DOWNHOLE LOGGING

Operations

LWD operations in Hole 1245A began at 0900 hr UTC on 18 July 2002, with tool initialization at the rig floor. LWD tools included the GeoVision Resistivity (resistivity at the bit [RAB]) with 9¹/₈-in button sleeve, measurement while drilling (MWD), NMR-MRP tool, and Vision Neutron Density tool (VND). Memory and battery life were increased to allow for up to 70 hr of continuous drilling. Hole 1245A was spudded at 0945 hr at 886.50 meters below rig floor (mbrf) water depth (drillers depth) on the western 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. Given calm heave conditions, the real-time data record was changed to increase the vertical resolution of the formation evaluation logs with less emphasis on high-resolution weight-on-bit and torque measurements. Mud pump noise affected the data transmission to a lesser extent than at Site 1244. LWD tools were pulled to 85 m clear of the seafloor at 0745 hr on 19 July for the dynamic positioning move to Site 1246. Total bit run was ~23 hr.

Logging Quality

Figure F47 shows the quality control logs for Hole 1245A. 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 meter), which was obtained over 86% 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 1245A. However, the quality of the RAB images in the interval from 17 to 25 mbsf in Hole 1245A (Fig. F48) is degraded by an apparent problem associated with low rates of bit rotation. The NMR-MRP porosity data were enhanced by using slow drilling rates, but the data-sampling resolution is less than that of the RAB, with one measurement every ~15 cm.

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 96% of the total section in Hole 1245A. Only the uppermost 24 mbsf of the hole shows washouts >1 in. The density correction, calculated from the difference between the short- and long-spaced density measurements, generally varies from 0 to 0.12 g/cm³ (Fig. F47), 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 ±0.017 g/cm³. The interval below 280 mbsf shows minor washouts resulting from borehole breakouts, with DCAL measurements up to 1 in; density measurements in this interval are slightly degraded.

Time-after-bit (TAB) measurements are 10 ± 3 min for ring resistivity and gamma ray logs. However, the TAB for the density and neutron porosity logs was more variable, ranging from 70 to 100 min (Fig. F47). In general, the TAB values remain relatively constant, coinciding with steady ROP while drilling over most of the hole, although some large variations in ROP are observed just below the seafloor and again at a depth of 62 mbsf.

The depths relative to seafloor were fixed for all of the LWD logs 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 1245A, it was determined that the gamma ray logging pick for the seafloor was at a depth of 882.0 mbrf. The rig floor logging datum was located 10.9 m above sea level.

Operations

Hole 1245E was originally planned to be RCB drilled and cored to a depth of 600 mbsf. With increasing hole stability problems, we decided to stop drilling at 550 mbsf and proceed with conventional wireline logging (CWL). The drill string became stuck shortly after starting the hole-conditioning program. The drill string was eventually freed after more than 14 hr of effort. It was determined that the hole should be safe and open to a depth of ~350 mbsf, which became the target depth for logging. Rig-up for logging operations began at 0300 hr on 15 August and final rig-down for the CWL operations was complete by 1630 on 16 August. See Table T23 for detailed information on the Hole 1245E CWL program.

CWL operations in Hole 1245E 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 T23). The triple combo tool string initially encountered a borehole bridge at a depth of ~320 mbsf that the tool string could not pass; TD of Hole 1245E was therefore redefined to be ~320 mbsf. Good quality data were acquired during the main uphole log pass (see below), and the tool was run back down to TD for a second pass. Before the start of the second log pass, several checkshots were performed with the QSST tool, but the signal-to-noise ratio was poor. QSST shot attempts were made with the WHC on and off, and the tool was lowered up and down and reseated at TD to improve coupling with the borehole wall, while at the same time extra cable slack was added. After stacking several QSST checkshots, the signal-to-noise ratio showed some improvement and a one-way traveltime of 758 ms was recorded at the TD of Hole 1245E (320 mbsf). To calculate an interval velocity with depth, a 32-m uphole shift is necessary to take into account the positioning of the QSST tool at the top of the triple combo tool string. The repeat pass of the triple combo tool string was performed over an interval from 320 mbsf up into the drill pipe at 73 mbsf. TAP tool data and associated depth data were recorded without any problems. The triple combo logging run ended with the rig-down of the tool string, which was completed at 1100 hr on 15 August.

For the second CWL run in Hole 1245E, 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 319 mbsf on two consecutive passes. For the first pass, the FMS button electrical current setting was too high and FMS images appeared very dark during real-time acquisition; this was rectified for the second pass. The FMS calipers showed that parts of the hole were elliptical in shape, one arm reading an average of ~12 in and the other up to 16 in, consistent with the density log caliper recorded on the triple combo run. Both passes of the FMS-sonic tool string appeared to track the same path up the borehole. DSI modes used for the first pass were standard-frequency monopole, low-frequency lower dipole, and standard frequency for the upper dipole. DSI modes used for the second pass were the same, except that the monopole was 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 compressional velocities. The lower-frequency monopole used in the second pass improved the quality of the automatic STC selections, but further reprocessing was still required.

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 tool string runs in Hole 1245E are of high quality (Figs. F49, F50, F51, F52). The hole conditions were generally good with density caliper measurement readings around 12 in, on average. Numerous relatively short washed-out intervals up to 16 in diameter are evident throughout the hole. Comparison of logs from successive passes shows good repeatability of the data, with only several notable small depth mismatches.

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 1245E was 883 mbrf for all of the CWL runs.

Data from Holes 1245A and 1245E show excellent quality LWD and CWL logs. The presence of gas hydrate was identified from ~50 to 131 mbsf by high resistivities and acoustic velocities and RAB image anomalies, allowing quantitative estimates of gas hydrate saturations. Interbedded layers of low and high density and variable natural gamma ray intensity are observed within and below the GHSZ, which may indicate lithologic changes associated with turbidites and the presence of a relatively thick free gas-saturated sand.

Figure F51 shows a comparison of downhole LWD and CWL data from Holes 1245A and 1245E, using the gamma ray, neutron porosity, density, photoelectric factor, and deep resistivity logs. The highly variable CWL log data within the upper 85 mbsf of Hole 1245E was obtained through the drill pipe. In general, the LWD and CWL data from each hole, as depicted in Figure F51, match relatively well, exhibiting similar curve shapes and absolute logging values. The CWL (Hole 1245E) and LWD (Hole 1245A) GRA density logs, however, are characterized by numerous low density anomalies that do not correlate between the two holes. However, both density logs in Figure F51 still compare favorably with the core-derived density data. In addition, the CWL (Hole 1245E) and LWD (Hole 1245A) 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.

The logged section in Holes 1245A and 1245E is divided into three "logging units," based on obvious changes in the LWD and CWL gamma ray, density, electrical resistivity (Figs. F48, F49, F50), and acoustic transit-time measurements (Fig. F52).

Logging Unit 1 (0-48 mbsf) is characterized by increasing resistivities and densities with depth as measured by the LWD tools. However, this trend in the downhole logging data is probably in part due to degraded log measurements within the enlarged portion of the near surface borehole as shown in Figure F47. The base of logging Unit 1 does not exactly coincide with the base of lithostratigraphic Unit I (0-31 mbsf), which is mainly composed of clay. The transition from logging Unit 1 to 2 is defined by a sharp increase in electrical resistivity.

Logging Unit 2 (48-131 mbsf) is characterized by zones of distinct high resistivities and moderate to high V_P , with peak resistivity values exceeding 2 m and V_P recorded at >1.61 km/s. The LWD-measured densities increase with depth in logging Unit 2 (1.65 g/cm³ at the top to near 1.80 g/cm³ at the bottom). In Hole 1245E (Fig. F52), 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 <1.56 km/s and corresponds to the depth of the BSR at this site. Logging Unit 2 is included in lithostratigraphic Unit II (31-212 mbsf), which is characterized as a diatom-bearing clay to silty clay interval with some sand-rich sections (see "Lithostratigraphic Unit II" in "Lithostratigraphic Units" in "Lithostratigraphy").

Logging Unit 3 (131-380 mbsf; TD of Hole 1245A) correlates with the lower part of lithostratigraphic Unit II (31-212 mbsf) and the upper part of lithostratigraphic Unit III (212-419 mbsf), both of which contain clay to silty clay turbidite sequences (see "Lithostratigraphic Units" in "Lithostratigraphy"). The gamma ray logs from this unit show a characteristic cyclicity that reflects the interbedded sand-silt-clay turbidite sequences. A 4-m-thick anomalous interval, characterized by variable V_P (ranging from 1.57 to 1.65 km/s), variable resistivity (ranging from ~0.8 to ~1.8 m), and low density (<1.5 g/cm³) is present in logging Unit 3 within the depth interval from 177 to 181 mbsf, which suggests the presence of a free gas-bearing sand. This apparent free gas-bearing interval corresponds to the seismic Horizon A (see "Introduction"). Toward the bottom of the hole (below 220 mbsf), the RAB resistivity images (Fig. F48) show borehole breakouts consistently at northwest-southeast orientations in the borehole.

Both the RAB and FMS tools produce high-resolution images of the electrical resistivity characteristics of the borehole wall that can be used for detailed sedimentological and structural interpretations. It is also possible to use the RAB and FMS tools to make high-resolution electrical 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. 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 F53 is characterized by light (high resistivity) to dark (low resistivity) bands, which in many cases can be traced across the display. The light continuous high-resistivity bands likely indicate that gas hydrate fills low- to high-angle fractures and nearly flatlying stratigraphic horizons in Hole 1245A.

In Figure F54, we have cross correlated a RAB image (Hole 1245A) and a FMS image (Hole 1245E) 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 F54, 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 F54 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.

As described above, logging Unit 3 (131-380 mbsf) correlates with the lower part of lithostratigraphic Unit II (31-212 mbsf) and the upper part of lithostratigraphic Unit III (212-419 mbsf), which are described as interbedded clay to silty clay turbidite sequences (see "Lithostratigraphic Units" in "Lithostratigraphy"). The RAB and FMS images in Figure F55, from logging Unit 3 in Holes 1245A and 1245E, 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, in which the conductive drilling fluids have penetrated more deeply into the formation.

Sediment porosities 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 nuclear magnetic resonance logs have been used to calculate sediment porosities from Hole 1245A. 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-derived measurements of bulk density in Hole 1245A (Fig. F48) are relatively consistent within logging Units 2 and 3, with values ranging from ~1.7 g/cm³ near the top of logging Unit 2 (depth = 48 mbsf) to >1.9 g/cm³ at the bottom of logging Unit 3 (depth = 380 mbsf). The LWD log-derived density measurements (_b) from Hole 1245A 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 1245A ranged from an average value at the seafloor of 2.69 to ~2.71 g/cm³ at the bottom of hole (see "Physical Properties"). The density log-derived porosities from Hole 1245A range from ~70% near the seafloor to ~50% at the bottom of logging Unit 3 (Fig. F56).

The LWD neutron porosity log from Hole 1245A (Fig. F56) yielded sediment porosities ranging from an average value at the top of the logged section of ~70% to ~60% in logging Unit 3. The "total" sediment porosities derived by the NMR-MRP tool in Hole 1245A (Fig. F56) ranged from ~70% near the seafloor to ~30% 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 LWD-derived porosities in Figure F56 reveals that the density log-derived porosities are generally similar to the core porosities in logging Units 2 through 3 (48-380 mbsf). However, the density log-derived porosities are generally higher than the core-derived porosities in logging Unit 1. The neutron and NMR-MRP-derived log porosities are generally similar to the core-derived porosities in logging Unit 2, but the neutron log porosities are higher than the core-derived porosities throughout most of logging Unit 3, whereas the NMR-MRP-log porosities are lower than the core-derived porosities in most of logging Unit 3.

The presence of gas hydrate at Site 1245 was documented by direct core sampling, with 11 gas hydrate samples recovered from Holes 1245B and 1245C within the depth interval from 54.10 to 129.26 mbsf. 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 and, possibly, in logging Unit 1. As previously discussed in "Downhole Logging" in the "Explanatory Notes" chapter, the presence of gas hydrate is generally characterized by increases in log-measured electrical resistivities and acoustic velocities. Logging Unit 2 at Site 1245 is characterized by a distinct stepwise increase in both electrical resistivities and acoustic velocities. In addition, the LWD resistivity tool reveals several thin high-resistivity zones within logging Unit 1 (0-48 mbsf), suggesting possible presence of gas hydrate.

Resistivity log data have been used to quantify the amount of gas hydrate at Site 1245. 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 (^m) from the LWD density tool to calculate water saturations in Hole 1245A. 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 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 1245 were a = 1, m = 2.8, and n = 1.9386.

Archie's Relation yielded water saturations (Fig. F57) ranging from an average minimum value of ~70% to a maximum of 100% in logging Unit 2 (48-131 mbsf) from Hole 1245A, which implies the gas hydrate saturations in logging Unit 2 range from 0% to 30%. 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. F57) correspond to zones that exhibit low acoustic velocities on the downhole recorded acoustic wireline logs (Fig. F52), which are indicative of free gas-bearing sediments.

The LDEO TAP tool was deployed on the triple combo tool string in Hole 1245E (Fig. F58). 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 temperature. 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 F58 reveals several gradient changes that were caused by borehole temperature anomalies. The temperature anomaly at 88 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 downgoing temperature log at a depth ~120 mbsf is near the depth of the BSR (131 mbsf) at this site. The anomalous temperature measurement at a depth of ~185 mbsf on the first pass downgoing temperature log is near the depth of seismic Horizon A, which may indicate gas flowing into the borehole.