DOWNHOLE LOGGING

Operations

In the Leg 204 Scientific Prospectus, Hole 1252A was designated as an alternate site (proposed site Site HR5a), which was selected to examine the nature of the accretionary complex in the area of Hydrate Ridge. With only 4 days remaining until the end of Leg 204, Hole 1252A was spudded at 1130 hr Universal Time Coordinated on 30 August 2002 at a water depth of 1051.0 meters below rig floor (mbrf). Hole 1252A was APC and XCB cored to a depth of 259.8 mbsf (drillers depth). Rig-up for conventional wireline logging (CWL) operations began at 1150 hr on 31 August, and final rig down for the CWL operations was completed by 0030 hr on 1 September. See Table T10 for detailed information on the Hole 1252A CWL program.

CWL operations in Hole 1252A began with the deployment of the triple combination (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 T10). The triple combo tool string initially reached the TD of the hole at 262 mbsf, which is 2.2 m deeper than the drillers determined TD for the hole. Excellent quality data were acquired during the main uphole pass (see below), and the tool was run back into the hole for a second logging pass. The second upward pass of the triple combo tool string started at a depth of 184 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 lowerings of the triple combo tool string. The triple combo logging run ended with the rig-down of the tool string being completed at 1810 hr on 31 August.

For the second CWL run in Hole 1252A, the FMS-sonic (FMS/Dipole Sonic Imager [DSI]/Scintillation Gamma Ray Tool [SGT]) tool string was deployed. The FMS-sonic string reached a maximum depth of 262 mbsf on two consecutive passes. The two FMS-sonic runs confirmed the excellent condition of the hole. The FMS images and sonic waveforms recorded from the two passes were of very high quality. During the first pass, 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, the monopole and the upper dipole were set at their standard frequencies, and the lower 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 pick accurate V_P . Some adjustment of the STC parameters allowed for improved P-wave picks, but further reprocessing is required. The quality of the recorded shear wave data was very high, but it will also require additional processing.

Logging Quality

All logging data in Hole 1252A are of very high quality (Figs. F26, F27, F28). The hole conditions were extremely good, with hole diameters seldom exceeding 12.5 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 anomalous data from the top of each log (70-78 mbsf) in Figures F26 and F27 were obtained through the drill pipe, which was set at a depth of 75 mbsf during the CWL logging program.

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 1252A was 1051 mbrf for all of the CWL runs.

Interpretation of Wireline Logging

Seismic data reveal a relatively complex geologic setting for this site (see Figs. F5, F8 in the "Leg 204 Summary" chapter). Analysis of log data from Hole 1252A shows that the contact between the folded accretionary complex and overlying sedimentary section is marked by an ~7-m-thick zone at ~114-121 mbsf that is characterized by anomalous gamma ray, density, and resistivity log values. Our initial interpretation is that this anomalous zone contains a mixture of mostly glauconite and possibly authigenic carbonate, which was probably deposited on top of the underlying accretionary complex as the anticline was developing. The integration of available core data and downhole logs also reveals the presence of at least one significant carbonate horizon near the crest of the anticline.

Logging Units

The logged section in Hole 1252A is divided into two "logging units" on the basis of obvious changes in the CWL gamma ray, bulk density, electrical resistivity, and acoustic transit-time measurements (Figs. F26, F27, F28). For comparison to other sites cored during Leg 204, the specific logging units defined in Hole 1252A are unique to this site. However, as described below, the boundary between logging Units 1 and 2 marks the contact between the sediments of the accretionary complex and relatively undeformed overlying sediments. This same boundary at other sites has been identified as the contact between logging Units 3 and 4.

Logging Unit 1 (75-121 mbsf) (base of pipe was set at 75 mbsf) is characterized by variable gamma ray, density, and resistivity log values. The most distinct interval is an ~7-m-thick zone (114-121 mbsf) near the base of logging Unit 1, in which the gamma ray log exceeds 110 American Petroleum Institute gamma ray units (gAPI), the density log measurements average ~1.9 g/cm³, and the deep reading resistivity log exceeds 1.6 m. Further analysis of the spectral gamma ray logging data from Hole 1252A (Fig. F27) shows that this anomalous gamma ray zone also contains relatively high concentrations of uranium and potassium. The high gamma ray (potassium) and density of this anomalous section suggest the presence of glauconite, which was confirmed by the shipboard sedimentologists (see "Lithostratigraphy"). Based on the comparison of the available CWL data from Hole 1252 with the regional seismic data (see "Introduction"), it appears that this glauconite-rich section was deposited on an unconformity marking the top of the accretionary complex. We have placed the boundary between logging Units 1 and 2 at the base of the anomalous gamma ray interval at 121 mbsf. Logging Unit 1 also contains several conspicuous high-resistivity intervals from 94 to 100 mbsf that may contain gas hydrate.

Logging Unit 2 (121-260 mbsf; TD of Hole 1252A), reflecting the upper portion of the deformed sediments of the accretionary complex, is characterized by almost constant gamma ray, neutron porosity, density, and resistivity log measurements with depth, which are not consistent with a normal compaction profile. Caliper logs (Figs. F26, F27) from Hole 1252A also show that, in comparison, logging Unit 2 exhibits more irregular borehole diameters, with the appearance of more numerous, relatively large washouts. Near the top of logging Unit 2 there is a relatively thin 1-m-thick interval (125-126 mbsf) characterized by distinct high resistivities (>2.0 m), high densities (>2.0 g/cm³), low neutron porosities (<30%), and low gamma ray values (<40 gAPI). This anomalous log interval appears to correlate with a distinct carbonate unit at 125 mbsf (see "Lithostratigraphy"). The base of the GHSZ at Site 1252, as interpreted from the regional seismic data, is at a depth of ~170 mbsf.

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. In Figure F29, we show an FMS image from near the boundary between logging Units 1 and 2 in Hole 1255A. This FMS image includes both the anomalously high gamma ray zone in logging Unit 1 (114-121 mbsf) and the apparent carbonate unit from near the top of logging Unit 2 (125-126 mbsf). The anomalously high gamma ray zone in logging Unit 1 appears to contain distinct nodules and layers of resistive material, probably glauconite with some carbonate, which would contribute to the high-density logging measurements within this interval. The carbonate near the top of logging Unit 2 appears as a single bright resistive layer with irregular upper and lower contacts.

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). Formation density and neutron data from the HLDT and the APS were used to assess the sediment porosities for Hole 1252A. Core-derived physical property data were used to both calibrate and evaluate the log-derived sediment porosities from Site 1252 (see "Physical Properties").

The HLDT density log data (Fig. F26) reveal several prominent high-density intervals within Hole 1252A, which yield relatively low density and neutron log-derived sediment porosities. The density log-derived porosities in Hole 1252A range from ~60% to ~70%, whereas neutron log porosities range from an average value at the top of the logged section of ~70% to ~65% in logging Unit 2. Comparison of core- and log-derived porosities in Figure F26 reveals that the density log-derived porosities are generally similar to the core porosities. However, the neutron porosities are generally higher than the core-derived porosities throughout most of the hole.

IR anomalies were observed in Hole 1252A at 82.75 and 98.60 mbsf, and samples were preserved as possible gas hydrate specimens. At the depth of these two samples, the downhole resistivity logs are characterized by distinct stepwise increases in both electrical resistivities and acoustic velocities. At all of the other Leg 204 core sites on Hydrate Ridge, we have used the LWD resistivity log data to quantify the amount of gas hydrate. We have also shown that the vertical resolution of the LWD and CWL resistivity logs differs greatly. The complete analysis and comparison of the LWD and CWL resistivity-derived gas hydrate concentrations on Hydrate Ridge, including gas hydrate quantification at Site 1252, will be completed postcruise.

The TAP tool was deployed on the triple combo tool string in Hole 1252A (Fig. F30). 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 F30 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 ~170 mbsf is near the depth of the BSR near this site.