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doi:10.2204/iodp.proc.308.104.2006 Downhole measurementsSite U1320 marked the second location drilled during Expedition 308 and the first time that downhole wireline logging operations were conducted. The primary logging objectives in Hole U1320A were to
Wireline loggingOperationsFollowing coring operations in Hole U1320A, the hole was conditioned for wireline logging by displacing 96 bbl of 8.9 ppg sepiolite mud, running a wiper trip, and placing the pipe at 62.5 mbsf (1542.9 mbrf). Three tool strings were deployed in the following order:
All wireline tool deployments were performed with the wireline heave compensator. Main and repeat passes were recorded with the triple combo and FMS-sonic tool strings to a total depth of 299.6 mbsf. Rig floor preparations for wireline logging operations began on 9 June 2005 at 2155 h and were completed by 10 June at 1730 h. In the first pass with the triple combo tool string, we encountered an obstruction at 173 mbsf that prevented the tool string from reaching the bottom of the hole. On the second pass, the triple combo tool string was successfully lowered to 299.6 mbsf. HLDS caliper readings of ~43 cm show washouts between 102 and 103 mbsf and from 167 to 173 mbsf. The FMS-sonic tool string deployment reached 299.6 mbsf with both main and repeat passes. The lockable float valve (LFV) temporarily obstructed tool string entry into the borehole, but this problem was overcome. During the first pass, data were collected at a logging speed of 275 m/h and the DSI was run in P- and S-wave (P&S) mode with a sampling rate of 15 Hz to acquire high-quality compressional wave velocity data. During the second pass, P&S, cross-dipole, and Stoneley modes were recorded at 3 Hz sampling rate. The LFV was not locked in position at the time of reentering the pipe; therefore, seawater was pumped down the pipe and the tool string was pulled safely into the pipe. The FMS-sonic tool string was retrieved to the rig floor, and rigdown was completed at 1004 h on 10 June 2005. The marine mammal watch began by placing observers at the bow and stern of the JOIDES Resolution 1 h prior to the check shot operations. The WST was rigged up during the initial stages of the marine mammal watch and lowered through the drill pipe at ~1000 m/h. The LFV obstructed lowering the tool into the borehole, and seawater was pumped to help open the LFV. Approximately 20 min was spent trying to bypass the washout zone at ~173 mbsf, after which the tool was successfully run down to 299.6 mbsf. A 30 min “soft shooting start” began during the tool’s descent and was completed when the tool reached 299.6 mbsf. Based on the HLDS caliper measurements, 14 stations in ~20 m intervals were selected for the check shot (Table T13). The GI seismic source had a 45 in3 generator chamber volume, 105 in3 injector chamber volume, and total pressure of 2000 psi. The recording length was 5 s with a sampling rate of 1 ms and a 40 ms delay time. At 1205 h (local time), a single common bottlenose dolphin was spotted 5 m off the port stern quarter. The GI source was shut down and the firing circuit secured. Following a 30 min observation period with no further marine mammal sightings, a 30 min “soft shooting start” began at 500 psi and finished at 2000 psi. The experiment lasted ~3 h with ~140 shots being fired. Log data qualityThe log data were depth-shifted using the FMS-sonic second pass as the reference depth curve with 1478 mbrf as the water depth. Environmental corrections were also made. Hole diameter was recorded by the hydraulic caliper on the HLDS and by the calipers on the FMS (Fig. F33). HNGS and SGT data were corrected for borehole diameter. APS and HLDS data were corrected for standoff and hole diameter. The caliper data show that the hole diameter in the interval between 176 and 296 mbsf, which corresponds to lithostratigraphic Unit V, was relatively uniform (~25 cm). Enlarged borehole intervals >30 cm were observed at 100, 110, and 170 mbsf. These zones correspond to alternating sand and silt/clay layers identified in lithostratigraphic Units I–IV. ResultsGamma radiationSGT and HNGS gamma ray measurements are in good agreement to 211 mbsf. Gamma ray data display large variations in the upper part of the borehole (65–176 mbsf), ranging from 10 to 110 gAPI (Fig. F33). Low gamma ray values (<60 gAPI) can be correlated to sand-prone layers in core samples, whereas clay layers display higher values (>70 gAPI). However, low gamma ray values observed in lithostratigraphic Unit III may be related to the high abundance of calcareous foraminifers and microfossils usually characterized by low radiogenic element concentration. The spectral measurements, consisting of contributions from the radioactive decay of 40K, 232Th, and 238U isotopes, show that the decay of 232Th is the main gamma ray contributor (Fig. F33). 232Th is commonly transported to depositional environments as clay fraction showing an affinity to terrestrial clay minerals. The range of 232Th values measured in Hole U1320A fall within values reported for illite and smectite (Rider, 1996). Clays observed in core samples support this interpretation (see “Lithostratigraphy”). ResistivityDeep-, medium-, and shallow-induction resistivity data from Hole U1320A show similar values except between 265 and 285 mbsf, where the shallow resistivity is slightly lower (Fig. F33). The lower shallow resistivity could be an indication of fluid invasion into a more permeable layer. Resistivity varies from 0.2 to 2 m between the top of the logged section and 176 mbsf. Low resistivity values (<0.7 m) recorded at 70, 76, 110, and 176 mbsf correspond to low gamma ray values and borehole washouts. The interval below 176 mbsf displays minimal variations in electrical resistivity, with the exception of changes at 180 and 285 mbsf, which correlate with seismic Reflectors R40 and R60. Preliminary correlations with the Hole U1320A lithostratigraphy shows that sand intervals have resistivity lower than ~0.7 m. DensityBulk density data were recorded in high-resolution mode. Enlarged borehole conditions degrade measurement quality; therefore, density data where caliper readings are >40 cm are low and unreliable. The bulk density logs display large variations between 65 and 176 mbsf, similar to the gamma ray data, with values ranging from 1.1 to 2.3 g/cm3 and prominent anomalies at 80, 110, 140, and 170 mbsf (Fig. F34). From 176 to 299.6 mbsf density is characterized by a systematic increase from 1.75 to 2.2 g/cm3. This increase is only interrupted by a sand-rich layer at 230 mbsf, where the density decreases to 1.8 g/cm3. Neutron porosityNeutron porosity data were recorded in high-resolution mode. Porosity data estimated from core data were compared to neutron porosity data (Fig. F34). Neutron porosity is inconsistent in the upper borehole section, varying from 42% to >70% (Fig. F34). High neutron porosity correlates with low density and compressional velocity, indicating possible washout zones. However, high porosity values may also reflect (1) fracture porosity that is not reflected in moisture and density measurements, which are measured on intact samples, or (2) hydrogen bound in minerals such as clays or in hydrocarbons, which contribute to the measurement; therefore, the raw neutron porosity values are often overestimates. Formation MicroScanner imagingPreliminary shipboard processing provided static and dynamic normalized FMS images (see “Downhole measurements” in the “Methods” chapter). FMS images (Fig. F35) show evidence for slump surfaces, high-angle faulting, and lithologic changes. The resistivity contrast between the sand and clay sediments compares well with the other log data. Where caliper values are high, FMS images are not reliable. Preliminary analyses of the FMS images show that in many intervals the borehole is irregular, resulting in an uneven contact of the FMS pads with the borehole wall. Nevertheless, some good-quality images provide information that could not be gleaned from the cores, particularly where sedimentary and structural features were severely disturbed by the extended core barrel (XCB) coring process (see “Lithostratigraphy;” Fig. F4). DiscussionHole U1320A wireline logs were separated into two zones on the basis of changes in velocity, resistivity, bulk density, and porosity. These zones correspond to major lithology changes identified in cores. Gamma ray data indicate interbedded sand, silt, and clay in the upper section of the borehole, which correlate with lithostratigraphic Units I, II, III, and IV. These units represent muddy and sandy turbidites to ~180 mbsf, roughly equivalent to the position of seismic Reflector R40 (Fig. F33). In general, sand-dominated layers have gamma radiation <60 gAPI and resistivity <1.1 Ωm, whereas layers dominated by clay have higher values. Alternating units measured with downhole logs can be correlated to seismic Reflectors R20, R30, and R40. A clear separation of thin-bedded intervals based on the measurements alone is difficult because of the limited vertical resolution of the logging tools. Nevertheless, within intervals of poor core recovery (70–120 and 170–190 mbsf), the log data provide an excellent means to infer lithology and define a nearly complete stratigraphic section for Site U1320. The largest variations in physical properties for the shallow section are observed in density (1.1–2.05 g/cm3) and neutron porosity (40%–70%). Washout zones ~3 m thick were encountered at 110 and 170 mbsf. These intervals are interpreted as sand-rich layers characterized by increased neutron porosity and caliper readings (~38 cm) and low density, gamma ray, resistivity, and sonic velocity values. The foraminifer-bearing clay, lithostratigraphic Unit III, has increased resistivity (>1.4 Ωm), bulk density (~1.9 g/cm3), and velocity (~1.65 km/s) (Fig. F36), which correlates to seismic Reflector R30. This unit also contains the 84 ka ash Layer Y8, which could not be distinguished in the wireline data. Below ~180 mbsf, porosity decreases systematically because of sediment compaction. Data from 299.6 to 170 mbsf are homogeneous and correlate to lithostratigraphic Unit V. This highly bioturbated unit is characterized by mud, clays, the absence of sand layers, and an overall homogeneous lithology (see “Lithostratigraphy”). Density and velocity data display a systematic increase downhole (1.7–2.1 g/cm3 and 1.5–1.8 km/s), interrupted at ~230 mbsf, near the projected depth of seismic Reflector R50. Below seismic Reflector R50, bulk density and sonic velocity values increase downhole but on a separate trend from the upper part of lithostratigraphic Unit V (Fig. F34). There are no obvious changes in lithology across this interval, despite the decrease in bulk density and sonic velocity. However, the section below seismic Reflector R50 shows a noticeable increase in methane content (see “Geochemistry and microbiology”). The change near seismic Reflector R50 may be related to a change in the compaction trend. Logging while drilling and measurement while drillingOperationsHole U1320B was the first LWD/MWD hole drilled during Expedition 308. Operations began with the initial makeup of the BHA, tool initialization, and calibration. The LWD tools (17.15 cm collars) included the GVR tool with a 23.18 cm button sleeve, MWD (PowerPulse) tool, Array Resistivity Compensated tool, and Vision Density Neutron (VDN) tool. Memory and battery life allowed for ~40 h of drilling without circulation; when circulating above 375 gpm, battery power was not used. Hole U1320B was spudded at 1479 mbrf with an initial pump rate of 125 gpm and bit rotation of 10 rpm. Below 1510 mbrf (31 mbsf) drilling proceeded with a flow rate of 300 gpm and bit rotation of 45 rpm. The mud pulsing system began transmitting data after achieving ~395 gpm between 1510 and 1528 mbrf (31–49 mbsf). Total depth (320 mbsf) was reached with an average ROP of ~25 m/h. Real-time data were transmitted to the surface at a rate of 24 Hz for 13 drilling hours. Hole U1320B was drilled 20 m deeper than Hole U1320A to ensure that all LWD/MWD sensors recorded measurements to the total depth of Hole U1320A. Logging data qualityFigure F37 shows the quality control logs for the Hole U1320B LWD data. The target ROP of 25 m/h (±5 m/h) was generally achieved except in the upper 8 m of the hole, where a rapid jet-in was necessary to start the hole. Density caliper log values range between 42 cm at the top of Hole U1320B where sediments were unconsolidated to 25 cm near the bottom of the hole where the sediments were more compacted. Several washouts were observed from 0 to 108 mbsf, where the caliper readings were >42 cm. The bulk density correction varies from –0.06 to 0.2 g/cm3 (mean = 0.06 g/cm3) (Fig. F37), which indicated good quality bulk density measurements. The depth (mbsf) for the LWD logs were calculated by identifying the gamma ray signal associated with the seafloor and shifting the logging data to that seafloor reference. For Hole U1320B, it was determined that the gamma ray log pick for the seafloor was at a depth of 1481 mbrf, which was 2 m deeper than the drillers’ depth. The rig floor logging datum was located 10.4 m above sea level for this hole. Annular pressure while drilling and equivalent circulating densityPressure within the borehole was monitored during MWD operations (see discussion in “Downhole measurements” in the “Methods” chapter) as APWD, APWD excess of hydrostatic (APWD*), and equivalent circulating density referenced to the seafloor (ECDrsf). APWD* increases slightly with depth. There are several excursions that correspond to low gamma ray values (interpreted as sand intervals) where the density-derived caliper measured enlarged borehole dimensions (Fig. F38). We interpret that sediment loading within the annulus caused these localized increases, which returned to normal conditions after the interval was drilled. Below 100 mbsf, both the ECDrsf and APWD* curves show uniform profiles with minor anomalies at ~143, 165, 209, and 270 mbsf (Fig. F38). The largest anomalies were 0.88 ppg and 0.23 MPa increases in the ECDrsf and APWD* curves. InterpretationHole quality in the upper 100 mbsf is highly variable with several caliper measurements ≥42 cm. Deeper than 100 mbsf, the caliper measured an average diameter of 26.4 cm. The GVR gamma ray log is also highly variable from 0 to 110 mbsf; below this depth gamma radiation is nearly constant (~74 gAPI) with only a 10 m interval (~165–175 mbsf) measuring a low value of 15.8 gAPI. From the gamma ray log, we interpret a series of sand and clay interlayers dominating the stratigraphy above ~110 mbsf. The interval from 165 to 175 mbsf reflects a sand interval that correlates with high caliper reading and seismic Reflector R40. GVR deep button resistivity increases from 0.3 to 2.8 m from the seafloor to 316 mbsf. The variations in resistivity are interpreted as variations in sand and clay content within units of variable thicknesses, especially above 110 mbsf and at ~165 mbsf. The VDN bulk density log increases from 1.0 g/cm3 at shallow depths to 2.1 g/cm3 near the bottom of the hole. Below 110 mbsf these data are consistent with variations in core MAD measurements (Fig. F39). Porosity decreases from 95% to 43%; these values are generally higher than those obtained from MAD measurements on core samples (Fig. F39), most likely due to hydrogen bound in clays that contributes to an overestimate in the logging measurement. These trends in the lower part of the hole record compaction where pore volume and water content are decreasing with depth because vertical effective stress is increasing. The density and porosity values in the shallower section reflect the presence of sand units and an enlarged borehole. The PEF log from Hole U1320B follows similar trends as those observed in the gamma ray, resistivity, and density. The PEF log shows a systematic increase from ~110 mbsf followed by a similar gradual decrease before reaching 165 mbsf. These variations are interpreted as gradual changes in clay/sand content (Fig. F39). GVR resistivity images show thin sand beds within lithostratigraphic Subunit IIE and a sharp contact with the top of lithostratigraphic Unit III that correlates to seismic Reflector R30 (Fig. F40). In addition, these images show steeply dipping beds within clay-rich lithostratigraphic Unit V (Fig. F41). Borehole breakouts have also been identified within lithostratigraphic Unit V (Fig. F42). Most of the steeply dipping beds have a north-south orientation, whereas the borehole breakouts have an east-west orientation suggesting a north-south maximum stress direction and deformation. Core-log-seismic integrationCore-log-seismic interpretationCompressional velocity was measured both with the DSI log and with the WST (check shot). DSI compressional velocities from the top of the logged section to 299.6 mbsf range from 1.53 to 1.82 km/s (Fig. F36). Between 70 and 176 mbsf, compressional velocities are more variable. The lowest velocities are at 110 and 176 mbsf and correspond to borehole washouts (Fig. F37). Seismic surfaces R20, R30, R40, R50, and R60 correspond to contrasts in velocity and density. In addition to the wireline sonic data, a check shot was completed (Figs. F36, F43; Table T13). The check shot velocities are compared with the predrill time-depth prediction (see Equation 1 in “Background and objectives” in the “Site U1319” chapter) in Figure F43. Between 80 and 200 mbsf the check shot velocities are slightly larger than the velocities predicted prior to drilling. Wireline and LWD data correlate well (Fig. F44). The largest variations in log responses occur from the seafloor to ~110 mbsf, which corresponds to lithostratigraphic Units I and II. These units consist of intercalated layers of clay and sand separated by bioturbated muddy intervals ranging between 1 and 10 m in thickness (see “Lithostratigraphy”). These variations are clearly defined in the LWD data. Seismic Reflectors R10–R40 correlate with low density, resistivity, gamma ray, and sonic velocity values, suggesting that these intervals represent transitions from mud-rich layers to sand-prone layers. LWD and wireline observations are linked to seismic data through a time-depth conversion using check shot data from Hole U1320A. We used LWD density data and the wireline sonic log to construct reflection coefficients for Hole U1320B (Fig. F45). A velocity of 1600 m/s was assumed from 0 to 50 mbsf. A 200 Hz minimum-phase Ricker wavelet was convolved with the reflection coefficients to create the synthetic seismogram. The successful correlation of events between the synthetic seismogram and the high-resolution seismic data is achieved from 0 to 140 mbsf; below 140 mbsf no reliable tie is established. Based on the time-depth model, six regional reflections (SF and seismic Reflectors R10, R30, R40, R50, and R60) mapped on high-resolution seismic data have been correlated with logging data from Holes U1320A and U1320B. Temperature and pressure measurementsTemperature/dual pressure probeTwo deployments of the T2P probe were completed in Hole 1320A (Table T14). The first deployment occurred at 126.3 mbsf (below Core 308-U1320A-15X) and the second at 213.0 mbsf (below Core 308-U1320A-24X). Both deployments used the tapered needle probe (Fig. F10 in the “Methods” chapter). T2P Deployment 3T2P Deployment 3 used the tapered needle probe. The tapered shaft is stronger than the straight shaft used in T2P Deployment 2 (Hole U1319A; 80.5 mbsf). A drill bit elevation of 1 m off hole bottom was employed when loading the colleted delivery system in the BHA. This was 11 m less standoff than that of T2P Deployment 2. It was hoped that a smaller drill bit standoff would aid vertical penetration of the probe into the formation and minimize bending of the needle probe. Details of the deployment are summarized in Table T15. The T2P recorded temperature and pressure at the tip; however, the shaft transducer did not record any data during the deployment (Fig. F46). The tip pressure increased during penetration into the sediment and a sharp pressure decrease occurred when the drill string was picked up. Pressure then dissipated until the probe was removed from the sediment. The pressure prior to pulling the tip was 14.24 MPa, which is significantly less than the hydrostatic pressure (16.04 MPa). Temperature recorded exhibits frictional heating during penetration into the formation when the drill bit was backed off and when the probe was pulled from the formation (Fig. F46). Heating when the drill bit was backed off may indicate that the probe was slightly pulled out of the sediment; this could create subhydrostatic pressure at the tip. The end temperature measurement was 7.23°C. We interpret this to be the equilibrium temperature in the sediment. T2P Deployment 4T2P Deployment 4 procedures were similar to those of Deployment 3 except we did not use the drill string to push the T2P into the formation. We relied on the weight of the tool string to push the tool into the formation. The deployment was completed after Core 308-U1320A-24X. Table T16 provides an event log for the deployment. Pressure was recorded at the tip and the shaft sensors throughout the deployment, and temperature was measured at the tip (Fig. F47). The tip pressure and temperature records increased during penetration into the formation and then dissipated. The final pressure recorded, 18.43 MPa, slightly exceeds hydrostatic (18.32 MPa). The final temperature recorded was 9.04°C. Shaft pressure did not increase during penetration into the sediment. One possible explanation for this was insufficient penetration of the shaft into the sediment. If the sediment stiffness was high, it may have prevented significant penetration of the tool by its own self-weight. To ensure penetration, future deployments were completed with the drill string pushing the probe into the sediment. Davis-Villinger Temperature-Pressure ProbeThe DVTPP was deployed immediately after Cores 308-1320A-23X (203.4 mbsf) and 32X (289.9 mbsf) (Table T14). Each deployment followed the standard DVTPP deployment procedure (see “Davis-Villinger Temperature-Pressure Probe” in “Downhole measurements” in the “Methods” chapter) to achieve 1 m of penetration into the sediment. The DVTPP deployments had similar pressure and temperature responses: (1) pressure and temperature increased penetration into the formation, (2) pressure dissipation curves were subhydrostatic, and (3) in situ temperature was measured successfully. At 203.4 mbsf, the pressure at the end of the deployment was 1 MPa less than hydrostatic (Fig. F48), whereas at 289.9 mbsf the end-pressure was ~2.5 MPa less than hydrostatic (Fig. F49). We interpreted these low pressures as caused by an internal leak in the probe. We reassembled the DVTPP to troubleshoot leaks. Equilibrium temperatures (10.20ºC at 203.4 mbsf and 11.04°C at 289.9 mbsf) are based on the last formation reading. Each of the temperature decay curves were of good quality. SummaryLogging operations at Site U1320 provided crucial information about the character and physical properties of the sediments. Core-log integration provided a detailed picture of the bedding style and lithofacies at this site and yielded the following insights:
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