IODP Expedition 302: Arctic Coring Expedition (ACEX): Paleoceanographic and tectonic evolution of the central Arctic Ocean
DOWNHOLE LOGGING SUMMARY
Expedition 302 Scientists
INTRODUCTION
On the 7th August 2004 the Oden (close-support icebreaker) and the Vidar Viking (drilling ship) departed Tromsø bound for a rendezvous with the Sovetskiy Soyuz (lead icebreaker) at the edge of the permanent Arctic pack ice and to transit from there to the drilling locations at approximately 88°N on the Lomonosov Ridge (Figure 1a).
During expedition 302 four sites, all located along a single multichannel seismic line AWI-91090, were cored to a total depth of 428 mbsf, in water depths of ~1300 m at ~88°N (Figure 1b).
One of the major challenges for ACEX was to hold station while drilling and logging as the continual pressure from the ever-moving pack ice, which at times was in excess of 4 m thick, tries to push the drilling vessel off-site. This required a detailed ice management plan utilising two support icebreakers. The Sovetskiy Soyuz was the lead icebreaker and broke up the largest ice floes as they approached the drill site, and the Oden would further reduce the size of these, keeping a channel of broken ice upstream of the Vidar Viking. The ice management plan was executed perfectly and was a great success providing extended on site operational windows.
The scientific objectives of ACEX were both paleoceanographic and tectonic.
The paleoceanographic goals included the following:
History of ice rafting including: the onset of IRD (ice rafted debris) into the Central Arctic Ocean; change in ice sources throughout the period of IRD deposition; timing of ice-sheet development and collapse;
Density structure of Arctic Ocean surface waters, nature of North Atlantic conveyor and onset of Northern Hemisphere glaciation;
Timing and consequences of the opening of the Bering Strait;
Land-sea links - response of the Arctic to Pliocene warm events;
Development of deep Fram Strait and deep water exchange between the Arctic Ocean and the world ocean;
History of biogenic sedimentation.
The tectonic goals included the following:
Investigate the nature and origin of the Lomonosov Ridge in order to establish the pre-Cenozoic environmental setting;
Study the history of rifting and the timing of tectonic events that affected the ridge.
TOOLS AND OPERATIONS
A suite of logging tools, similar to the standard ODP tool strings, was available throughout the operation on the Vidar Viking. The available tools comprised the following:
QAIT (array induction tool) - resistivity;
HLDS (Hostile Environment LithoDensity Sonde) - density;
APS (accelerator porosity sonde) - porosity;
HNGS (hostile environment natural gamma sonde) - total and spectral natural gamma ray;
SGT (scintillation gamma tool) - total natural gamma ray;
BHC (borehole compensated sonic) - acoustic p-wave velocity;
FMS (formation microscanner) - formation micro-resistivity imaging.
This selection of tools provided the opportunity to characterise the in situ geophysical and geological properties of the formation, provided ice and hole conditions permitted.
Hole M0004A was the deepest penetration achieved during ACEX and reached a total depth of 428 mbsf. The logging tools were moved to the rig floor and the first tool string (FMS-APS-NGT-SGT) and wireline were rigged-up simultaneously. The tools were run into the hole at slow speed in order to allow time for them to warm up (air temperatures were sub-zero). The tool failed to clear the BHA despite repeated attempts. A number of different tool string configurations were subsequently run into the hole, none managing to pass through the BHA, with the problem eventually identified as a blockage in the drill bit. At this stage the allocated logging time had been used up and logging in Hole M0004A was abandoned.
Hole M0004B reached a total depth of 218 mbsf, the pipe was pulled to 65 mbsf and the rig floor prepared for logging. Rigging of the wireline and tool string occurred concurrently, and was completed in just over 4 hours. The tool string comprised the FMS-BHC-NGT-SGT (Figure 2); the choice of tools was such that it could be run as a straight-through tool string without the need for articulated and eccentralized subs. The logging string was run slowly into the hole in order to warm the tools before powering them up. The tool string passed through the bit without incident and two complete passes were achieved. Logging operations for ACEX were then completed. The entire logging operation in M0004B was achieved in just over 9 hours a significant achievement given the conditions (air temperature ~-10°C) and the space limitations on the drill floor, and reflects the efficiency and team work of all involved (drilling crew, Schlumberger Engineer, Vidar Viking crew and ESO personnel).
Figure 3 shows the lithostratigraphy developed from the sites cored during ACEX. The significance of the loss of the logging in Hole M0004A is clearly observable as less than half the formation was logged in Hole M0004B. However, this was the first set of downhole log information that has ever been obtained in the Central Arctic Ocean, and may be important in filling some of the coring gaps over the logged section.
LOG DATA QUALITY
During "normal" IODP logging operations in the open ocean, ship’s heave and borehole condition (diameter and roughness) are the two controlling factors that affect the quality of wireline log data. One advantage of logging in pack-ice is that there is no ship’s heave, so the principal influence on log data quality is the condition of the borehole wall. If the borehole diameter is variable over short intervals, resulting from drilling induced washout, clay swelling, or borehole wall collapse, the logs from tools that require good contact with the borehole wall (i.e., FMS) may be degraded. Deep penetration measurements such as sonic velocity, which do not require contact with the borehole wall, are generally less sensitive to borehole conditions. Thin stratigraphic sections will also cause irregular log results. The use of a mud (guar gum), combined with a primarily fine-grained formation resulted in a good quality borehole wall. The short time interval between the termination of drilling and the start of logging operations also favoured good borehole conditions.
FMS caliper logs (two per pass) provide a method for assessing the borehole condition. Figure 4 shows the caliper logs from both passes. The bit OD is 9? in., so for much of the formation the hole diameter was under gauge and narrowed significantly between 75 and 90 mbsf, at 155 mbsf and again between 180-184 mbsf. The caliper logs indicate that the borehole conditions for the logged section were good and nowhere was the borehole washed out to the degree where it would adversely affect the tool response. This is supported by a favourable comparison of parameter magnitudes between passes, and a good depth match over much of the logged interval between passes (<± 1 m). This depth offset increases to 2.6 m at ~155 mbsf, but improved again in the bottom of the hole. The seafloor was accurately picked from the spike in gamma ray seen in the second logging pass as the tool string crossed the seafloor. The log depth offsets were removed by depth matching the passes.
As expected, the lack of heave and good borehole conditions facilitated the collection of good quality FMS data. However, the borehole wall was "marked" during the drilling/reaming/pipe pulling procedure and this drilling artifact is clearly visible on the FMS images (Figure 5), which in places obscures the details of interest. Further reprocessing and detailed core-log integration has the potential to improve the quality of the images.
SUMMARY
The downhole logs begin below the pipe (65 mbsf) and extend to the bottom of Hole M0004B (218 mbsf), providing data through part of Lithologic Units 1/3, 1/4, 1/5, and 1/6 (Figure 3). Figure 6 shows the natural gamma and p-wave velocity downhole logs. Interestingly, the Lithological Unit boundaries (1/3-1/4; 1/4-1/5; 1/5-1/6) are not clearly matched by distinct changes within downhole logs, or indeed any of the core petrophysical data sets.
The total natural gamma ray logs (Figure 6) display cyclicity at a number of depth frequencies (meter and larger depth-scale) but vary little across a constant baseline of around 80-85 GAPI. At ~153 mbsf the borehole rapidly becomes under-gauge (Figure 4) and this is closely associated with a peak in gamma ray and drop in velocity. This under-gauge interval is assumed to represent deformation of unltihified sediments into the borehole. Undrained shear strength measured on recovered core indicate that the sediment becomes slightly weaker from ~155 to ~ 200 mbsf. Just below ~155 mbsf the gamma log shows a change to much larger amplitude and longer frequency (depth) fluctuations. This characteristic signature peaks in a non-recovered section at ~180 mbsf. Below this layer the gamma log returns to low magnitude variations centered on a baseline similar to that above 155 mbsf, rising rapidly at the transition into Unit 1/4.
The velocity log shows a gradual increasing-with-depth trend down to about 136 mbsf, indicative of normal consolidation in unlithified sediments (Figure 3 and Figure 6). The consistency of the gamma log through this interval supports the consolidation interpretation i.e. there is no appreciable change in sediment composition. The core density log does not display an increase downhole through this interval. Below 136 mbsf, the velocity log increases in magnitude and decreases in frequency of cycles.
MSP-PETROPHYSICS
On MSP operations undertaken in IODP by the ECORD (European Consortium for Ocean Research Drilling) Science Operator (ESO), downhole logging and core physical property measurements are now grouped together under "Petrophysics". ESO provided a petrophysics container, for the offshore part of the operation, which was equipped with a GeoTek Multi Sensor Core Logging (MSCL) system, running gamma density, p-wave velocity, non-contact resistivity, and magnetic susceptibility (offering when necessary, 2 susceptibility loops for increased logging speed). On-shore, in the Bremen Core Repository, prior to the arrival of the science party whole core logging was completed with natural gamma ray (NGR) measurements being undertaken using a semi-automated GeoTek system, that provided the facility to log cores using significantly longer count times than is possible off-shore, resulting in high quality NGR data.
CONCLUSIONS
Although incomplete, logging coverage of the cored interval will assist in characterizing the physical properties of the Lomonosov Ridge sedimentary sequence. Detailed core-log integration will allow cores to be located in their correct stratigraphical position and the log data may be used to fill a few coring gaps to ~210 mbsf. The in situ p-wave velocity (from which density can be derived), along with density, provides data used to calculate impedance so that an accurate tie to the seismic stratigraphy can be made.
ACKNOWLEDGMENT
We would like to acknowledge the help and assistance of Schlumberger during the acquisition and processing of logging data.We thank Baker Hughes for supply of the logging data processing and interpretation software Recall, which was used for the offshore and onshore interpretation of the downhole logging data.
Brice Rea: Petrophysics Staff Scientist, Department of Geography and Environment, School of Geosciences, University of Aberdeen, Elphinstone Road, Aberdeen AB24 3UF. United Kingdom. (b.rea@abdn.ac.uk)
ADDITIONAL LEG-RELATED PUBLICATIONS:
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Scientific Prospectus
Preliminary Report
Proceedings of the Integrated Ocean Drilling Program
