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doi:10.2204/iodp.proc.308.103.2006

Downhole measurements

Site U1319 marked the first location drilled and the first time that logging while drilling (LWD) and measurement while drilling (MWD) operations were conducted during Expedition 308. The primary logging objectives in Hole U1319B were to

• Determine the extent and lateral variation of turbidite deposits,
• Correlate lithostratigraphic units to the southern flank of the basin, and
• Document the lateral change in petrophysical properties of sediments above seismic Reflector R60.

Logging while drilling and measurement while drilling

Operations

LWD operations began with the initial makeup of the BHA, tool initialization, and calibration. The LWD tools (17.1 cm collars) included the GeoVision Resistivity (GVR) tool with a 23.2 cm button sleeve, MWD (Powerpulse), Array Resistivity Compensated (ARC) tool, and Vision Density Neutron (VDN) tool. Memory and battery life allowed for ~40 h of drilling without circulation; when circulating at rates >375 gallons per min (gpm), battery power is not used. Hole U1319B was spudded at 1440.6 meters below rig floor (mbrf) 20 m north of Hole U1319A. Drilling proceeded at a rate of penetration (ROP) of ~30 m/h to a total depth of 180 mbsf. Real-time data were transmitted to the surface at 24 Hz during a total of 7 drilling hours. From 0–15 mbsf bit rotation was ~50 rpm and pump rates were ~80 gpm. At 1454 mbrf, pump rates were increased to ~395 gpm and the mud pulsing system began transmitting data to the surface. At 1616 mbrf, 150 bbl of 12.2 ppg water-based mud (seawater, sepiolite, and barite) was displaced into the hole. Hole U1319B was drilled 22.5 m deeper than Hole U1319A to ensure all MWD/LWD sensors recorded measurements to the total depth of Hole U1319A.

Logging data quality

Figure F39 shows the quality control logs for the Hole U1319B LWD data. The target ROP of 30 m/h (±5 m/h) was generally achieved except above 10 mbsf, where a rapid jet-in was necessary to start the hole. This ROP was sufficient to record 1 sample/4 cm over the majority of the hole. The quality of GVR images is good, and no significant resolution loss is observed with variation in ROP except from 0 to 11 mbsf where the images are degraded by the rapid ROP and low rotation rate of the bit. Time-after-bit (TAB) measurements are ~5 min for ring resistivity, 4 min for gamma ray logs, 43 min for density, and 46 min for neutron porosity logs.

The density caliper log (DCAV), which gives the diameter of the LWD borehole, is the best indicator of borehole conditions. The density caliper values range between 42.2 cm at the top of Hole U1319B where sediments are unconsolidated to ~25.4 cm toward the bottom of the hole where the sediments are more compacted. Only the uppermost 50 m of the hole has washouts >2.5 cm. A standoff of <2.5 cm between the tool and the borehole wall indicates high-quality density measurements with an accuracy of ±0.015 g/cm³. The bulk density correction (IDDR), calculated from the difference between the short- and long-spaced density measurements, varies from –0.05 to 0.16 g/cm³ (mean = 0.06 g/cm³) (Fig. F39), which shows that the bulk density measurements are of good quality.

The depths, in mbsf, for the LWD logs were fixed by identifying the gamma ray signal at the seafloor. For Hole U1319B, it was determined that the gamma ray log pick for the seafloor was at a depth of 1441.0 mbrf. The rig floor logging datum was 10.4 m above sea level for this hole.

Annular pressure while drilling and equivalent circulating density

Annular pressure is measured within the borehole (APWD) but is monitored, as annular pressure while drilling in excess of hydrostatic (APWD*) and equivalent circulating density referenced to the seafloor (ECD_rsf) (see discussion in “Array resistivity compensated tool” in “Downhole measurements” in the “Methods” chapter). There were no abrupt pressure increases during operations in Hole U1319B (Fig. F40). APWD increased linearly with depth from the seafloor to 160 mbsf. APWD* was <0.25 MPa for the majority of the borehole. ECD_rsf decreased rapidly in the shallow section and then decreased slowly from 60 to 160 mbsf. Two pressure increases were measured below 160 mbsf, caused by mud sweeps that were part of drilling operations.

Interpretation

MWD operations in Hole U1319B provided data coverage by all LWD tools to at least 157.5 mbsf (Fig. F41). Hole quality was excellent below 50 mbsf, where the diameter was <28 cm. The gamma ray log (GR) gradually increases with depth in the first 50 mbsf where hole diameter is large; below this depth GR is nearly constant (~75 gAPI). Deep button resistivity increases from 0.6 to 1.8 Ωm from the seafloor to 175 mbsf. Over this same depth interval, bulk density increases from 1.4 to 2.0 g/cm³ and porosity decreases from 75% to 50%. These trends represent normal compaction where pore volume and water content are decreasing with depth because vertical effective stress is increasing.

Deviations from this normal compaction trend exist at 25 mbsf where GR has a step decrease, from 30.5 to 31.5 mbsf where GR increases, and from 78 to 93 mbsf where bulk density decreases. The GR decrease at 25 mbsf correlates with the transition to foraminifer-bearing clay at the top of lithostratigraphic Unit III (see “Lithostratigraphy”), GR increases from 30.5 to 31.5 mbsf and correlates with fine laminae of sands identified in lithostratigraphic Unit IV (see “Lithostratigraphy”). The decrease in bulk density (78–93 mbsf) is consistent with decreases in gamma ray attenuation (GRA)- and moisture and density (MAD)-derived density measurements on core (see “Physical properties”). Overall, the resistivity and GR responses indicate that the sedimentary section on the edge of Brazos-Trinity Basin IV is condensed and minimally influenced by turbidite influx to the basin.

The PEF log from Hole U1319B gradually increases from 0 to 126 mbsf. Between 126 and 145 mbsf, PEF decreases slightly. We interpret these subtle variations as a change in silt/clay content. PEF anomalies are found at 148–149 and 153–155 mbsf, where values increase from a mean value of 3.4 b/e^– to 7.0 and 9.6 b/e^–, respectively. These anomalies do not correlate with any other physical measurement obtained with LWD tools and are likely caused by barite in the mud cake generated after 150 bbl of 12 ppg mud was displaced at the bottom of the hole. These mud sweeps are also imaged in the APWD log (Fig. F40).

GVR resistivity images show some steeply dipping features between 172 and 174 mbsf within mass transport deposits (Fig. F42). These images also show a potential erosional surface at 167 mbsf and several alternating resistive and conductive bands that could reflect laminations observed within the cores (Fig. F42).

We used LWD density data to construct a synthetic seismogram for Hole U1319B (Fig. F43). Reflection coefficients were calculated using the LWD density data and a constant compressional wave velocity of 1600 m/s. A 200 Hz minimum-phase Ricker wavelet was convolved with the reflection coefficient series to create the synthetic seismogram. The correlation of events between the synthetic seismogram and the high-resolution seismic data indicates that the time-depth model is appropriate for these sediments in the shallow section (e.g., seismic Reflectors R10–R40). A time-depth mismatch occurs at seismic Reflectors R50 and R60, where the synthetic reflections occur shallower than the same events in the high-resolution seismic data (Fig. F43). The overall quality of the time-depth model allows correlation of seismic reflections with observations in core and log data.

Six regional reflections (seafloor [SF] and seismic Reflectors R10, R30, R40, R50, and R60) mapped on high-resolution seismic data have been correlated with logging data in Hole U1319A. The sediments between seismic Reflectors R30 and R40 have a constant bulk density. Bulk density has a small, abrupt increase, and the GR also shows an increase at seismic Reflector R40. Seismic Reflector R50 correlates with subtle decreases in GR, resistivity, and bulk density. A minor increase in bulk density may be associated with the low amplitude of seismic Reflector R60; however, the time-depth tie at Reflector R60 is not well constrained. In general, the logging data indicate a relatively homogeneous mud-prone section that is normally compacted.

Temperature and pressure measurements

Advanced piston corer temperature tool

The advanced piston corer temperature (APCT) tool was deployed before collecting Core 308-U1319A-5H (42.5 mbsf) (Table T14). Temperature was measured in the sediment for 10 min to establish the temperature decay curve (Fig. F44). Extrapolation of the temperature decay curve with an assumed thermal conductivity of 1.24 W/(m·K) provides an equilibrium temperature of 5.86°C (Fig. F44). Data from the deployment are available in “Downhole” in “Supplementary material.”

Temperature/dual pressure probe

Two deployments of the temperature/​dual pressure probe (T2P) were completed in Hole U1319A (Table T14). The first deployment, completed in the water column, was the first sea deployment of the T2P. The deployment was intended as a pressure test for the instrument. The second deployment penetrated the sediment at 80.5 mbsf immediately beneath Core 308-U1319A-9H.

T2P Deployment 1

The T2P was deployed prior to Core 308-U1319A-1H to a depth of 1388 meters below sea level (mbsl). The drill bit was positioned at 1380 mbsl and the T2P was deployed while preparing the seafloor camera survey. The primary objectives of the deployment were to (1) pressure test the T2P, (2) check pressure transducer calibrations, and (3) confirm that the T2P could successfully pass through the lockable float valve (LFV) of the bottom-hole assembly (BHA). Data were continuously recorded in memory on the tool at 1 Hz sampling rate.

T2P Deployment 1 occurred in the drill pipe with the T2P connected to the colleted delivery system (CDS). The deployment methodology is provided in “Temperature/​dual pressure probe” in “Downhole measurements” in the “Methods” chapter. The T2P was lowered until the tip was at 511 mbsl, where a hydrostatic reference was recorded for 2 min. The tool was then lowered until the tip was at 1011 mbsl for another 2 min reference measurement. The T2P was then lowered through the LFV. The T2P tip reached a maximum depth of 1388 mbsl (–41.6 mbsf), where a 5 min reference was recorded. References were also taken during retrieval of the T2P when the tip was at 1010 and 511 mbsl. No drilling fluid was circulated during the deployment. Table T15 provides the time-event log for T2P Deployment 1.

The pressure test deployment was successful. The tool recorded pressure and temperature for the entire deployment (Fig. F45) and passed through the LFV without problem. The tip pressures were consistent with hydrostatic pressure assuming an average fluid density of 1.024 g/cm³ (Fig. F45). The shaft pressures were higher. The offset in pressure at the shaft is interpreted to reflect an errant calibration factor. The temperature record showed a downhole decrease in temperature to 4.82°C at 1388 mbsl. Comparison of the pressures and temperatures during deployment and recovery documented <2% drift during the experiment. Measurements during retrieval drifted upward for fluid pressure and downward for temperature.

T2P Deployment 2

The second T2P deployment occurred after Core 308-U1319A-9H; therefore, the T2P measured pressure and temperature at 80.5 mbsf in sediment recovered in Core 308-U1319A-10H. The deployment occurred with the drill bit initially raised to 68.5 mbsf. Once the T2P was latched in the CDS, the drill string pushed the T2P into the formation (deployment procedure is described in “Temperature/​dual pressure probe” in “Downhole measurements” in the “Methods” chapter). Formation pressure and temperature were monitored for 30 min before retrieving the T2P (Fig. F46). Overpull of 7000 lb was measured when pulling the T2P out of the hole. This overpull was interpreted to be the force required to unseat the CDS from the BHA, not the force required to pull the T2P out of the formation. Drilling fluid was circulated during the deployment; however, circulation was stopped when taking hydrostatic reference measurements, when pushing the probe into the sediment, and for the first 10 min that the T2P was in the sediment. Table T16 provides a timeline for the deployment.

When the T2P was recovered on the rig floor, the protective shroud was not covering the tip. We interpret that the shroud never reseated over the tip during retrieval of the tool. The tip of the tool was damaged (Fig. F47). Damage was interpreted to result from bending of the tip during penetration followed by a straightening of the tip when the T2P was pulled into the BHA. Secondary inspection of the tool revealed that the drive tube was bent slightly.

Pressure and temperature data were recorded throughout the deployment, despite the damage to the tip. The pressure and temperature increased before the drill string pushed the T2P into the formation (Fig. F46; Table T16). This response reflects the tip of the tool entering the formation while engaging the CDS in the BHA; it indicates that the weight of the tool is sufficient to cause penetration in the formation. The shaft pressure and temperature increased after the drill string pushed the T2P into the sediment. The tip pressure decreased during penetration.

After 30 min in the sediment, the pressure at the tip was 15.95 MPa, whereas the shaft pressure was 15.5 MPa. Hydrostatic pressure at 80.5 mbsf was 15.2 MPa. The equilibrium temperature was 7.3°C (Fig. F46). These preliminary pressure and temperature interpretations should be viewed cautiously because of the damage incurred during the deployment.

Most likely, the T2P and drive tube were damaged because the T2P did not enter the sediment vertically. One possible reason the shaft was not vertical when it penetrated is the 12 m distance between the drill bit and the bottom of the hole. Future deployments were modified with the drill bit <2 m off the bottom of the hole. The goal is to achieve vertical penetration of the probe and thus preserve the integrity of the T2P and the recorded data.

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