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doi:10.2204/iodp.proc.308.106.2006

Downhole measurements

The first MWD/LWD hole in the Ursa Basin was Hole U1322A. The primary objectives were to document the vertical and lateral variations in rock properties in the Ursa Basin and to provide key parameters controlling generation of overpressure, rates of sedimentation, and timing of MTDs. The drilling operations proceeded with careful pressure monitoring because of the risk of encountering shallow-water flows by penetrating sand-prone, overpressured sediments (see “Background and objectives”).

Logging while drilling and measurement while drilling

Operations

LWD/MWD operations at Hole U1322A began with the initial makeup of the BHA, tool initialization, and calibration. The LWD tools (17.15 cm collars) included the GeoVision Resistivity (GVR) tool, MWD (PowerPulse) tool, Array Resistivity Compensated (ARC) tool, and Vision Density Neutron (VDN) tool. Because of the potential of encountering unstable and overpressured zones, the LWD BHA used in the Ursa Basin sites included a 13.97 cm full-hole (FH) pin bottom connection, 13.97 cm FH boxtop connection, and 9.63 m long jar section above the VDN to aid in removing tools in the event of a collapsed hole. Hole U1322A was spudded at 1330 mbrf (seafloor depth) with an initial pump rate of ~55 gpm and bit rotation of 10 rpm. The bit was jetted to 3.8 mbsf, and the VIT camera was recovered. Drilling proceeded to 200 mbsf at an average ROP of 30 m/h (Fig. F41). Drilling continued with a reduced ROP of 20 m/h below 200 mbsf to avoid exceeding the target depth of 238 mbsf.

Logging data quality

LWD log quality data are displayed in Figure F41. Borehole conditions were good with a maximum density-derived caliper log (DCAV) of 33 cm at ~35 mbsf. The caliper data below 120 mbsf are close to bit size and only show a slight enlargement (~30 cm) at 200 mbsf. The bulk density correction values vary only minimally from –0.04 to 0.14 g/cm³ (average = 0.02 g/cm³), indicating reliable bulk density measurements.

The depths in mbsf for LWD logs were fixed by identifying the gamma ray signal associated with the seafloor and shifting the logging data according to the seafloor. The gamma ray log pick for the seafloor was at 1330 mbrf. The rig floor logging datum was 10.5 m above sea level.

Annular pressure while drilling and equivalent circulating density

Pressure within the borehole was monitored during MWD operations (see discussion in “Array Resistivity Compensated Tool” in “Downhole measurements” in the “Methods” chapter) as annular pressure while drilling (APWD), annular pressure in excess of hydrostatic (APWD*), and equivalent circulating density referenced to the seafloor (ECD_rsf) (Fig. F42). ECD_rsf decreases from the top to the bottom of the borehole. APWD* increases slightly with depth to the bottom of the borehole, indicative of a gradual increase in pressure that is necessary to overcome pipe friction (or wall drag). There is no record of shallow-water flow into the borehole. The ECD_rsf and APWD* logs vary systematically below 120 mbsf.

Results

LWD/MWD operations in Hole U1322A reached 238 mbsf without encountering any major sand units. Hole quality was good (average diameter = 26.9 cm) for most of the borehole, and caliper measurements are only slightly increased above 40 mbsf (Fig. F43). These increased caliper values correlate with low gamma ray data, suggesting an increased silt/​sand content in the sediments. However, gamma ray measurements have near-constant values of ~70 gAPI throughout the borehole. This is only interrupted at ~215 mbsf, where gamma radiation increases to 90 gAPI. This increase is not reflected in resistivity, which remains relatively monotonous throughout the borehole. The only notable resistivity increase, from ~1 to ~3.5 Ωm, occurs at 160 mbsf.

Bulk density increases from 1.0 to 1.6 g/cm³ at shallow depths and then increases gradually to 2.0 g/cm³ at the bottom of Hole U1322A. This corresponds with the decrease in neutron porosity from 83% to 43%. These trends are also observed in the photoelectric factor (PEF) values and imply similar material throughout the borehole.

Core-log-seismic integration

LWD observations are linked to seismic data through a time-depth conversion constructed from regional check shot data collected prior to Expedition 308 (Equation 1). The TWT from sea level is related to t_bsf by adding t_bsf to the TWT from sea level to the seafloor (1.766 s) (Table T1) as picked on high-resolution seismic data.

The synthetic seismogram for Hole U1322A was constructed using LWD density data (Fig. F44). Reflection coefficients were calculated using 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 coefficients to create the synthetic seismogram. The correlation between the synthetic seismogram and the high-resolution seismic data matches from 0 to 100 mbsf. A time-depth mismatch occurs at seismic Reflector S30 and continues to seismic Reflector S50-1322, where the synthetic reflections occur slightly shallower than the same events in the high-resolution seismic data (Fig. F44). Nevertheless, the overall quality of the time-depth model allows an approximate correlation of seismic reflections with observations in core and log data.

LWD logs display subtle variations throughout the borehole, and the strongest contrast is observed in resistivity and gamma ray logs. These correspond to seismic Reflectors S10 and S30 and support the definition between lithostratigraphic Subunits IA and IB and lithostratigraphic Units I and II (Fig. F43). Seismic Reflector S10 also correlates with small decreases in resistivity and gamma ray within a silt interval, suggesting that increased sand content creates the acoustic impedance contrast. Although seismic Reflector S30 corresponds to low resistivity, the gamma ray contrast is less prominent. The largest variations in log responses occur from the seafloor to ~35 mbsf, which correspond to lithostratigraphic Subunit IA. This unit consists of intercalated layers of clay, foraminifer-bearing layers, and silt. These variations are identified in the LWD data and allow extrapolation between core and seismic observations.

GeoVision resistivity imaging

Resistivity images of Hole U1322A are of good quality and allow identification of several structures (Figs. F45, F46). Based on visual core descriptions, Hole U1322B was divided into lithostratigraphic Units I and II, which are dominated by clay with occasional interbedded silt units (see “Lithostratigraphy”). These units are further divided into subunits that reflect the occurrence of either undisturbed or contorted and faulted sediments. Lithostratigraphic Subunit IB was characterized as an MTD with small faults, folded beds, and steep dipping surfaces; these features are also imaged by the GVR data (Fig. F45). Diversely sized high- and low-resistivity oval-shaped patches likely represent the hinge line of these folds. The resistivity contrasts are the result of cutting through layers of different resistivity composing the fold. These folds are clearly revealed by mirrored sinusoids centered on an oval-shaped area of high or low resistivity, depending on the imaged material.

Lithostratigraphic Subunit IB contains several reverse and normal faults. However, these are mostly on the centimeter scale (see “Lithostratigraphy”) and are problematic to identify in the GVR images. The change between disturbed Subunit IB and laminated Subunit IC is defined by an increase in resistivity (Fig. F45).

East-west-oriented borehole breakouts imaged by the GVR indicate the direction of the minimal horizontal stress (Figs. F45, F46). Consequently, the maximum horizontal stress direction in Hole U1322A is north–south. However, Ursa Basin slopes to the southeast, and it remains unclear if these breakouts are the expression of the local or regional stress field. Similar to Subunit IB, lithostratigraphic Unit II MTDs contain steeply dipping folds and highly disturbed sediment layers, expressed by variable resistivity (Fig. F46). Horizontal and even bedding of MTDs 2-3 and 2-5 immediately above and below MTD 2-4 are also indicated by resistivity measurements and support the lithostratigraphic division.

Temperature and pressure measurements

Advanced piston corer temperature tool

The APCT tool was deployed twice in Hole U1322B (Table T16). Temperature was measured in the sediment for 10 min. The temperature decay curves are extrapolated with an assumed thermal conductivity of 1.2 W/(m·K) to estimate in situ temperatures. The first deployment was at 80.0 mbsf (Core 308-U1322B-9H) and provided an in situ temperature of 6.44°C (Fig. F47). The second deployment (108.5 mbsf; Core 308-U1322B-12H) yielded an equilibrium temperature of 6.56°C (Fig. F48).

Temperature/Dual Pressure Probe

Eleven deployments of the T2P probe were completed at Site U1322 (Table T16). These measurements provided numerous constraints on in situ temperature and formation pressure.

T2P Deployment 17

T2P Deployment 17 recorded pressure and temperature increases while penetrating the sediment; however, upon removal of the bit from the bottom of the hole the pressure signals decreased (Table T17; Fig. F49). These pressure decreases were interpreted to reflect pulling the T2P probe tip partly out of the sediment when picking up the drill bit off the bottom of the hole. We interpret that the colleted delivery system (CDS) did not completely decouple the drill string from the tool. Thus, when the drill string was raised, it dislodged the probe from the weak formation. The pressures were nearly constant at subhydrostatic after backing off the bit and thus did not provide any constraint on in situ conditions. Hydrostatic pressure is 13.68 MPa. The temperature profile showed a spike associated with penetration and a second spike with backing off the drill bit (Table T17; Fig. F49) followed by a temperature decay. The probe appeared to reach equilibrium with the sediment at 4.90°C.

T2P Deployment 19

During T2P Deployment 19, the pressure and temperature sensors recorded increases during landing of the CDS in the BHA and during penetration into the sediment (Table T18; Fig. F50). A temperature increase was also observed when the drill bit was picked up from the bottom of the hole. This was coincident with large decreases in tip and shaft pressures. After this decrease, the tip and shaft pressures equilibrated to 15.2 MPa (Fig. F50). The hydrostatic pressure is 14.61 MPa. It was interpreted that picking up the drill string also raised the T2P. It is unclear what the constant pressure of 15.2 MPa represented. Temperature measurements decayed quickly after frictional heating caused by raising the bit. The temperature decayed to a final value of 7.69°C.

T2P Deployment 20

Pressure and temperature increased during landing of the CDS in the BHA during T2P Deployment 20. However, there was significant noise in the data while landing and while pushing into the formation (Table T19; Fig. F51). This noise resulted from landing the tool with the bit 12 m off the bottom of the hole, then pulling the tool back into the pipe, and then landing the CDS with the bit 3 m off the bottom of the hole. Fluids were circulated during landing of the CDS in the BHA, which also could have created noise. After pushing the tool into the formation and picking up the bit, tip and shaft pressures decreased. The tip then slowly increased to a final value of 15.4 MPa. The shaft pressure was nearly constant at 15.6 MPa. Hydrostatic pressure was 14.84 MPa. The temperature dissipation was smooth after picking up the bit. The equilibrium temperature was 8.31°C.

T2P Deployment 21

T2P Deployment 21 (Table T20) recorded only 3 min of data. Inspection of the data acquisition unit documented that the memory card was not properly seated; thus, data were not recorded. The memory card was replaced, and the quick-release button for the card was removed. This modification decreased the probability that the memory card could be accidentally ejected during tool assembly or deployment.

T2P Deployment 22

During T2P Deployment 22 (Table T21) an internal leak flooded the electronics connecting the pressure transducers and the thermistor to the data acquisition unit. This shorted all circuits, and no data were recorded. A leak at an O-ring seal between the drive tube and the drive tube nut caused the malfunction. The poor O-ring seal was the result of the drive tube being out-of-round from damage in a previous deployment. The deformed drive tube was removed from use. All electrical components were cleaned, dried, and tested.

T2P Deployment 23

Temperature and pressures increased with landing of the CDS in the BHA and with penetration into the formation during T2P Deployment 23 (Table T22; Fig. F52). The tip pressure decreased when the bit was picked up and then gradually increased to a final pressure of 14.4 MPa. The shaft pressure decreased when the bit was pulled up and then dissipated to 15.4 MPa. Hydrostatic pressure was 14.76 MPa. After initial frictional heating, the temperature decayed to an equilibrium value of 8.07°C.

T2P Deployment 24

While landing the CDS in the BHA during T2P Deployment 24, pressures and temperature increased. Increases also occurred when the tool was pushed into the formation (Table T23; Fig. F53). Pressure and temperature dissipated while the tool was in the formation. The tip pressure reached a final pressure of 15.8 MPa and the shaft pressure decreased to 17.3 MPa. The hydrostatic reference is 15.26 MPa. The temperature profile decreased to a final value of 8.81°C.

T2P Deployment 25

Similar to previous shallow deployments, pressure and temperature increased during T2P Deployment 25 when the probe was pushed into the formation, and then pressure dropped rapidly when the drill bit was picked up (Table T24; Fig. F54). The tip and shaft pressures then rapidly equilibrated at 13.5 and 13.6 MPa, less than hydrostatic (13.65 MPa). The temperature decreased rapidly while picking up the bit, after which temperature was nearly constant at 4.93°C. The pressure and temperature measurements suggest that the probe was not in good communication with the formation after picking up the bit; measurements may have been influenced by communication with fluid at the bottom of Hole U1322D.

T2P Deployment 26

Pressure and temperature data increased during penetration of the tool into the formation during T2P Deployment 26 (Table T25; Fig. F55). Tip and shaft pressures dropped when the bit was picked up and then were nearly constant at 13.7 and 13.8 MPa. Hydrostatic pressure was 13.95 MPa. Pressure and temperature perturbations occurred around 0727 h (Fig. F55) but were not related to deployment activities (Table T25). The temperature profile decreased rapidly and showed multiple step increases and decreases before reaching a final temperature of 5.31°C.

T2P Deployment 27

Penetration of the probe into the formation created pressure and temperature increases during T2P Deployment 27 (Table T26; Fig. F56). After picking up the bit, the tip pressure rapidly decreased to a near-constant value of 14.0 MPa, which is less than hydrostatic pressure (14.25 MPa). The shaft pressure dissipated to a final value of 14.6 MPa. Smooth temperature decay was recorded. The final temperature (6.79°C) was assumed to be in equilibrium with the formation.

T2P Deployment 28

T2P Deployment 28 had large pressure and temperature increases with penetration and pressure drops associated with picking up the bit (Table T27; Fig. F57). Pressures at the shaft and tip were near constant at 14.2 and 14.3 MPa after picking the bit up. Hydrostatic pressure was 14.60 MPa. Temperature decayed to a formation temperature of 6.95°C.

Data recording stopped during recovery of the probe. Inspection of the tool revealed that the tip broke and the drive tube bent during deployment. Damage to the probe likely occurred while pushing into the formation, and then the tip was broken when pulling out of the formation. The tool then would have flooded and data recording stopped. Because of the damage during deployment, the end pressure and temperature records should be interpreted with caution.

Davis-Villinger Temperature-Pressure Probe

Five DVTPP deployments were completed at Site U1322 (Table T16) to provide additional constraints on formation pressure and temperature. Similar to the T2P data, the DVTPP provided multiple temperature measurements but only a few reliable pressure measurements.

DVTPP Deployment 16

Temperature and pressure increased during DVTPP Deployment 16 penetration (Fig. F58). The pressure then decreased rapidly to ~15.9 MPa, which is higher than hydrostatic pressure (14.93 MPa). Temperature dropped rapidly and then slowly increased while the tool was in the formation. These data were interpreted to record a system leak and communication with borehole fluids.

DVTPP Deployment 17

Pressure and temperature responses for DVTPP Deployment 17 were good during penetration and were followed by dissipation (Fig. F59). The pressure dissipated to a final value of 14.8 MPa but had not equilibrated with the formation. Hydrostatic pressure was 14.25 MPa. The temperature equilibrated with the formation at 7.04°C.

DVTPP Deployment 18

Pressure increased during penetration during DVTPP Deployment 18 and then decreased abruptly when the bit was picked up (Fig. F60). The decrease may have been caused by pulling the tool up when the bit was picked up. The pressure then increased to 16.4 MPa, where the hydrostatic reference was 15.46 MPa. Temperature increased during penetration and then dissipated while the tool was in the formation (Fig. F60). The final temperature was 9.19°C. This may be slightly above in situ conditions, as the temperature was still decreasing.

DVTPP Deployment 19

Pressure and temperature profiles for DVTPP Deployment 19 showed good penetration and dissipation responses (Fig. F61). Pressure dissipated to 16.6 MPa but had not equilibrated with the sediment, where hydrostatic pressure was 15.64 MPa. Equilibrium temperature was 9.30°C.

DVTPP Deployment 20

Similar to DVTPP Deployment 19, pressure and temperature responses provided constraints on in situ conditions during DVTPP Deployment 20. After penetration, the pressure dissipated to 16.4 MPa and the temperature dissipated to 8.96°C (Fig. F62). Hydrostatic pressure was 15.01 MPa. The temperature was assumed to be in equilibrium with the formation.

Temperature and pressure summary

The combined APCT, T2P, and DVTPP tool measurements helped to constrain the temperature and the pressure field at Site U1322. Temperature increased linearly with depth (Fig. F63). A regression of the temperatures provided a thermal gradient of 26.2°C/km.

The pressure profile at Site U1322 was harder to constrain. Pressure dissipation curves provided maximum estimates of in situ pressure between 100 and 238 mbsf (Fig. F64). Pressure increases with time were also observed at Site U1322. It was believed that these reflect pore fluids equilibrating with a void around the pressure sensor. This void may have been caused when the bit was picked up if the bit movement pulled up the DVTPP or T2P. Incorporation of these increases can also provide a lower bound on formation pressure (Fig. F65). In general, the observations confirmed overpressure at Site U1322.

Summary

Logging data obtained at Site U1322 provided crucial information about the character of the sediments in this region and their physical properties as summarized below.

• The logging data change at depth is consistent with the core observations between lithostratigraphic Units I and II. Unit I is characterized by alternating laminated silt-clay layers and two MTDs, whereas Unit II has silt and sand alternating with MTDs.
• The main structures observed by GVR data in Hole U1322A are folded beds and steeply dipping surfaces, interrupted by undisturbed laminated units. GVR imaging also supports the division into subunits (see “Lithostratigraphy”).
• A synthetic seismogram for Hole U1322A was constructed using the LWD density data. The correlation between the synthetic seismogram and the high-resolution seismic matches only in the uppermost 100 mbsf.
• In situ pressure and temperature measurements constrained the thermal gradient at 26.2°C/km at Site U1322 and documented overpressure.

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