Logging Summary
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IODP Expedition 329: |
South Pacific Gyre
Microbiology
Expedition 329
Scientific Party
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Introduction |
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Figure 1.
Location map showing the sites
drilled during Expedition 329.
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The main objectives of the downhole
measurement program for Expedition 329
were to document crustal physical
properties, define structural and
lithologic boundaries as a function of
depth, and identify alteration in the
basaltic basement from the passage of
fluids. Downhole logging measurements
complement core measurements by
determining the thickness of lithologic
units in intervals where core recovery
was poor. Borehole logging and core-log
integration are invaluable for
reconstructing recovery gaps and
estimating bulk geochemical and
structural characteristics of deep
basement drill sites. Logging data from
the South Pacific Gyre will help
characterize current physical properties
that may set important constraints on
the downhole microbial community.
Expedition 329 logging operations were
planned for four sites: the three deep
basement sites (U1365, U1367 and U1368)
and the deepest of the sediment sites
(U1371) (Figure
1).
The purpose of this plan was to fully
characterize the nature of the basaltic
basement particularly looking at
alteration, structure and fluid flow to
study potential habitability of the
basement for microbial life.
Unfortunately due to poor hole
conditions and weather only one site of
the four was able to be logged, Site
U1368. Two tool-strings were
deployed at the site: triple combo and
FMS-gamma.
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Operations
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Site U1368 was drilled to a depth of
104.5 meters below sea floor, ~ 75
meters into basaltic basement. Prior to
the start of logging operations the bit
was released, the BHA was raised to a
depth of 34 mbsf and the hole flushed to
clear debris. No mud was used for
logging operations. The triple combo was
the first toolstring deployed and
contained natural and spectral gamma ray
(HNGS), density (HLDS), GPIT (magnetic
orientation), and resistivity
(DIT-E). One downlog and two
complete uplogs were completed. The
second deployment consisted of FMS
(resistivity images) and spectral and
natural gamma (HNGS). The sonic tool
(DSI) was not run to reduce the length
of the toolstring in the relatively
shallow hole. This toolstring also
completed one downlog and two uplogs
without incident.
Logging
Results
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Figure 2.
FMS images showing highly
fractured rubbly pillows and less
fractured more massive pillows. |
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Figure
3. Comparison of
percent potassium natural gamma
ray measurements from downhole
logs (purple) and whole round
cores (green).
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Preliminary interpretation of the
downhole log data subdivided Hole U1368F
into two units: Unit I covering the
sediment sequence and Unit II covering
the underlying basaltic basement
sequence. FMS images show the basement
sequence consists of a thick sequence of
rubbly pillow lavas, with one or two
larger meter scale lava flows (Figure 2).
Formation MicroScanner (FMS) images were
obtained for the open-hole interval
between 39 and 103 m WMSF. The diameter
of hole from the FMS calipers varied
between 18.8 and 35.1 cm. High quality
FMS images were obtained in much of the
borehole; only intervals that were
washed out showed poor quality images.
Standard, computed, and individual
spectral contributions from 40K, 238U,
and 232Th were part of the gamma ray
measurements obtained in Hole U1368A
with the HNGS. Downhole gamma ray
measurements covered a total of 95 m of
the lithostratigraphic sequence in Hole
U1368F both in open hole and through
drill pipe. Natural gamma measurements
made downhole compare well with results
of measurements on whole round cores (Figure 3).
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Helen Evans: Logging
Staff Scientist, Borehole Research Group
Lamont-Doherty Earth Observatory of
Columbia University, PO Box 1000, 61
Route 9W, Palisades, NY 10964, USA
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