DOWNHOLE LOGGING

Operations

LWD operations at Site 1250 consisted of drilling two dedicated LWD boreholes in Holes 1250A and 1250B. LWD operations in Hole 1250B were conducted because of a RAB tool failure in Hole 1250A. Site 1250 LWD operations began at 0330 hr Universal Time Coordinated (UTC) on 22 July 2002 by spudding Hole 1250A at a water depth (drillers depth) of 807.00 meters below rig floor (mbrf) on the crest of southern Hydrate Ridge. The LWD tools deployed in Hole 1250A included the GeoVision Resistivity (GVR; RAB), MWD (Powerpulse), Nuclear Magnetic Resonance (NMR-MRP) tool, and Vision Neutron Density (VND) tool. Drilling proceeded at reduced ROP of 15 m/hr and low fluid circulation rates of 15 spm to minimize formation washout in the unconsolidated sediments below seafloor. No real-time MWD or NMR-MRP data were recorded over this interval, as the pump rate was insufficient to activate the turbines in the downhole tools. The ROP was increased to ~25 m/hr, fluid circulation was returned to more normal levels at a bit depth of 20 mbsf, and real-time MWD and NMR-MRP data were recorded to TD (210 mbsf). The LWD tools were pulled to the rig floor at 2215 hr on 22 July for a total bit run of ~18 hr. Data from four sequential holes (Holes 1247A, 1248A, 1249A, and 1250A) were downloaded from the LWD tools, which amounted to ~76 MB of binary data. At this point, it was determined that the battery power in the RAB tool had been unexpectedly depleted during the logging run in Hole 1250A, and resistivity and gamma data were not recorded for this hole. Drilling time for the entire four-hole suite was 65 hr.

As noted above, the reason for drilling Hole 1250B was the loss of the RAB data in Hole 1250A. LWD operations for Hole 1250B began with initialization of Azimuthal Density Neutron (ADN), RAB, and NMR-MRP tools and running the bottom-hole assembly to 60 m above the seafloor at 2400 hr on 23 July. Hole 1250B was also spudded at a water depth of 807.00 mbrf (drillers depth). Drilling proceeded to 25 mbsf with an average ROP of 20 m/hr and pump-stroke circulation rate of 15 spm. No real-time MWD or NRM-MRP data were recorded over this interval. The ROP was increased to a relatively high rate of 50 m/hr at 25 mbsf and maintained to TD at 180 mbsf in an attempt to speed up operations. The pipe was pulled up from the BOH to 160 mbsf without rotating to evaluate the effect of drilling motion on the NMR-MRP log. The tools were recovered at the rig floor at 1400 on 23 July and laid down prior to downloading data from Hole 1250B. For Hole 1250B the total bit run was ~8 hr.

Logging Quality

Figure F34 shows the quality control logs for Holes 1250A and 1250B. As noted above, the target ROP for the two LWD logging runs at Site 1250 were different (in Hole 1250A, target ROP below 20 mbsf was ~25 m/hr; in Hole 1250B, target ROP below 25 mbsf was ~50 m/hr), which provided an opportunity to compare the effects of variable drilling conditions on the LWD data obtained in both holes. The actual recorded ROP below ~20 mbsf in Hole 1250A was consistent at 25 m/hr (±5 m/hr). However, the measured ROP in Hole 1250B was more variable below 25 mbsf, with measured ROP from 30 to 70 m/hr. In Hole 1250A, an ROP of 25 m/hr was sufficient to record one sample per 4-cm interval (~25 samples per meter). With a ROP of 50 m/hr in Hole 1250B, a measurement was obtained every 10 cm (~15 samples per meter). The quality of the RAB image from Hole 1250B is quite high, and no significant resolution loss was observed with variation in ROPs. The increased pump rates below 20 mbsf (Hole 1250A) and 25 mbsf (Hole 1250B) yielded enhanced NRM-MRP porosity data, 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 borehole wall 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 drilled section in Hole 1250A, and ~86% of Hole 1250B is characterized by DCAL measurements <1 in. Only the uppermost ~18 m of Hole 1250B contains relatively thick washout intervals of >1 in. The density correction, calculated from the difference between the short- and long-spaced density measurements, varies from 0 to ~0.12 g/cm³ in both Holes 1250A and 1250B (Fig. F34), which suggests very high 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.

Below 30 mbsf in Hole 1250A, the time-after-bit (TAB) measurements were 90 ± 10 min for the density and neutron porosity logs (Fig. F34). The recorded TABs for Hole 1250B (below 20 mbsf) were significantly shorter (55 ± 10 min) than those measured in Hole 1259A because of the higher ROP in Hole 1250B.

The recorded LWD data from Hole 1250B are of very high quality. The density and neutron logs from Hole 1250B closely match the logs obtained from Hole 1250A. There is minimal reduction in vertical resolution resulting from the faster ROP in Hole 1250B, and the borehole is in relatively good shape throughout the shallow interval of both LWD holes drilled at Site 1250.

The depths relative to seafloor for the LWD logs from Holes 1250A and 1250B 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 Holes 1250A and 1250B, it was determined that the gamma ray log pick for the seafloor was at the same depth of 806.0 m below the rig floor. The rig floor logging datum was located 11.0 m above sea level for 1250A and 11.1 m above sea level for Hole 1250B.

Operations

Hole 1250F was APC and XCB cored to a depth of 180 mbsf (drillers depth). Rig-up for conventional wireline logging (CWL) operations began at 0340 hr on 26 August and final rig-down was completed by 1405 hr on 26 August. See Table T19 for detailed information on the Hole 1250F CWL program.

CWL operations in Hole 1250F began with the deployment of the triple combo tool string (TAP/DIT/HLDT/APS/HNGS/QSST) (Table T19). The triple combo tool string initially reached a depth of 179 mbsf without difficulty and with no sticking problems. Excellent quality data were acquired during the main uphole pass, and the tool was run back to the BOH for a second pass. The second pass also reached a TD of 179 mbsf, and excellent quality data were recorded on the second ascent. The TAP temperature data and associated depth data were recorded without problems during both of the triple combo tool string lowerings. The caliper from the HLDT showed the hole to be in extremely good condition, with the hole diameter seldom >13 in. After completing the second pass, the triple string combo tool was again lowered to a depth of 179 mbsf to obtain several checkshots (10 tacked shots) with the QSST. A one-way traveltime of 632 ms was recorded at TD (179 mbsf). To calculate a checkshot interval velocity with depth, a 32-m uphole shift is necessary to take into account the position 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 0950 hr on 26 August.

For the second CWL run in Hole 1250F, the FMS-sonic tool string (FMS/DSI/SGT) was deployed. The FMS-sonic tool string reached a maximum depth of 182 mbsf on two consecutive passes. The two FMS-sonic tool string 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. However, the FMS-sonic tool string did not initially reenter the drill pipe (set at 58 mbsf) during the first ascent, which required the end of the first pass to be aborted prematurely. It was determined that the lockable flapper valve (LFV) had closed, preventing the FMS-sonic string from reentering the drill pipe. The drilling rig mud pumps were used to pump open the LFV. During the first pass of the FMS-sonic string, the DSI tool 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, a Stoneley wave mode was used instead of the upper dipole; the monopole was set at the standard frequency; and the lower dipole was set at the standard frequency. The recorded sonic waveforms from both lowerings of the DSI are of very high quality, but the very low velocity of the 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 compressional (P)- wave velocity (V_P), 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.

Logging Quality

All logging data from the triple combo and FMS-sonic runs in Hole 1250F are of very high quality (Figs. F35, F36, F37, F38). The hole conditions were excellent, with an almost straight HLDT caliper measurement averaging about 12.2 in. 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 1250F was 807 mbrf for all of the CWL runs.

The well log data plots for Holes 1250A, 1250B, and 1250F in Figures F35, F36, F38, and F39 show excellent quality LWD and CWL logs. Lower pump rates through the shallow subsurface section of Holes 1250A and 1250B greatly reduced the effect of borehole washout on the LWD logs in the near-surface unconsolidated sediments. The downhole LWD and CWL logs dramatically highlight gas hydrate-bearing sediments with high resistivities, high acoustic velocities, and RAB image anomalies. Resistivity, acoustic, and density log variations below the zone of predicted gas hydrate stability (~111 mbsf) may indicate lithologic changes and the presence of free gas. Unlike many other sites on Hydrate Ridge, no borehole breakouts were observed in the RAB images from Hole 1250B.

Figure F37 shows a comparison of downhole LWD and CWL data from Holes 1250B and 1250F, using the gamma ray, neutron porosity, density, photoelectric cross section, and deep resistivity logs. The highly variable CWL logging data within the upper 63 mbsf of Hole 1250F was obtained through the drill pipe. A comparison of similar logging signatures in Figure F37 reveals that the LWD logging data from Hole 1250B are ~3 m deeper than the CWL data from Hole 1250F. This depth difference is best shown with the deep resistivity and density logs from the two holes. At a depth of ~89 mbsf in Hole 1250F the CWL density and resistivity logs show a distinct increase in value; however, this same log response is at a depth of ~92 mbsf on the LWD data from Hole 1250B. The offset could possibly be explained by local variability in the geology of this site; however, Hole 1250F was located only 15 m west of Hole 1250B. This apparent depth discrepancy will be further examined after the cruise. After taking into account the apparent depth difference between Holes 1250B and 1250F, it can be seen that the LWD and CWL data from each hole match relatively well, exhibiting similar curve shapes and absolute log values. The CWL- (Hole 1250F) and LWD-recorded (Hole 1250B) resistivity logs, however, exhibit 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.

The logged section in Holes 1250A, 1250B, and 1250F has been divided into three "logging units" on the basis of obvious changes in the LWD gamma ray, density, electrical resistivity (Fig. F39), and acoustic transit-time measurements (Fig. F38).

Logging Unit 1 (0-29 mbsf) is characterized by a 10-m-thick high-resistivity zone (2-12 mbsf), with a measured RAB peak value exceeding 2.5 m. Logging Unit 1 is also characterized by increasing densities with depth as measured by the LWD tools. However, this trend in the downhole recorded density data is probably due, in part, to degraded log measurements within the enlarged portion of the near surface of both boreholes as shown in Figure F34. Logging Unit 1 is located entirely within lithostratigraphic Units I and II (0-45 mbsf), which is composed of finely interbedded diatom-rich clay and silty clay sediments. In Hole 1250B, the transition from logging Unit 1 to 2 is defined by an increase in the measured resistivities (from ~0.8 to ~1.1 m) and a subtle increase in LWD-derived density (from ~1.58 to ~1.68 g/cm³).

Logging Unit 2 (29-111 mbsf) is characterized by zones of distinct high resistivities and high acoustic velocities, with the resistivity in one relatively thin zone >3.0 m (at a depth of ~100 mbsf) and V_P recorded at >1.63 km/s. The gamma ray log in logging Unit 2 shows a characteristic cyclicity that may reflect the interbedded nature of the sand, silt, and clay turbidite sequences described by the shipboard sedimentologists for Lithostratigraphic Unit III (45-100 mbsf) (see "Lithostratigraphic Unit III" in "Lithostratigraphic Units" in "Lithostratigraphy"). The downhole log-measured densities generally increase with depth in logging Unit 2 (1.65 g/cm³ at the top to near 1.8 g/cm³ at the bottom). In Hole 1250F (Fig. F38), the V_P log has been used to precisely select the depth of the boundary between logging Units 2 and 3, which is considered to be the base of the deepest gas hydrate presence as inferred form the available LWD and CWL data. The boundary between logging Units 2 and 3 is marked by a subtle drop in resistivity (to ~0.5 m) and V_P (to ~1.45 km/s). The boundary between logging Units 2 and 3 corresponds, roughly, to the depth of the BSR at this site.

Logging Unit 3 (111-210 mbsf, TD of Hole 1250A) coincides with lithostratigraphic Unit II (45-148 mbsf), which is again described as an interbedded sand, silt, and clay turbidite sequence (see "Lithostratigraphic Unit II" in "Lithostratigraphic Unit" in "Lithostratigraphy"). In Hole 1250B, logging Unit 3 is generally characterized by more uniform resistivities compared to Unit 2. A 2-m-thick anomalous interval, characterized by variable V_P (ranging from ~1.50 to ~1.55 km/s), variable resistivities (ranging from ~1.1 to ~1.7 m), and low densities (>1.5 g/cm³) occurs in logging Unit 3 within the depth interval from 146 to 151 mbsf, which suggests the presence of a free gas-saturated sand. This apparent free gas-bearing interval corresponds to the seismic Horizon A (see "Introduction").

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. The RAB and the FMS tools 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 hydrate occurrences. The resolution of the RAB images is considerably lower than the resolution of the FMS images. 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 F40, we have cross correlated a RAB image (Hole 1250B) and an FMS image (Hole 1250F) from the stratigraphic interval that contains Horizon A, which has been identified as a prominent regional seismic reflector (see "Introduction"). In this figure, 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.

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 NRM-MRP logs have been used to calculate sediment porosities for Holes 1250A and 1250B. Core-derived physical property data, including porosities (see "Physical Properties"), were used to both calibrate and evaluate the log-derived sediment porosities.

The VND LWD log-derived measurements of density in Holes 1250A and 1250B (Fig. F39) increase with depth and are relatively consistent within both holes, with values ranging from ~1.6 near the top of each hole to >1.85 g/cm³ near the BOHs. The density log measurements are slightly degraded in the very top of each hole. The LWD log-derived density measurements from Holes 1250A and 1250B 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 log density porosity calculation. The core-derived grain densities (_m) in Hole 1250A ranged from an average value at the seafloor of 2.73 to ~2.68 g/cm³ at the BOH, whereas the core-derived grain densities (_m) in Hole 1250B ranged from an average value of 2.71 g/cm³ at the top of the hole to about 2.68 g/cm³ at TD (see "Physical Properties"). The density log-derived porosities in Holes 1250A and 1250B were similar and ranged from ~50% to 75% (Figs. F41).

The LWD neutron porosity logs from Holes 1250A and 1250B (Fig. F41) yielded sediment porosities ranging from an average value at the top of the logged section of ~60% to ~50% at the BOH. The "total" sediment porosities derived by the NMR-MRP tool in Holes 1250A and 1250B (Fig. F41) ranged from about 70% near the seafloor to about 40% near the BOH. The NMR-MRP porosity logs from Holes 1250A and 1250B exhibit several relatively thin (2-3 m thick) intervals of significantly low porosities (at depths of about 60, 66, 80, and 82 mbsf). These apparent decreases in NMR-MRP porosities can be attributed to the presence of gas hydrate. Porosity logs in gas hydrate-bearing reservoirs are subject to error because most downhole porosity devices are calibrated to the physical properties of water-bearing sediments (as reviewed by Collett and Ladd, 2000). Therefore, downhole log-derived porosities need to be corrected for the presence of gas hydrate. The required correction for density and neutron-derived porosities is relatively small. But NMR-MRP porosities are more significantly affected by gas hydrate. The effect of gas hydrate on the downhole log-derived porosities from Site 1250 will be further examined after the cruise.

The comparison of core- and log-derived porosities in Figure F41 reveals that the neutron-, density-, and NMR-MRP-derived porosities are generally similar to the core porosities in logging Units 2-3 (29-210 mbsf). However, the density-derived porosities are slightly higher than the core-derived porosities in logging Unit 1.

The presence of gas hydrate at Site 1250 was documented by direct sampling, with numerous specimens of gas hydrate being recovered in Holes 1250C-1250E from near the seafloor to a depth of 86.35 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 Units 1 and 2. As previously discussed (see "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 1250 contains distinct zones that exhibit stepwise increases in both electrical resistivities and V_P . As discussed above, a portion of logging Unit 1 is also characterized by high resistivity measurements. Because the drill pipe was set at a depth of 73 mbsf for CWL logging, no acoustic logging data were collected from logging Unit 1.

Resistivity log data from Hole 1250B have been used to quantify the amount of gas hydrate at Site 1250. For the purpose of discussion, it is assumed that the high resistivities measured in logging Units 1 and 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 1250B. It should be noted that gas hydrate saturation (S_h) is the measurement of the percentage of pore space 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 for Site 1250 were a = 1, m = 2.8, and n = 1.9386.

Archie's Relation yielded water saturations (Fig. F42) ranging from a minimum value of only ~50% in logging Unit 1 and near the bottom of logging Unit 2, to a maximum of 100% in portions of logging Units 1 and 2 (0-111 mbsf), which implies the gas hydrate saturation in Hole 1250A ranges from 0% to 50%. The low water saturations shown in logging Unit 3 (Fig. F42) below the GHSZ probably 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 1250F (Fig. F43). 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 F43 reveals several gradient changes, which were caused by borehole temperature anomalies. The temperature anomaly at ~73 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 ~130 mbsf is ~20 m below the depth of the BSR (111 mbsf) at this site.