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doi:10.2204/iodp.proc.308.107.2006

Downhole measurements

Site U1323, located between Sites U1322 and U1324, was a dedicated LWD/MWD site. The principal objectives at this site were to characterize key physical properties and compare and correlate with Sites U1322 and U1324.

Logging while drilling and measurement while drilling

Operations

Operations in Hole U1323A used the same LWD/MWD tool configuration, bottom-hole assembly, and procedures as Site U1322 (see “Operations” in the “Site U1322” chapter). The hole was started using a rapid jet-in penetration and pump rates of ~12 strokes per minute (spm) from seafloor to ~5 mbsf before retrieving the VIT camera. After the VIT camera was recovered, bit rotation and pump rates of 50 rpm and 12 spm were used to 25 mbsf while maintaining an ROP of 30 m/h. From 25 to 35 mbsf, pump rates were increased to 80 spm. From 35 to 247 mbsf, an ROP of 30 m/h and pump rates of at least 80 spm were maintained.

The GeoVision Resistivity tool had low battery power and would not record data when the pumps were turned off. At 198 mbsf, a 3 m thick sand layer was encountered and APWD increased by ~1 MPa. A residual backpressure of ~1 MPa was also observed by the driller when the mud pumps were shut down. We pumped 10.5 ppg mud in the hole. After a wiper trip, the overpressure stabilized and drilling operations continued. At 242 mbsf, a rapid drop in gamma radiation was observed and drilling operations were terminated to prevent penetrating an overpressured sand.

Logging data quality

Figure F2 shows the quality control logs for Hole U1323A. The target ROP of 30 m/h (±5 m/h) was generally achieved. The density-derived caliper log is >24 cm for most of Hole U1323A, suggesting unstable borehole conditions from 0 to 130 mbsf and from 180 to 209 mbsf. These zones had an increased bulk density correction that varied from –0.15 to 0.16 g/cm³ (Fig. F2). This is greater than the larger corrections in Hole U1322A.

LWD logs were depth-shifted by identifying the gamma ray signal associated with the seafloor. The seafloor pick was 1271.1 mbrf, 0.1 m deeper than the drillers depth. The rig floor datum was located 10.5 m above sea level.

Annular pressure while drilling and equivalent circulating density

Annular 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 in excess of hydrostatic (APWD*) and equivalent circulating density referenced to the seafloor (ECD_rsf) (see “Downhole measurements” in the “Methods” chapter). ECD_rsf decreases and APWD* increases from 0 to 150 mbsf (Fig. F3). From 150 to 198 mbsf, the ECD_rsf log gradually increases where an overpressured silty sand interval was encountered (Fig. F3). At 198 mbsf, APWD* increased by ~1 MPa over a 3 m thick interval. After pumping weighted mud and running a wiper trip, drilling operations resumed. APWD* below 198 mbsf increased gradually. ECD_rsf decreased gradually, showing no significant anomalies, thus suggesting that we did not drill through any additional overpressured units.

Results

LWD/MWD operations in Hole U1323A reached 247 mbsf. Hole diameter averaged 29.4 cm with enlarged conditions from 0 to 130 and 180 to 209 mbsf (Fig. F4). Gamma ray (GR) measurements increased with depth, averaging ~70 gAPI throughout the borehole. The GR signature showed a consistent increase from the seafloor followed by a similar decrease until reaching a low at 40 mbsf. A GR decrease to 30 gAPI indicates the presence of an overpressured sand at ~198 mbsf. At ~244 mbsf the GR values dropped to 47 gAPI, indicating the top of a second sand unit (Fig. F4).

The resistivity log showed a general increase with depth until ~198 mbsf, where a decrease in resistivity marks the top of the overpressured sand unit. Overall, resistivity ranged from 0.3 to 1.5 Ωm (mean = 1.1 Ωm). Bulk density increased gradually with depth from 1.3 to 1.9 g/cm³, corresponding to neutron porosity values ranging from 90% to 47%. Photoelectric factor (PEF) values range from 2.1 × 10^–24 to 4.1 × 10^–24 b/e^– to a depth of 184 mbsf. Below this depth, PEF increased as a result of heavy mud in the borehole. Heavy mud containing barite was used below 198 mbsf.

Interpretation of units

The stratigraphic succession drilled at Site U1323 was divided into logging Units 1, 2, and 3 based on gamma ray and resistivity logs. Logging Unit 1 is further divided into Subunits 1a, 1b, 1c, and 1d.

Logging Unit 1 (0–197 mbsf)

Logging Unit 1 is interpreted as clay with several silty intervals and two MTDs. The inferred subunits were classified using the following criteria.

Subunit 1a (0–40 mbsf)

Logging Subunit 1a extends from 0 to 40 mbsf, just below seismic Reflector S10 (Fig. F5). This subunit was defined on the basis of gamma radiation, which increases from 0 to 20 mbsf followed by a decrease until 40 mbsf. Increasing resistivity values define the base of this subunit. Subunit 1a is characterized by an alternation of low- and high-amplitude reflectors in seismic data. Subunit 1a correlates with lithostratigraphic Subunit IA at Sites U1324 and U1322. Based on the observations at Sites U1324 and U1322, this subunit is interpreted as clay with some silty intervals at the base. At Site U1322, this was interpreted as a hemipelagic drape and very distal turbidites from channel-levee systems to the west.

Subunit 1b (40–51 mbsf)

Logging Subunit 1b is characterized by constant gamma radiation and an increase in resistivity with two peaks corresponding to relatively high amplitude seismic reflectors within a zone of chaotic and discontinuous reflectors (Fig. F5). Based on the resistivity response and seismic character, Subunit 1b is interpreted as a 10 m thick MTD. This interpretation is linked to Sites U1322 and U1324, where reverse/​normal faults and folds are observed in correlative sediments.

Subunit 1c (51–97 mbsf)

The base of logging Subunit 1c corresponds to regional seismic Reflector S20 (Fig. F5). The gamma ray profile is ~70 gAPI, resistivity increases steadily throughout the interval with minor variations, and seismic facies have parallel low- to medium-amplitude reflectors. Subunit 1c is interpreted mostly as clay and mud, based on the uniform gamma radiation and resistivity. A decrease in the gamma ray log at ~58 mbsf suggests a silt interval. In addition, this subunit has lower density than the overlying and underlying subunits (Fig. F4). This inferred subunit correlates with lithostratigraphic Subunit IC at Sites U1324 and U1322, which is composed of couplets of different-colored clay with a minor amount of silt laminae.

Subunit 1d (97–195 mbsf)

Logging Subunit 1d is based on gamma ray and resistivity data that remain fairly consistent throughout this interval. The acoustic character of the subunit is transparent or chaotic, with some isolated medium-amplitude reflections and some locally subparallel reflections toward the base of the subunit. Based on these observations, Subunit 1d is interpreted as an interval of MTDs with few silt beds. The top and bottom of Subunit 1d correspond to seismic Reflectors S20 and S30.

Logging Unit 2 (195–220 mbsf)

The top of logging Unit 2 is characterized by a sharp decrease in gamma radiation from 80 to 30 gAPI and a covariant response in resistivity. These responses are interpreted as the presence of sand units (Figs. F4, F5). The top and bottom of logging Unit 2 correspond to seismic Reflectors S30 and S40-1323. Several seismic reflectors between S30 and S40-1323 are subparallel with good lateral continuity that could represent transitions from mud/clay beds to silt/sand-rich layers. Logging Unit 2 cannot be correlated to Site U1324 or Site U1322.

Logging Unit 3 (220–242 mbsf)

Logging Unit 3 is characterized by increasing gamma radiation with depth, which suggests an increase in mud/clay content (Figs. F4, F5). Variations are observed at ~224 mbsf, suggesting increased silt content and, at 242 mbsf, indicating increased sand content. The seismic data in this interval show discontinuous reflectors. The base of the unit is characterized by a high-amplitude reflector that correlates with sand at 242 mbsf.

Physical properties from logging data

LWD bulk density data were used to derive porosity (Equation 1 in the “Site U1321” chapter) (Fig. F6A). Bulk density increases gradually with depth from 1.3 to 1.9 g/cm³, which corresponds to porosity from 78% to 45%. The upper 40 m of the sequence (logging Subunit 1a) is characterized by relatively low bulk density that increases from 1.3 to 1.7 g/cm³ and calculated porosity from 78% to 55% (Fig. F6A). Logging Subunit 1b has small variations in bulk density and porosity. Bulk density within logging Subunit 1c gradually increases from 1.6 to 1.7 g/cm³. Logging Subunit 1d, an MTD, shows increasing bulk density from 90 to 120 mbsf and constant bulk density of 1.9 g/cm³ below 170 mbsf. Low density observed at 120 and 135 mbsf is attributed to large borehole diameter. The top of Subunit 1d is marked by a density increase of 0.02 g/cm³, whereas at the base of the subunit bulk density drops by ~0.05 g/cm³ (Fig. F6A). A comparison of LWD-calculated porosity data versus hydrostatic vertical effective stress from Sites U1322, U1323, and U1324 indicates that the sedimentary sequence drilled at the three sites follows the same trend (Fig. F6C). Thus, the data suggest that the sedimentary successions at all three sites have similar consolidation states.

Core-log-seismic integration

LWD observations are linked to seismic data through a time-depth conversion using check shot data from Hole U1324A. Reflection coefficients were calculated using the LWD density data and a constant compressional wave velocity of 1600 m/s. A 150 Hz minimum-phase Ricker wavelet was convolved with the reflection coefficients to create the synthetic seismogram (Fig. F7). The correlation between the synthetic seismogram and the high-resolution seismic matches only in the uppermost 90 mbsf. A time-depth mismatch occurs below seismic Reflector S20, where the synthetic reflections occur shallower than the same events in the high-resolution three-dimensional seismic data (Fig. F7).

Summary

Site U1323 downhole measurements provide insights on lithofacies for correlation with Sites U1322 and U1324. Our preliminary interpretation allows for the following conclusions:

• Logging Unit 1 was interpreted to be a mud- and/or clay-banded unit with intervals of silt beds and MTDs. Chaotic intervals in seismic reflection profiles are interpreted as MTDs and intervals of continuous reflectors as clay.
• Logging Unit 2 is interpreted to contain silty beds and two sand beds, defined by low resistivity and gamma ray values. Seismic facies analysis suggests that these sands are associated with the Southwest Pass Canyon eastern levee.
• Logging Unit 3 was interpreted as a MTD with some silty sand beds.

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