DOWNHOLE LOGGING

Operations

LWD operations at Site 1251 began with replacing both the ADN tool and the RAB batteries prior to running pipe to the seafloor at 2300 hr Universal Time Coordinated (UTC) on 22 July 2002. Hole 1251A was spudded at a water depth of 1228.00 meters below rig floor (mbrf) ~3.2 nmi east of southern Hydrate Ridge. The LWD tool string included the GeoVision Resistivity (GVR) (RAB), MWD (Powerpulse), NMR-MRP tool, and Vision Neutron Density (VND). An attempt to limit the initial ROP in the upper part of the hole to 25 m/hr was only partially successful. For the most part, the upper 30 mbsf of Hole 1251A was drilled at various ROP, ranging from values as low as 20 to as high as 70 m/hr. At a bit depth of ~30 mbsf, the ROP was increased and maintained at a high rate of ~50 m/hr to the bottom of the hole (BOH) at 380 mbsf. As a result of the higher ROP, the vertical resolution of the RAB tool images was reduced to ~10 cm and the NMR spectral data were also slightly degraded. The LWD tools were pulled to the rig floor at 1700 hr on 23 July for a total bit run of ~18 hr. Upon recovery, the LWD data from Hole 1251A were downloaded and the ship was offset to Site 1250.

Logging Quality

In general, the recorded LWD data from Hole 1251A are of good quality. There is minimal reduction in vertical resolution as a result of the faster ROP, and the borehole is in good shape through the shallow interval. Figure F38 shows the quality control logs for Hole 1251A. As discussed above, the ROP in the uppermost ~30 m of Hole 1251A was highly variable; however, below 30 mbsf the ROP was maintained at a relatively constant rate of 50 m/hr (±10 m/hr). This is sufficient to record one sample per 10-cm interval, which was obtained over 92% of the total section of the hole. The quality of RAB images is, thus, quite high, and no significant resolution loss is observed with variation in ROP in Hole 1251A.

The differential caliper log (DCAL), which gives the distance between the tool sensor and the wall of the borehole as recorded by the LWD density tool, is the best indicator of borehole conditions. The differential caliper values are <1 in over 90% of the total section in Hole 1251A. Only the uppermost ~38 m (0-38 mbsf) of the hole and the bottom ~70 m (310-380 mbsf) of the hole show washouts >1 in. The density correction (DRHO), calculated from the difference between the short- and long-spaced density measurements, varies from 0 to 0.175 g/cm³ (Fig. F38), which shows the good quality of the density measurements above ~280 mbsf. A standoff of <1 in between the tool and the borehole wall indicates high-quality density measurements with an accuracy of ±0.015 g/cm³.

Time-after-bit (TAB) measurements for most of Hole 1251A are 5 ± 2 min for ring resistivity and gamma ray logs and 50 ±10 min for density and neutron porosity logs (Fig. F38). TAB values remain relatively constant over most of the hole, coinciding with the relatively consistent ROP while drilling, although some large variations in ROP were observed near the top of the hole.

The depths relative to seafloor for all of the LWD logs were fixed by identifying the gamma ray signal associated with the seafloor and shifting the logging data to the appropriate depth as determined by the drillers pipe tallies. For Hole 1251A, it was determined that the gamma ray log pick for the seafloor was at a depth of 1216.5 mbrf. The rig floor logging datum was located 11.1 m above sea level.

Operations

Hole 1251H was planned as a dedicated conventional wireline logging (CWL) hole and was to be drilled with a 9.875-in tricone rotary bit to a depth 445 mbsf. The drilling target was reached much faster than anticipated, but the cleaning and conditioning of the hole proved to be difficult and time consuming. Rig-up for logging started at 2200 hr on 17 August, 14 hr after reaching the initial TD of the hole (445 mbsf). The first lowering of the triple combo tool string could not pass an obstruction at 240.5 mbsf; each subsequent log run to the bottom of the hole encountered more fill or obstructions. The final run of the FMS-sonic tool could not pass below 209.5 mbsf. The final rig down of the FMS-sonic tool was complete by 1145 hr on 18 August. See Table T20 for detailed information on the Hole 1251 CWL program.

CWL operations in Hole 1251H began with the deployment of the triple combo tool string (TAP/DIT/HLDT/APS/HNGS/QSST) (Table T20). The triple combo tool string initially encountered a borehole bridge at a depth of 240.5 mbsf that it could not pass, which corresponds to the depth of the shallowest significant borehole breakouts on the LWD RAB images from Hole 1251A (Fig. F39). The quality of the data acquired during the main and repeat uplog pass of the triple combo tool string was moderately degraded because of the washed-out nature of the borehole (see "Logging Quality"). Before the start of the second log pass, several checkshots were attempted with the QSST, but the signal-to-noise ratio was poor, preventing the acquisition of useful data. The repeat pass of the triple combo tool string was conducted over an interval from 225.5 mbsf up to the bottom of the pipe (78.5 mbsf). TAP and depth data were recorded without any problems. The triple combo logging run ended with the rig-down of the tool string being completed at 0645 hr on 18 August.

For the second CWL run in Hole 1251H, the FMS-sonic tool string (FMS/DSI/SGT) was deployed. The FMS-sonic tool string reached a maximum depth of 207.5 mbsf on two consecutive passes. The obstruction at 207.5 mbsf was near the depth of the BSR for this site (estimated at 205 mbsf) (see "Introduction"). The length of the FMS-sonic tool string (31 m) prevented the recording of acoustic logging data across the estimated depth of the BSR. The FMS calipers also showed that most of the hole was severely enlarged, consistent with the HLDT caliper recorded on the triple combo run. DSI modes used for the first pass were the standard-frequency monopole, low-frequency lower dipole, and standard-frequency upper dipole. DSI modes used for the second pass were the same, except for the monopole run at low frequency. The recorded sonic waveforms are of very high quality, particularly the dipole recordings, but the very low velocity of this formation made it difficult for the automatic slowness/time coherence (STC) picking program to select accurate V_P . By increasing the maximum slowness of the dipole in the STC processing, we were able to accurately pick the shear velocity in the dipole waveforms. Despite an irregular hole, acoustic data from Hole 1251H are of good quality, but the DSI-derived V_P will require further reprocessing.

A final run was made for seismic experiments, which will be discussed elsewhere.

Logging Quality

The quality of the CWL data from Hole 1251H was moderately degraded by the size and uneven nature of the borehole (Figs. F40, F41, F42, F43). The triple combo caliper log from Hole 1251H (Fig. F40) shows borehole diameters greater than the 17.6-in maximum range of the caliper for a significant portion of the hole. The HLDT/APS/HNGS-derived gamma ray, density, and neutron porosity data are particularly susceptible to the adverse effects from large and irregular hole diameters. The gamma ray logs (Figs. F40, F41) from Hole 1251H were not significantly affected by the size of the borehole. However, the density log in Hole 1251H was severely degraded within the depth interval from 120 to 197 mbsf. Relative to the density log measurements, the neutron log does not appear as severely degraded. The DIT and DSI also appear to yield useful resistivity and acoustic data (as discussed above).

The absolute depths, relative to seafloor, for all of the CWL logs were fixed by identifying the gamma ray signal associated with the seafloor and depth shifting the logging data appropriately. The gamma ray pick for the seafloor in Hole 1251H was 1219.5 mbrf for both CWL runs.

Data from Holes 1251A and 1251H show excellent quality LWD logs and moderately degraded CWL logs. Analysis of LWD resistivity data and CWL acoustic and resistivity data suggests the presence of gas hydrate within two intervals at Site 1251, from 90 to 115 mbsf and 185 to 205 mbsf. Low- to high-density interbedding is observed throughout the hole below 130 mbsf, which likely indicates lithologic changes associated with turbidite sequences. The RAB images in Hole 1251A revealed borehole breakouts in the lower portion of the hole (300-380 mbsf) that are subparallel to similar features in Hole 1244D. NMR-MRP data were transmitted to shore for processing to estimate bound-fluid volume and total free-fluid porosity and for comparison with neutron, density, and core porosity estimates.

Logging-While-Drilling and Wireline Logging Comparison

Figure F42 shows a comparison of downhole LWD and CWL data from Holes 1251A and 1251H using the gamma ray, neutron porosity, density, photoelectric factor, and deep resistivity logs. The highly variable CWL data within the upper 85 mbsf of Hole 1251H was obtained through the drill pipe. Comparison of similar log signatures on Figure F42 reveals that the LWD data from Hole 1251A are offset by ~3 m relative to CWL data from Hole 1251H. This depth difference is best shown with the deep resistivity logs from the two holes. At a depth of ~196 mbsf in Hole 1251H, the CWL resistivity log shows a distinct increase in value; however, this same log response is at a depth of ~199 mbsf on the LWD data from Hole 1251A. This offset could possibly be explained by local variability in the geology of this site; however, Hole 1251H was located only 20 m south of Hole 1251A (Fig. F1). This apparent depth discrepancy will be further examined after the cruise. When considering the apparent depth difference between the two logged holes, the LWD and CWL gamma ray and resistivity logs as depicted in Figure F42 match relatively well, exhibiting similar curve shapes and absolute log values. The CWL (Hole 1251H) and LWD (Hole 1251A) density and neutron logs, however, are still characterized by numerous mismatches and anomalies that do not correlate between the two holes. As discussed above, the CWL was degraded by enlarged borehole conditions. Within intervals of relatively in-gauge hole, such as between 85 and 120 mbsf, the CWL and LWD density logs compare favorably with the core-derived density data. The CWL- (Hole 1251H) and LWD-recorded (Hole 1251A) resistivity logs exhibit some differences in measured values with depth and a difference in the apparent vertical resolution of each device, with the LWD RAB tool yielding a log with a higher vertical resolution.

Logging Units

The logged sequence in Holes 1251A and 1251H is divided into four "logging units" on the basis of obvious changes in the LWD and CWL gamma ray, density, electrical resistivity (Figs. F39, F40, F41), and acoustic velocity (Fig. F43).

Logging Unit 1 (0-36 mbsf) is characterized by increasing resistivities, densities, and gamma ray values with depth as measured by the LWD tools. However, this trend in the downhole logging data is probably due, in part, to degraded log measurements within the enlarged portion of the near-surface borehole as shown in Figure F38. The base of logging Unit 1 appears to coincide with the base of lithostratigraphic Subunit IB (34 mbsf), which is composed of clay to silty clay. The transition from logging Unit 1 to 2 is defined by an increase in electrical resistivity and formation densities.

Logging Unit 2 (36-205 mbsf) is characterized by zones of distinct high resistivity and somewhat higher V_P, with peak resistivity values exceeding 2 m and V_P recorded at >1.59 km/s. The gamma ray log in this unit shows a characteristic cyclicity of values that may reflect the silty clay to clay interbedded turbidite sequences as described by the shipboard sedimentologists for Lithostratigraphic Subunits IC, IIA, and the upper part of Subunit IIB (34-300 mbsf) (see "Lithostratigraphy"). The downhole LWD-measured densities increase with depth in logging Unit 2 (1.6 at the top to near 1.85 g/cm³ at the bottom). Since the CWL acoustic transit-time log was not able to pass below the expected depth of the BSR (~193 mbsf) at this site, other downhole logging data were used to identify the base of the deepest LWD-CWL inferred presence of gas hydrate. The RAB resistivity logs have been used to select the depth of the boundary between logging Units 2 and 3, which is marked by a relatively dramatic drop in resistivity of >1 m at a depth of 205 mbsf. Also noted on the LWD density log is a subtle drop in density at the contact between Units 2 and 3, which roughly corresponds to the depth of the BSR at this site. Because of an offset in resistivity logs between Holes 1251A and 1251H (as discussed above), the contact between logging Units 2 and 3 in Hole 1251H would be at 202 mbsf (Figure F42).

Logging Unit 3 (205-304 mbsf) correlates with the lower part of lithostratigraphic Subunit IIB (178-300 mbsf), which is described as an interbedded clay, silty clay, to sand turbidite sequence. Logging Unit 3 is generally characterized by numerous intervals of varying thickness that exhibit high resistivities (>1.7 m) and low densities (<1.7 g/cm³), which collectively suggest the presence of free gas-saturated sediments. The transition from Unit 3 to Unit 4 is marked by an abrupt drop in density (from ~1.9 to 1.7 g/cm³) (Fig. F39), which appears to mark the contact with the underlying deformed sediments of the accretionary complex.

Logging Unit 4 (304-380 mbsf; TD of Hole 1251A), reflecting the upper portion of the deformed sediments of the accretionary complex, is characterized by highly variable resistivity and density measurements that are the result of enlarged borehole breakouts (with DCAL values >1 in). These breakouts appear consistently with nearly a north-south orientation in the borehole.

Resistivity-at-the-Bit and Formation MicroScanner Images

The RAB tool produces high-resolution images of the electrical resistivity characteristics of the borehole wall that can be used for detailed sedimentological and structural interpretations. The RAB tool can also be used to make high-resolution electrical images of gas hydrates in the borehole, thus yielding information about the nature and texture of gas hydrates in sediments. During Leg 204, the RAB images proved to be a useful tool for evaluation of borehole breakouts, which are the product of differential horizontal stresses acting on the borehole. In Figure F44, the RAB image from Hole 1251A shows a dominant set of parallel borehole breakouts oriented approximately north-south.

Logging Porosities

Sediment porosity can be determined from analyses of recovered cores and from numerous borehole measurements (see "Physical Properties" and "Downhole Logging" both in the "Explanatory Notes"). Data from the LWD density, neutron, and NMR-MRP logs have been used to calculate sediment porosities for Hole 1251A. Core-derived physical property data, including porosity (see "Physical Properties"), have been used to both calibrate and evaluate the log-derived sediment porosities.

The VND LWD log-derived measurements of density in Hole 1251A (Fig. F39) are relatively consistent throughout most of the hole, with values ranging from ~1.5 near the seafloor to over 1.9 g/cm³ at the bottom of logging Unit 3 at 304 mbsf. The density log measurements are degraded in logging Unit 4, as discussed earlier in this chapter. The LWD log-derived density measurements (_b) from Hole 1251A were used to calculate sediment porosities () using the standard density-porosity relation,

= (_m - _b)/(_m - _w).

Water density (_w) was assumed to be constant and equal to 1.05 g/cm³; however, variable core-derived grain/matrix densities (_m) were assumed for each logging density-porosity calculation. The core-derived grain densities (_m) in Hole 1251A ranged from an average value at the seafloor of 2.69 to ~ 2.71 g/cm³ at the bottom of the hole (see "Physical Properties"). The density log-derived porosities in logging Units 1 through 3 (0-304 mbsf) of Hole 1251A range from ~45% to 70% (Fig. F45). However, the density logging porosities in logging Unit 4 (304-380 mbsf) are more variable, ranging from 45% to 72%, which is in part controlled by poor borehole conditions.

The LWD neutron porosity log from Hole 1251A (Fig. F45) yielded sediment porosities ranging from an average value at the top of the logged section of ~70% to ~60% in logging Unit 4. The "total" sediment porosities derived by the LWD NMR tool in Hole 1251A (Fig. F45) ranged from ~75% near the seafloor to ~55% near the bottom of the hole.

In studies of downhole logging data, it is common to compare porosity data from different sources to evaluate the results of particular measurements. The comparison of core- and log-derived porosities in Figure F45 reveals that the density log-derived porosities are generally similar to the core porosities in logging Units 2 through 3 (36-304 mbsf). However, the density log-derived porosities are generally higher than the core-derived porosities in logging Units 1 and 4. The neutron- and NMR-MRP-derived log porosities are generally similar to the core-derived porosities in logging Units 1 and 2, but the neutron and NMR-MRP log porosities are higher than the core-derived porosities throughout most of logging Units 3 and 4. The NMR-MRP porosity log also exhibits numerous low-porosity zones throughout the entire hole, which will be further evaluated after the cruise.

Gas Hydrate

Several specimens suspected of containing gas hydrate were preserved from Hole 1251C at depths of 175.4, 178.11, 189.87, and 190.72 mbsf (Fig. F46). Despite the limited samples of gas hydrates, it was inferred, based on geochemical pore water analyses (see "Interstitial Water Geochemistry"), IR image analysis of cores (see "Physical Properties"), and downhole-logging data that disseminated gas hydrate is present in portions of logging Unit 2. As previously discussed in "Downhole Logging" in the "Explanatory Notes" chapter, the presence of gas hydrate is generally characterized by increases in logging-measured electrical resistivities and acoustic velocities. Certain zones in logging Unit 2 at Site 1251 are characterized by distinct stepwise increases in both electrical resistivities and acoustic velocities.

Resistivity log data were used to quantify the amount of gas hydrate at Site 1251. For the purpose of discussion, it is assumed that the high resistivities and velocities measured in logging Unit 2 are due to the presence of gas hydrate. Archie's Relation,

S_w = (aR_w/^mR_t)^1/n

(see "Downhole Logging" in the "Explanatory Notes" chapter), was used with resistivity data (R_t) from the LWD RAB tool and porosity data () from the LWD density tool to calculate water saturations in Hole 1251A. It should be noted that gas hydrate saturation (S_h) is a measurement of the percentage of pore space in sediment occupied by gas hydrate, which is the mathematical complement of Archie-derived S_w , with

S_h = 1 - S_w.

For Archie's Relation, the formation water resistivity (R_w) was calculated from recovered core water samples and the Archie a and m variables were calculated using a crossplot technique, which compares the downhole log-derived resistivities and density porosities. See Collett and Ladd (2000) for details on how to calculate the required formation water resistivities and Archie variables. The values used for Site 1251 were a = 1, m = 2.8, and n = 1.9386.

Archie's Relation yielded water saturations (Fig. F46) ranging from an average minimum value of ~82% to a maximum value of 100% in logging Unit 2 (31-205 mbsf) of Hole 1251A, which implies the gas hydrate saturations in logging Unit 2 range from 0% to 18%. The low water saturations shown in logging Unit 3 that fall below the predicted base of the methane hydrate stability zone (Fig. F46) are indicative of free gas-bearing sediments.

Temperature Data

The TAP tool was deployed on the triple combo tool string in Hole 1251H (Fig. F47). During the process of coring and drilling, cold seawater is circulated in the hole, cooling the formation surrounding the borehole. Once drilling ceases, the temperature of the fluids in the borehole gradually rebounds to the in situ equilibrium formation temperatures. Thus, the temperature data from the TAP tool cannot be easily used to assess the nature of the in situ equilibrium temperatures. However, the plot of the first pass downgoing temperature profile in Figure F47 reveals several gradient changes, which were caused by borehole temperature anomalies. The temperature anomaly at ~78.5 mbsf is the base of the drill pipe during the initial descent of the triple combo tool string. The break in the slope of the first pass downgoing temperature log at a depth ~210 mbsf is near the depth of the BSR (205 mbsf) at this site.