DOWNHOLE LOGGING

Operations

Leg 204 LWD operations began on 16 July 2002 at 1330 hr, with initial BHA makeup, tool initialization, and calibration. The LWD tools (6-in collars) included the GeoVision Resistivity (GVR) (RAB) with a 9¹/₈-in button sleeve, MWD (Powerpulse), NMR-MRP tool, and VND tool. The U.S. Department of Energy provided funding support to deploy the NMR-MRP tool. Memory and battery life allowed for at least 24 hr of continuous drilling. Hole 1244D was spudded at ~2300 hr at 906.00 meters below rig floor (mbrf) water depth (drillers depth) on the eastern flank of Hydrate Ridge. Drilling proceeded at ~25 m/hr to TD at 380 mbsf without difficulty, and real-time data were transmitted to the surface at a rate of 6 Hz. Some extraneous pump noise affected the data transmission for 2-5 min after each pipe addition but caused minimal real-time data loss. From the BOH, the tools were pulled out of the hole without rotating to ~355 mbsf to evaluate the effect of drilling motion on the NMR-MRP log. LWD tools and data were retrieved at the rig floor at ~0130 hr on 18 July. The total bit run was ~38 hr.

Logging Quality

Figure F39 shows the quality control logs for Hole 1244D. The target rate of penetration (ROP) of 25 m/hr (±5 m/hr) in the interval from the seafloor to TD was generally achieved. This is sufficient to record one sample per 4-cm interval (~25 samples per m), which was obtained over 85% of the total section of the hole. Using slow drilling rates enhanced the NMR-MRP porosity data, and the data sampling resolution is approximately one sample per 15-cm interval. The quality of RAB images is good, and no significant resolution loss is observed with variation in ROP in Hole 1244D. However, the quality of the RAB images in the upper 38 m of Hole 1244D (Fig. F40) is degraded by an apparent problem associated with low rates of bit rotation.

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 92% of the total section in Hole 1244D. Only the uppermost 38 m of the hole shows washouts >1 in. The bulk density correction (DRHO), calculated from the difference between the short- and long-spaced density measurements, varies from 0 to 0.1 g/cm³ (Fig. F39), which shows the good quality of the density measurements. 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. The interval below 250 mbsf shows minor washouts resulting from borehole breakouts, with differential caliper measurements up to 1 in. Density measurements below 250 mbsf are not accurate.

Time-after-bit (TAB) measurements are 10 ± 2 min for ring resistivity and gamma ray logs and 87 ± 5 min for density and neutron porosity logs (Fig. F39). TAB values remain relatively constant, coinciding with steady ROP while drilling over most of the hole, although some large variations in ROP are observed during pipe additions toward the bottom of the hole. Large values on the DCAL and DRHO logs near the bottom of the hole also indicate that the borehole was enlarged.

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 log data to the appropriate depth as determined by the drillers pipe tallies. For Hole 1244D, it was determined that the gamma ray log pick for the seafloor was at a depth of 906.0 mbrf. The rig floor logging datum was located 10.9 m above sea level for this hole.

Operations

Hole 1244E was APC cored to a depth of 135.8 mbsf and then drilled to a TD of 250 mbsf. Rig-up for conventional wireline logging (CWL) operations began at 2230 Universal Time Coordinated (UCT) on 20 August and final rig-down was completed by 0945 UCT on 21 August. See Table T22 for detailed information on the Hole 1244E CWL program.

CWL operations in Hole 1244E began with the deployment of the triple combo tool (Temperature/Acceleration/Pressure [TAP] tool/Dual Induction Tool [DIT]/Hostile Environment Litho-Density Tool [HLDT]/Accelerator Porosity Sonde [APS]/Hostile Environment Gamma Ray Sonde [HNGS]/Inline Checkshot Tool [QSST]) (Table T22). The triple combo tool string initially reached TD of the hole (250 mbsf) without difficulty and with no sticking problems. Excellent quality data were acquired during the main uphole pass (see below), and the tool was run back to the bottom of the hole for a second log pass. Before the start of the second log pass, several checkshots were performed with the QSST. A one-way traveltime of 733 ms was recorded at the TD of Hole 1244E (250 mbsf). To calculate a checkshot interval velocity with depth, a 32-m uphole shift is necessary to take into account the positioning of the QSST at the top of the triple combo tool string. The repeat pass of the triple combo tool was performed over an interval from 245.5 up into the drill pipe at a depth of 72.5 mbsf. TAP tool temperature data and associated depth data were recorded without problems. The triple combo logging run ended with the rig-down of the tool string, which was completed at ~0515 hr on 21 August.

For the second CWL run in Hole 1244E, the FMS-sonic string (FMS/Dipole Shear Sonic Imager [DSI]/Scintillation Gamma Ray Tool [SGT]) was deployed. The FMS-sonic tool string reached maximum depth of 250.5 mbsf on two consecutive passes. The two FMS-sonic runs confirmed the excellent condition of the hole, as observed during the triple combo log run. Both passes of the FMS-sonic tool string appeared to follow a helicoidal path throughout the entire borehole. DSI modes used in Hole 1244E were medium-frequency monopole for the first pass, low-frequency monopole for the second pass, and for both passes low-frequency mode for the lower dipole standard frequency for the upper dipole. 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 . Some adjustment of the STC parameters allowed for improved P-wave picks but further reprocessing was required. The quality of the recorded shear (S-) wave data was very high, but it will also require additional processing.

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

Logging Quality

All logging data from the triple combo and FMS-sonic runs in Hole 1244E are of very high quality (Figs. F41, F42, F43, F44). The hole conditions were excellent, with an almost straight HLDT caliper measurement from 11.4 to 12.0 in, on average. Comparison of logs from successive passes shows good repeatability of the data. The two passes of the FMS calipers also showed that the hole was nearly cylindrical, consistent with the HLDT log caliper recorded on the triple combo runs.

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 log data appropriately. The gamma ray pick for the seafloor in Hole 1244E was 904.5 mbrf for all of the CWL runs.

Holes 1244D and 1244E show excellent quality LWD and CWL logs. The presence of gas hydrate was identified from ~40 to 127 mbsf by high resistivities, high acoustic velocities, and RAB image anomalies, allowing quantitative estimates of gas hydrate saturations. RAB imaging also revealed evidence of fractures filled with gas hydrate. Low- and high-density interbedding is observed below the GHSZ (~127 mbsf), which may indicate lithologic changes within lithostratigraphic Unit II associated with turbidites, ash layers, and/or the presence of gas. Borehole breakouts, which result from horizontal stress differences, are observed in the lower portion of the hole. 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.

Comparison of Logging-While-Drilling and Wireline Logs

Figure F43 shows a comparison of downhole LWD and CWL data from Holes 1244D and 1244E, using the gamma ray, neutron porosity, density, photoelectric factor, and deep-resistivity logs. The highly variable CWL logging data within the upper 75 mbsf of Hole 1244E was obtained through the drill pipe. Comparison of similar logging signatures on Figure F43 reveals that the CWL logging data from Hole 1244E are ~4 m deeper than the LWD from Hole 1244D. This difference is best shown with the deep resistivity and density logs from the two holes. At a depth of ~156 mbsf in Hole 1244D, the LWD density and resistivity logs show a distinct increase in value; however, this same log response is at a depth of ~160 mbsf on the CWL data from Hole 1244E. This difference is best explained by local variability in the geology of this site, with Hole 1244E being drilled more than 90 m southeast of Hole 1244D. After taking into account the apparent depth offset between Holes 1244D and 1244E, it can be seen that the LWD and CWL data from each hole match relatively well (Fig. F43), exhibiting similar curve shapes and absolute log values. The CWL (Hole 1244E) and LWD (Hole 1244D) resistivity logs exhibit differences in measured values with depth and a difference in the apparent vertical resolution of each device, with the RAB (LWD) tool yielding a log with a higher vertical resolution.

Logging Units

The logged section in Holes 1244D and 1244E is divided into four "logging units" on the basis of obvious changes in the LWD and CWL gamma ray, density, electrical resistivity (Figs. F40, F41, F42), and acoustic transit-time measurements (Fig. F44).

Logging Unit 1 (0-43 mbsf) is characterized by increasing resistivities and densities with depth as measured by the LWD tools. However, this trend in the downhole log data is probably due in part to degraded log measurements within the enlarged portion of the near-surface borehole as shown in Figure F39 and evidenced by the discrepancy between the core and LWD log density data between 0 and 30 mbsf (Fig. F40). The base of logging Unit 1 does not coincide with the base of lithostratigraphic Unit I (0-62 mbsf), which is composed of clay to silty clay sediments. The transition from of logging Unit 1 to 2 is defined by a sharp increase in electrical resistivity.

Logging Unit 2 (43-127 mbsf) is characterized by zones of distinct high resistivities and high compressional velocities (V_P), with peak resistivity values exceeding 2 m and V_P recorded at >1.65 km/s. The gamma ray logs in this unit show a characteristic cyclicity of values that may reflect the interbedded sand and clay turbidite sequences as described by the shipboard sedimentologists for lithostratigraphic Unit II (62-250 mbsf) (see "Lithostratigraphic Unit II" in "Lithostratigraphic Units" in "Lithostratigraphy"). The downhole log-measured density increases with depth in logging Unit 2 (1.7 g/cm³ at the top to near 2.0 g/cm³ at the bottom). In Hole 1244E (Fig. F44), the acoustic transit-time log has been used to precisely select the depth of the boundary between logging Units 2 and 3, which is marked by a drop in V_P to <1.55 km/s. Also noted on the LWD density log is a subtle drop in bulk density at the contact from logging Unit 2 into 3, which corresponds to the depth of the BSR at this site.

Logging Unit 3 (127-247 mbsf) correlates with the lower part of lithostratigraphic Unit II (62-250 mbsf), which is described as an interbedded sand, silt, and clay turbidite sequence. Logging Unit 3 is generally characterized by lower and more uniform resistivities and acoustic velocities compared to logging Unit 2. The transition from logging Unit 3 to 4 is marked by an abrupt drop in electrical resistivity (from ~1.3 to 1.0 m) and bulk density (from ~1.9 to 1.6 g/cm³) (Fig. F40), which appears to mark the contact with the deformed sediments of the accretionary complex.

Logging Unit 4 (247-380 mbsf; TD of Hole 1244D), reflecting the upper portion of the deformed sediments of the accretionary complex, is characterized by highly variable resistivity and density measurements that are in part the result of enlarged borehole breakouts (with DCAL values >1 in) but also reflect lithologic variation. The breakouts appear consistent with a northeast-southwest orientation in the borehole.

Resistivity-at-the-Bit and Formation MicroScanner Images

Both the RAB and FMS tools produce high-resolution images of the electrical resistivity of the borehole wall that can be used for detailed sedimentological and structural interpretations. It is also possible to use the RAB and the FMS tools 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. The resolution of the images from the RAB tool is considerably lower than the resolution of the images from the FMS. For example, the RAB images have about a 5- to 10-cm vertical resolution, whereas the FMS tool can resolve features such as microfractures with widths <1 cm. However, the RAB tool provides 360° coverage of the borehole, whereas FMS images cover only ~30% of the hole.

The RAB image in Figure F45 is from within the zone of expected gas hydrate stability in Hole 1244D. This RAB image is characterized by light (high resistivity) to dark (low resistivity) bands, which, in many cases, can be traced across the display as sine waves. These light continuous high-resistivity sine wave bands likely represent gas hydrate occupying moderately to steeply dipping fractures (east-northeast dip direction) in Hole 1244D.

In Figure F46, we have cross correlated a RAB image (Hole 1244D) and an FMS image (Hole 1244E) of an ash layer, as identified by the shipboard sedimentologists at a depth of ~216 mbsf in Hole 1244C (see "Lithostratigraphic Unit II" in "Lithostratigraphic Units" in "Lithostratigraphy"). As shown in Figure F46, the ash layer, imaged by the RAB and FMS tools, appears as a complex interbedded zone of high and low resistivities. More detailed examination of the FMS image shows distinct lateral variability within this ash layer.

As described above, logging Unit 3 (127-247 mbsf) correlates with the lower part of lithostratigraphic Unit II (62-250 mbsf) (see "Lithostratigraphic Unit II" in "Lithostratigraphic Units" in "Lithostratigraphy"), which is described as an interbedded sand, silt, and clay turbidite sequence. The RAB and FMS images in Figure F47, from logging Unit 3 in Holes 1244D and 1244E, are characterized by interbedded light (resistive) and dark (conductive) layers. The darker, more conductive layers usually represent the more porous, coarser-grained fraction of the turbidite sequence where the conductive drilling fluids have penetrated more deeply into the formation.

During Leg 204, the RAB images proved to be a very useful tool with which to evaluate the occurrence of borehole breakouts, which are the product of differential horizontal stress acting on the borehole. In Figure F48, the RAB image from Hole 1244D shows a dominant set of parallel borehole breakouts oriented approximately east-northeast to west-southwest.

Logging Porosities

Sediment porosities can be determined from analyses of recovered cores and from numerous borehole measurements (see and "Downhole Logging" in the "Explanatory Notes" chapter). Data from the LWD density, neutron, and nuclear magnetic resonance logs have been used to calculate sediment porosities for Hole 1244D. Core-derived physical property data, including porosities (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 1244D (Fig. F40) are relatively consistent within the upper 285 mbsf of the hole, with values ranging from ~1.4 g/cm³ near the seafloor to over 1.8 g/cm³ at the bottom of logging Unit 3 at 247 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 1244D were used to calculate sediment porosities () using the standard density-porosity relation,

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

Water densities (_w) were assumed to be constant and equal to 1.05 g/cm³; however, variable core-derived grain/matrix densities were assumed for each log density-porosity calculation. The core-derived grain densities (_m) for Hole 1244D ranged from an average value at the seafloor of 2.69 g/cm³ to about 2.71 g/cm³ at the bottom of the hole (see "Physical Properties"). The density log-derived porosities in logging Units 1-3 (0-247 mbsf) of Hole 1244D range from ~40% to 70% (Fig. F49). However, the density log porosities in logging Unit 4 (247-380 mbsf) are more variable, ranging from 35% to 95%, which is in part controlled by poor borehole conditions.

The LWD neutron porosity log from Hole 1244D (Fig. F49) 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 NMR-MRP tool in Hole 1244D (Fig. F49) ranged from ~80% near the seafloor to ~0% near the bottom of the hole.

In studies of downhole log data it is common to compare porosity data from different sources to evaluate the results of particular measurements. The comparison of core-derived and LWD log-derived porosities in Figure F49 reveals that the density log-derived porosities are generally similar to the core porosities in logging Units 1 though 3 (0-247 mbsf). However, the density log-derived porosities are generally higher than the core porosities in Unit 4. The neutron log-derived porosities are generally similar to the core-derived porosities in logging Units 1 and 2, but the neutron log porosities are higher than the core-derived porosities throughout most of logging Unit 3. The NMR-MRP porosities are generally lower than the core-derived porosities in logging Units 2 through 3. The NMR-MRP porosities in logging Unit 1 (above ~25 mbsf) appear to have been significantly degraded by washouts in the upper part of the hole. 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

The presence of gas hydrates at Site 1244 was documented by direct sampling, with several pieces of gas hydrate being recovered in Holes 1244C and 1244E within the depth interval from 49.94 to 126.70 mbsf. Despite these limited occurrences of visible gas hydrates, it was inferred, based on geochemical core analyses (see "Interstitial Water Geochemistry"), IR image analysis of cores (see "Physical Properties"), and downhole logging data, that disseminated gas hydrate is present in logging Unit 2. As previously discussed in "Downhole Logging" in the "Explanatory Notes" chapter, gas hydrate occurrences are generally characterized by increases in log-measured electrical resistivities and acoustic velocities. Logging Unit 2 at Site 1244 is characterized by a distinct stepwise increase in both electrical resistivities and acoustic velocities. In addition, the RAB images reveal several thin high-resistivity zones within logging Unit 1 (0-43 mbsf), suggesting the possible occurrence of gas hydrate.

Resistivity log data have been used to quantify the amount of gas hydrate at Site 1244. For the purpose of this 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 1244D. It should be noted that gas hydrate saturation (S_h) is the measurement of the percentage of pore space in sediment occupied by gas hydrate, which is the mathematical complement of Archie-derived pore water saturations (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 the details on how to calculate the required formation water resistivities and Archie variables. The values used at Site 1244 were: a = 1, m = 2.8, and u = 1.9386.

The Archie Relation yielded water saturations (Fig. F50) ranging from an average minimum value of ~75% to a maximum of 100% in logging Unit 2 (43-127 mbsf) of Hole 1244D, which implies the gas hydrate saturations in logging Unit 2 range from 0% to 25%. It also appears that logging Unit 1 may contain several thin gas hydrate-bearing intervals. However, the low water saturations shown in logging Unit 3 (Fig. F50) correspond to zones that exhibit low acoustic velocities on the downhole-recorded acoustic wireline logs (Fig. F44), which is indicative of free gas-bearing sediments.

Temperature Data

The LDEO TAP tool was deployed on the triple combo tool string in Hole 1244E (Figs. F48, F51). 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 rebound to the in situ equilibrium formation temperatures. Thus, the temperature data from the TAP tool can not 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 F51 reveals several gradient changes that were caused by borehole temperature anomalies. The temperature anomaly at 87.5 mbsf is the base of the drill pipe during the initial descent of the triple combo tool string. The break in the slope of first pass down going temperature log at a depth ~120 mbsf is near the depth of the BSR (127 mbsf) at this site.