes to 1.41 g/cm³ at 37.5 m LSF and 1.32 g/cm³ at 45.5 m LSF. These spikes are probably caused by hole washouts, as caliper data suggest. Measured density drops from ~1.6 g/cm³ at 74 m LSF to ~1.2 g/cm³ at 79 m LSF at the boundary between logging Units I and II. In Unit II the density values decrease from ~1.25 to ~1.10 g/cm³, which is far too low to represent the formation density. These low values are associated with high caliper values and indicate serious hole enlargement in this zone. Near the top of Unit III (155.9 m LSF), density is 2.00 g/cm³. In this unit the density log shows a nearly constant value of ~1.93 g/cm³ from ~152 to 381 m LSF, with some low density spikes related to high caliper (e.g., 247.8–251.9 and 300.5–302.6 m LSF). Below 381.0 m LSF the caliper value significantly increases again, resulting in unreliably low density measurements. These high-caliper zones could be fault-related damage regions. Zones with low caliper values produce higher density values (e.g., 402.0–415.7, 450.5–467.7, and 469.6–489.4 m LSF). Intervals with high density may indicate zones of enhanced compaction or less pervasive disruption by faulting and/or fracturing.

Density-derived porosity

Figure F12 shows a comparison of the two available calipers (ECAL and ADIA), the neutron porosity, and the density-derived porosity calculated from the RHOB log (see the “Expedition 314 methods” chapter). A more systematic comparison of those two porosities was made through cross-plotting (Fig. F13). Those figures show that in logging Unit I, the density-derived porosity log is near constant (65%) and is higher than the neutron porosity log by as much as 10%. In logging Unit II, the few data available are not considered reliable because both of the calipers show values higher than 10 inches. In logging Unit III, the density-derived porosity is almost constant (~45%) and is systematically lower than the neutron porosity by 10%, except for one distinct zone from 470 to 493 m LSF, where the density-derived porosity is close to 35%, and one zone of washouts from 415 to 450 m LSF, where the neutron porosity becomes higher.

Neutron porosity

The TNUC log in Hole C0003A shows scatter throughout the entire depth range (Fig. F14). Both raw data and smoothed data using a 4.5 m running average are presented. Overall, the neutron porosity is nearly constant with depth within each logging unit. The ADIA data show severe washouts in logging Units I, II, and in the bottom part of Unit III. The porosity is likely overestimated in such high-caliper zones.

The porosity log shows a nearly constant value of ~58% in logging Unit I and above and increases sharply from 54.8% at 74 m LSF to 65.9% at 78 m LSF at the boundary between logging Units I and II. In logging Unit II the porosity is nearly constant with depth and is characterized by slight long-wavelength variations with values ranging from ~62% to <70%. These values are probably strongly affected by the poor hole quality. The sharp cutoff of porosity values <70% reflects an artificial threshold in the data acquisition. Porosity drops from 68.5% at 148 m LSF to 50.5% at 154 m LSF at the boundary between logging Units II and III. The porosity remains nearly constant with some small fluctuations, ranging between ~47.5% and ~56% above 381 m LSF in logging Unit III. Below 381 m LSF where caliper values are relatively high, the porosity log exhibits fluctuations ranging from ~46% to ~61%. However, those values may not be reliable because of washouts.

Resistivity and estimated porosity

Resistivity logs

Figure F15 shows three different resistivity measurements: ring, bit, and deep button resistivity. The overall downhole trends are identical for the three logs. Superposition of the bit and ring resistivity measurements shows very good agreement between them in logging Unit III and for the few data points of logging Unit I and above. In logging Unit II the ring resistivity is significantly lower than the bit resistivity by as much as 0.2 ΩWm but this corresponds to a zone of major washout shown by both ECAL and ADIA calipers. This is confirmed by the cross-plot between bit and ring resistivity (Fig. F16). The difference between those two measurements is small in logging Unit III.

The following description of resistivity is based on the bit resistivity shown in Figure F15. Very few resistivity values in logging Unit I and above are reliable. At the bottom of this section (from 55 to 76.6 m LSF) we can distinguish a constant resistivity value of 0.9 Ωm. In logging Unit II, the bit resistivity measurement, which should be the measurement least affected by washouts, slightly increases from 0.9 to 1.1 Ωm. The logging Unit II/III boundary is marked by a sharp increasing step of resistivity from 1.1 to 1.8 Ωm, followed by a decreasing step from 1.8 to 1.6 Ωm. From 152 to 320 m LSF, the resistivity data show a near-constant value of 1.6 Ωm. At 320 m LSF, we observed a small decreasing step from 1.6 to 1.5 Ωm. It is followed, from 330 to 420 m LSF, by an increasing trend of resistivity from 1.5 to 1.7 Ωm, and from 420 m to 460 m LSF by a decreasing trend of resistivity from 1.7 to 1.3 Ωm, which corresponds to the fractured zone identified in “Log characterization and lithologic interpretation.” Below 460 m LSF, two zones of relatively high resistivity values (1.7 Ωm between 475 and 485 m LSF and 1.5 Ωm between 505 and 507 m LSF) depart from a trend of constant resistivity values of ~1.5 Ωm.

Estimation of temperature profile

The temperature gradient was estimated from porosity-dependent rock thermal conductivity. The depth-dependent thermal conductivity (K) at this site was obtained using the following equation:

where K_g is the grain thermal conductivity (2.85 W/[m·K] from ODP Leg 190 Site 1173 [Moore, Taira, Klaus, et al., 2001]), K_w is the water thermal conductivity (0.60 W/[m·K]), and ϕ is the thermal neutron porosity. Calculated K is ~1.0 W/(m·K) in logging Unit I and II and ~1.3 W/(m·K) in logging Unit III. Surface temperature of 2°C and surface heat flow of 60 mW/m² were assumed. The resulting temperature reaches 28°C at 535 m LSF.

Estimation of porosity from resistivity

The seawater electrical resistivity was calculated as a function of temperature (see the “Expedition 314 methods” chapter) and used to evaluate both ring and bit formation factors. Ring and bit formation factors are normally related to porosity by Archie’s law. Formation factor versus density and porosity cross-plots (Fig. F17) show that the data points from logging Units I (and above) and III fit well to an Archie’s law curve, with a = 1 and m = 2.4. The resistivity-derived porosity profiles were then calculated with these parameters. It should be noted that lithologic variations, not taken into account in this estimation, could affect the resulting porosity, as well as the choice of Archie’s law constants. Unit II and suspected damaged intervals in Unit III are outside of the curve because of the larger density-derived porosity values caused by hole washouts. Comparison of the resistivity-derived porosity calculated with Archie’s law to the density-derived porosity is also presented in Figure F18. On Figure F18, the density-derived porosity log is also shown for comparison; however, this log has been filtered to remove data from portions of the hole where caliper diameter is >9.5 inches.

Resistivity-derived porosity generally decreases with depth. Bit and ring resistivity–derived porosity show very similar trends except in logging Unit II. In the upper part of logging Unit I, resistivity-derived porosity slowly decreases from 63% at 60 m LSF to 62% at 69 m LSF and is followed by a sharp decrease at its base reaching 57% at 75 m LSF. The boundary between logging Units I and II corresponds to a 7% resistivity-derived porosity increase, reaching 63% at 80 m LSF. Bit resistivity–derived porosity in logging Unit II decreases to an average value of 56% at its base (150 m LSF). Fluctuations of 2%–7% amplitude and 15 m wavelength are superimposed on this decreasing trend. In this logging unit, the difference between ring and bit resistivity–derived porosity is very large (up to 15%), whereas the difference is small in the other logging units. Logging Unit II is characterized by very high caliper values, suggesting strong washouts. We suggest that the time delay between the two measurements allowed for washout development and resulted in high overestimation of the ring resistivity–derived porosity.

At the transition from logging Unit II to III resistivity-derived porosity sharply decreases from 56% to 47% within a 9 m interval, with a local 44% minimum in between. Resistivity-derived porosity decreases slowly between 47% at 158 m LSF and 41% at 416 m LSF, with a slight 1% increase between 320 and 330 m LSF. Variations with smaller amplitude can also be observed. At 416 m LSF, it begins to increase slowly to reach 43% at 461 m LSF. In this high caliper value interval, density-derived porosity has been filtered out, but the bit resistivity-derived porosity does not seem to be affected by the washout. The lowermost 37 m of the hole shows one sharp 2% amplitude fluctuation over 10 m followed by a second 3% amplitude fluctuation over the remaining 27 m.

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