m).
LWD operations at Site 1247 began at 0430 hr Universal Time Coordinated (UTC) on 20 July 2002, with tool initialization at the rig floor. LWD tools included the Geo Vision Resistivity (GVR) RAB with 91/8-in button sleeve, MWD, the NMR-MRP tool, and the VND. Batteries had sufficient remaining life and were not changed. Hole 1247A was spudded at 0715 hr at 845.00 meters below rig floor (mbrf) water depth (drillers depth) on the northwestern crest of Southern Hydrate Ridge. Drilling proceeded at ~25 m/hr to TD at 270 mbsf at 2130 hr on 20 July. Heave conditions had increased since the LWD operations at Site 1246, and the real-time data record was changed to increase the time resolution of weight-on-bit and torque measurements for heave analysis. LWD tools were pulled to ~60 m clear of the seafloor at 2315 hr on 20 July for the dynamic positioning move to Site 1248. Total bit run was ~18 hr.
Figure F32 shows the quality control logs for Hole 1247A. The target ROP of 25 m/hr (±5 m/hr) in the interval from the seafloor to TD was generally achieved. This was sufficient to record one sample per 4-cm interval (~25 samples per meter), which was obtained over 96% 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 1247A. However, the quality of the RAB images in the upper 15 mbsf of Hole 1247A (Fig. F33) is degraded by an apparent problem associated with low rates of bit rotation. The NRM-MRP porosity data were enhanced by using a slow drilling rate, with a data sampling resolution of approximately one sample per 15-cm interval.
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 DCAL values are <1 in over 96% of the total sections in Hole 1247A. Only the uppermost 47 mbsf of the hole contains washouts >1 in. The density correction, calculated from the difference between the short- and long-spaced density measurements, varies from 0 to 0.13 g/cm3 (Fig. F32), which generally suggests good-quality density measurements. A standoff of <1 in between the tool and the borehole wall also indicates high-quality density measurements, with an accuracy of ±0.015 g/cm3.
Time-after-bit (TAB) measurements are 12 ± 2 min for ring resistivity and gamma ray logs and 87 ± 5 min for density and neutron porosity logs (Fig. F32). TAB values remain relatively constant over the interval, coinciding with the steady ROP while drilling over most 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 log data to the appropriate depth, as determined by the drillers pipe tallies. For Hole 1247A, it was determined that the gamma ray log pick for the seafloor was at a depth of 847 mbrf. The rig floor logging datum was located 10.9 m above sea level for this hole.
Hole 1247B was APC and XCB cored to a depth of 220 mbsf (drillers depth). Rig-up for conventional wireline logging (CWL) operations began at 0035 hr on 24 August, and final rig-down was completed by 1215 hr on 24 August. See Table T15 for detailed information on the Hole 1247B CWL program.
CWL operations in Hole 1247B began with the deployment of the triple combo tool string (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 T15). The triple combo tool string initially reached the TD of the hole (220 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 (BOH) for a second log pass. The second pass also reached a TD of 220 mbsf, and excellent-quality data were recorded on the second ascent. TAP tool temperature data and associated depth data were recorded without problems during both lowerings of the triple combo tool string. After completing the second pass, the triple combo tool string was again lowered to a depth of 220 mbsf to obtain several checkshots with the QSST. A one-way traveltime of 678.5 ms was recorded at the TD of Hole 1247B (220 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 triple combo logging run ended with the rig-down of the tool string being completed at 0715 hr on 24 August.
For the second CWL run in Hole 1247B, the FMS-sonic tool string (FMS/Dipole Sonic Imager [DSI]/Scintillation Gamma Ray Tool [SGT]) was deployed. The FMS-sonic tool string reached a maximum depth of 220 mbsf on two consecutive passes. The two FMS-sonic runs confirmed the excellent condition of the hole, as observed during the triple combo logging run. The FMS images and sonic waveforms recorded from the two lowerings of the FMS-sonic tool string were of very high quality. During the first pass of the FMS-sonic tool string, the DSI was set at a low-frequency mode for the lower dipole, standard frequency for the upper dipole and low frequency for the monopole. During the second pass of the DSI, the monopole and the lower dipole were set at their standard frequencies, and the upper dipole was set at a low frequency. The recorded sonic waveforms from both lowerings of the DSI 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 compressional velocities. Some adjustment of the STC parameters allowed for improved VP picks, but further reprocessing is required. The quality of the recorded shear 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.
All logging data from the triple combo and FMS-sonic runs in Hole 1247B are of very high quality (Figs. F34, F35, F36, F37). The hole conditions were excellent, with an almost straight HLDT caliper measurement from 11.8 to 12.5 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 logging data appropriately. The gamma ray pick for the seafloor in Hole 1247B was 846 mbrf for all of the CWL runs.
Data from Holes 1247A and 1247B show excellent quality LWD and CWL logs. Low ROPs and reduced pump rates during LWD operations in Hole 1247A greatly reduced the effect of borehole washouts observed at other sites drilled earlier during Leg 204. No sliding tests were conducted to evaluate downhole LWD tool motion at this site. The downhole LWD and CWL logs reveal in situ gas hydrate as high-resistivity zones and RAB image anomalies. The high-resistivity nature of the gas hydrate-bearing interval in Hole 1247A allowed quantitative estimates of gas hydrate saturations. Resistivity and density log variations below the GHSZ (~128 mbsf) appear to indicate lithologic changes and the possible presence of free gas. Borehole breakouts, which result from subsurface horizontal stress differences, are observed in the lower portion of the hole. NRM-MRP logs were also reprocessed to compute bound fluid volume and total free fluid porosities.
Figure F36 shows a comparison of downhole LWD and CWL data from Holes 1247A and 1247B, using the gamma ray, neutron porosity, density, photoelectric factor, and deep resistivity logs. The highly variable CWL log data within the upper 78 mbsf of Hole 1247B was obtained through the drill pipe. Comparison of similar log signatures on Figure F36 reveals that the CWL logging data from Hole 1247B matches the LWD logging data from Hole 1247A. The LWD and CWL data from each hole exhibit similar curve shapes and absolute log values. The CWL (Hole 1247B) and LWD (Hole 1247A) resistivity logs, however, 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.
The logged section in Holes 1247A and 1247B is divided into four "logging units" on the basis of obvious changes in the LWD and CWL gamma ray, density, electrical resistivity (Figs. F33, F34, F35), and acoustic velocity (Fig. F37).
Logging Unit 1 (0-37 mbsf) is characterized by increasing gamma ray values, resistivity, and density with depth as measured by the LWD tools. However, this trend in the downhole logging data is probably due in part to degraded logging measurements within the enlarged portion of the near-surface borehole as shown in Figure F32. The base of logging Unit 1 does not exactly coincide with the base of lithostratigraphic Unit I (0-27 mbsf), which is composed of silty clay sediments. The transition from logging Unit 1 to 2 is defined by a sharp increase in LWD-derived density (from ~1.55 to ~1.70 g/cm3) and a relatively subtle increase in resistivity (from ~1.0 to ~1.1 m).
Logging Unit 2 (37-128 mbsf) is characterized by zones of distinct high resistivities and high acoustic velocities, with measured peak resistivity values >1.8 m and VP > 1.70 km/s. The gamma ray log in this unit shows a characteristic cyclicity of values that may reflect the interbedded sand and clay turbidite sequences as described by the shipboard sedimentologists for lithostratigraphic Units II and III (27-220.62 mbsf) (see "Lithostratigraphic
Units" in "Lithostratigraphy"). The downhole logging-measured density increases with depth in logging Unit 2 (from 1.6 at the top to near 1.9 g/cm3 at the bottom). In Hole 1247B (Fig. F37), 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 an abrupt drop in VP to <1.53 km/s. Also noted on the density log is a subtle drop in density at the contact between logging Unit 2 and 3 (of ~0.1 g/cm3), which corresponds to the estimated depth of the BSR at this site.
Logging Unit 3 (128-178 mbsf) correlates with the lower part of lithostratigraphic Unit III (60-220.65 mbsf), which is described as a diatom- and foraminifer-bearing silty clay turbidite sequence. Logging Unit 3 is generally characterized by lower and more uniform resistivities compared to Unit 2. The transition from logging Unit 3 to 4 is marked by an abrupt drop in gamma ray values (from ~65 to 55 American Petroleum Institute gamma ray units [gAPI]) and a more subtle drop in density (Fig. F33), which appears to mark the contact with the deformed sediments of the accretionary complex. A 2-m-thick anomalous interval, characterized by variable VP (ranging from 1.51 to 1.54 km/s), variable resistivity (ranging from ~1.1 to ~1.9 m), and low density (<1.45 g/cm3) is present in logging Unit 3, within the depth interval from 163 to 165 mbsf, which collectively suggests the presence of a free gas-bearing sand. This apparent free gas-bearing interval corresponds to seismic Horizon A (see "Introduction").
Logging Unit 4 (178-270 mbsf; TD of Hole 1247A), reflecting the upper portion of the deformed sediments of the accretionary complex, is characterized by almost constant gamma ray and density log measurements with depth that are not consistent with a normal compaction profile.
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. The RAB and the FMS tools can also be used to make high-resolution images of gas hydrates in the borehole, thus yielding information about the nature and texture of gas hydrate in sediments. The resolution of the images from the RAB tool is considerably lower than the resolution of the images from the FMS. 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.
In Figure F38, we have cross correlated a RAB image (Hole 1247A) and an FMS image (Hole 1247B) from the stratigraphic interval that contains Horizon A, which has been identified as a prominent regional seismic reflector (see "Introduction"). As shown on the FMS and RAB images in Figure F38, Horizon A appears as a complex interbedded zone of high and low resistivities. More detailed examination of the FMS image shows distinct lateral variability within this interval and apparent fine-scale sedimentologic structures. The comparison of the deep- and shallow-measuring RAB images in Figure F38 also shows evidence of geologic controls on the infiltration of conductive drilling fluids into the formation, which appears more prevalent in the shallow measuring RAB image.
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" chapter). Data from the LWD density, neutron, and NMR-MRP logs have been used to calculate sediment porosities for Hole 1247A. 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 1247A (Fig. F33) are relatively consistent within logging Units 2 and 3, with values ranging from ~1.6 near the top of Unit 2 (37 mbsf) to >1.8 g/cm3 at the bottom of Unit 4 at 250 mbsf. The density log measurements are degraded in logging Unit 1 as discussed earlier in this chapter. The LWD log-derived density measurements from Hole 1247A 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 1247B ranged from an average value at the seafloor of 2.70 to ~2.73 g/cm3 at the BOH (see "Physical Properties"). The density log-derived porosities in logging Units 2 through 4 (37-270 mbsf) of Hole 1247A range from ~46% to 62% (Fig. F39). However, the density log porosities in logging Unit 1 (0-37 mbsf) are more variable, ranging from 67% to 88%, which is in part controlled by poor borehole conditions.
The LWD neutron-porosity log from Hole 1247A (Fig. F39) yielded sediment porosities ranging from an average value at the top of the logged section of ~68% to ~55% in logging Unit 4. The "total" sediment porosity derived by the NMR-MRP tool in Hole 1247A (Fig. F39) ranged from ~80% near the seafloor to ~45% near the bottom of the hole.
In studies of downhole logging data it is common to combine and compare porosity data from different sources to evaluate results and assess the accuracy of a particular measurement. The comparison of core-derived and LWD log-derived porosities in Figure F39 reveals that the density and NMR-MRP-derived porosities are generally similar to the core porosities in most of logging Units 2 and 3 (37-178 mbsf). However, the density and NMR-MRP log-derived porosities are generally higher than the core-derived porosities in logging Unit 1 and the upper portion of logging Unit 2, which can be attributed to downhole logging data that have been degraded by enlarged borehole conditions. The neutron porosities are generally higher than the core-derived porosities throughout most of the hole.
The presence of gas hydrate at Site 1247 was documented by direct sampling, with one specimen of gas hydrate recovered in Hole 1247B at a depth of 93.01 mbsf and a significant IR anomaly perhaps related to gas hydrate observed at 113.09 mbsf. Despite this limited occurrence of gas hydrates, it was inferred, based on geochemical core analyses (see "Interstitial Water Geochemistry"), IR 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, the presence of gas hydrate is generally characterized by increases in logging-measured electrical resistivities and acoustic velocities. Logging Unit 2 at Site 1247 is characterized by a distinct stepwise increase in both electrical resistivities and acoustic velocities. In addition, the LWD resistivity tool revealed several thin high-resistivity zones within logging Unit 1 (0-37 mbsf), which suggests the possible presence of gas hydrate.
LWD resistivity log data have been used to quantify the amount of gas hydrate at Site 1247. For the purpose of discussion, it is assumed that the high resistivity measured in logging Unit 2 is a result of the presence of gas hydrate. Archie's Relation,
mRt)1/n
(see "Downhole Logging" in the "Explanatory Notes" chapter), was used with resistivity data (Rt) from the LWD RAB tool and porosity data (m) from the LWD density tool to calculate water saturations in Hole 1247A. It should be noted that gas hydrate saturation (Sh) is the measurement of the percentage of pore space in sediment occupied by gas hydrate, which is the mathematical complement of Archie-derived Sw , with
For Archie's Relation, the formation water resistivity (Rw) was calculated from recovered core-water samples and the Archie a and m variables were calculated using a crossplot technique which compares the downhole logging-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 for Site 1247 were a = 1, m = 2.8, and n = 1.9386.
Archie's Relation yielded water saturations (Fig. F40) ranging from an average minimum value as low as of ~85% to a maximum of 100% in logging Unit 2 (37-128 mbsf) of Hole 1247A, which implies the gas hydrate saturations in logging Unit 2 range from 0% to 15%. Figure F40 also reveals that logging Unit 1 may contain a small amount of gas hydrate. However, the low water saturations shown in logging Unit 3 (Fig. F40) may indicate the presence of free gas-bearing sediments (as discussed previously in this chapter).
The TAP tool was deployed on the triple combo tool string in Hole 1247B (Fig. F41). 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 F41 reveals several gradient changes that were caused by borehole temperature anomalies. The temperature anomaly at ~90 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 of ~140 mbsf is near the depth of the BSR (128 mbsf) at this site.
