Logging Summary

IODP Expedition 306:

North Atlantic Climate 2

Introduction

Figure 1. Map of Expedition 306 site locations in the North Atlantic.

The primary objective of Expedition 306 was developing high-resolution late Neogene–Quaternary climate proxy records in the North Atlantic and putting these into a PAC (Paleointensity Assisted Chronology) framework. This PAC is constructed from combination of geomagnetic paleointensity, stable isotope, and detrital layer stratigraphies. Sites drilled during Expedition 306 are located north of the Azores near the Mid-Atlantic Ridge (Sites U1312 and U1313 (reoccupation of ODP Sites 608 and 607, respectively), on the middle of the Gardar Drift (Site U1314), and a CORK emplacement at Site U1315 with associated downhole logging at nearby ODP Hole 642E (Figure 1). Several possible locations for drilling on Eirik Drift in Labrador Sea had to be abandoned due to poor weather.

The primary logging objectives of Expedition 306 were Sites U1313 and one of 2 Eirik Drift sites. The goal at the two sites was to provide corrected depth-scale information from core-log integration to account for core expansion and overall quality control of the spliced record. Due to high sedimentation rates at most of the Expedition 306 sites, a secondary goal was to look at millennial-scale changes that would be identifiable in Formation MircoScanner (FMS) data and high-resolution Multi-sensor Gamma ray tool (MGT) data. When the Eirik Drift sites were abandoned, Site U1314 was chosen but, again, due to poor weather, no logging was possible.

Logging Operations

Ultimately, only Site U1313 was logged as part of Expedition 306. Downhole logging operations were carried out after completing coring of Hole U1313B to a depth of 302 mbsf (3727 meters below rig floor (mbrf)) and displaced with sepiolite mud. The drill pipe was raised to 65.3 mbsf (3489.6 mbrf) prior to logging. During logging operations, the sea state was fairly calm with a typical heave of 2m or less. The initial plan was to use two tool string configurations, the triple combo” with an additional Multi-sensor Gamma-ray Tool (MGT) and the Formation Micro Scanner (FMS)–sonic. However, shortly after deploying the triple combo-MGT, power problems forced us to bring the tool string back on deck for examination. It was determined that the MGT tool was leaking and had caused damage to the telemetry cartridge below. The MGT was removed from the tool string and a new telemetry cartridge was installed on the tool string. Following the repairs, the triple combo was deployed successfully to the bottom of the borehole at 300 mbsf (3725.3 mbrf). This leak in the housing of the MGT caused significant delays and there was no time available for any additional toolstrings.

As part of a CORK emplacement project at Site U1315 that deployed a long term (5 years) bottom water temperature monitoring experiment, we did have the unique opportunity to reoccupy and log ODP 642E again after 20 years. The plan was to use two tool string configurations, the triple combination (triple combo) with an additional General Purpose Inclinometer Tool (GPIT) and the FMS–sonic. The Lamont Borehole Research Group’s (LDEO-BRG) Temperature, Acceleration, and Pressure (TAP) tool was deployed with triple combo and we logged down slowly stopping every 5-10 m over the upper 100 m and then logged continuously at 1800 ft/hr down to total depth of 588 mbsf. While collecting the downhole temperature data, we also logged down with the triple combo tool string. At 588 mbsf, we reached an impassable obstruction and stopped the downhole logging. We then logged the hole up into casing to a depth of 335 mbsf. After the triple combo, the FMS-sonic tool string was also deployed to ~580 mbsf after again reaching the same hole obstruction as before. The second pass of the FMS-Sonic was only able to reach a total depth of ~440 mbsf before reaching an obstruction. So, a shortened second run was made from that depth into casing until 310 mbsf.

Logging Results

Figure 2. Comparison of core and log physical properties from Hole U1313B. (A.) Bulk density for the interval 80 to 300 mbsf. (B.) Gamma ray radiation for the interval 0 to 300 mbsf. cps = counts per second; gAPI = American Petroleum Institute gamma ray units.

Figure 3. Detailed comparisons of core and log physical properties from upper 70 mbsf of Hole U1313B. (A) Core and log gamma-ray. (B) Core “L”-(~CaCO3) and log gamma-ray.

Figure 4. Linear correlation of log gamma-ray (interval 0-225 mbsf) from Hole U1313B and benthic oxygen isotope stack over last 5.4 Ma (Lisiecki and Raymo, 2005). Both data sets are shown with scales inverted so that warm interglacials/low gamma-ray (ie low Th =low clay) intervals are shown as prominent peaks. The correlation has only two tiepoints at 0 and 5.2 Ma with no stretching or squeezing of log data depths.

Logging Highlights: Hole U1313B

The successful deployment of the triple combo tool string at U1313B provided complete coverage of the 300m section and provided very good physical property and lithologic information for density, porosity, natural gamma-ray, resistivity and photoelectric effect. Corresponding core physical property measurements were very consistent with in situ downhole data (Figure 2).

Due to the high detrital content of the core, we were able to use the downhole natural gamma measurements recorded through the cased portion of the hole (0-70 mbsf) for detailed stratigraphic correlation (Figure 3). While the signal was attenuated by 4-5X, the correlation was critical for correlation in this critical Quaternary section at this locations. Also of special note is the dramatically consistent linear correlation of downhole natural gamma-ray (upper 225 mbsf) with the recent Lisiecki and Raymo (2005) benthic oxygen isotope record over the last 5.4 Ma (Figure 4). The consistency of downhole data with both core data and age models will allow mapping of spliced core record to actual depth resulting in more accurate sedimentation rate calculations as well as more detailed age/depth models.

Figure 5. Comparison of old and new physical properties from Hole 642E.
Caliper (in), Porosity (%), Gamma-ray and Thorium (gAPI = American Petroleum Institute gamma ray units). All new data is shown in blue and old data in red.

Figure 6. Detailed resistivity profile over an interval from 530 to 550 mbsf at Hole 642E showing a pattern of basalt flows with lower resistivity at the top and increasing towards the bottom. An enlarged portion of a FMS image showing volcaniclastic (basaltic vitric tuff) interval beneath one flow and at the top of the next between 546 to 548 mbsf.

Figure 7. Detailed resistivity profile over an interval from 530 to 550 mbsf at Hole 642E showing the pattern of basalt flows with a lower resistivity at the top and increasing towards the bottom. A blowup of a FMS image showing fine-grained basalt interval between 542 to 545 mbsf.

Logging highlights: Hole 642E

A) Log(New)-Log(Old) Comparisons

An important part of re-visiting this ODP legacy site is an evaluation of hole conditions after 20 years. The rotary bit size used for coring this site was 9.75 in. The original caliper log is plotted against 2 calipers from the FMS tool (Figure 5). As can be seen, the original caliper (density tool) was not very reliable showing a much larger than bit size hole for almost the entire length of the cored interval. Most of the intervals with hole sizes larger than 12 in the new FMS caliper logs correspond to high porosity-low resistivity zones.

A comparison of porosity logs shows a very good correlation downhole. The overall variability of porosity is much larger (10 to 95%) than the original measurements (15 to 70%)(Figure 5) and is attributed to a perhaps more sensitive porosity sonde.

B) FMS/Sonic Logging

FMS imaging of the hole yielded good results and will allow easy correlation to existing core data and filling in the gaps (~60% of the formation). Examples from the volcaniclastic and fine-grained basalt intervals are shown in Figures 6 and 7, respectively.

The fine-scale (cm) resistivity data will allow high-resolution studies of fracture density of basalts and porosity within the sequence. Combined with new shear wave data from the Sonic tool, it should be possible to construct more reliable permeability estimates as well as revised synthetic seismograms that may yield better depth-velocity correlations.

C) Temperature Log at ODP Hole 642E

Figure 8. Temperature log profile versus depth at Hole 642E collected using TAP tool.

A temperature log (Figure 8) was obtained at Hole 642E using the L-DEO-BRG TAP tool. This tool logs at a rate of 1 Hz, has a precision of 5 mK and an accuracy of 1 K. The temperature was logged on the way down. The TAP tool was held off bottom for a few minutes and indicates a bottom water temperature of approximately 0.2° C. The 10 m of the borehole has a very steep gradient (~2500 °C/km). Below this section the borehole has a relatively low gradient of approximately 22° C/km. The borehole is cased to a depth 390 mbsf. At a depth of approximately 500 mbsf a positive temperature excursion may indicate inflow. The temperature log as a whole indicates significant fluid discharge that may be as much as 10’s of meters per year. This excursion may correlate with a high permeability zone indicated in the other logs.

Sean Higgins : Logging Staff Scientist, Borehole Research Group, Lamont-Doherty Earth Observatory of Columbia University, PO Box 1000, 61 Route 9W, Palisades NY 10964, USA