IODP Expedition 310: Tahiti Sea Level
Downhole Logging Summary
Introduction
Offshore downhole-logging operations for expedition 310 were provided
by the University of Montpellier (Laboratoire de Tectonophysique) who
form part of the European Petrophysics Consortium (EPC), which is part
of the European Science Operator (ESO). The EPC is primarily concerned
with the planning, acquisition, quality control and assurance, science
support and education outreach related to petrophysical measurements on
mission specific platforms. Petrophysics in relation to this consortium
includes both downhole measurements (i.e. borehole logging) and physical
properties (continuous and discrete) measured on recovered core (e.g.
Magnetic susceptibility, Acoustic properties, Density and Porosity).
The set of borehole geophysical instruments was constrained by the scientific
objectives, the coring technique used (HQ diameter boreholes) and the
geological setting of the expedition. Slimline logging tools of various
manufactures (Table
1) were used in combination with a MountSopris Instruments (MSI) acquisition
system (winch) and Advanced Logging Technology (ALT Ltd) and MSI loggers
and operating systems (ALTlogger and MSlog software); all tools were run
individually. The acquisition of downhole geophysical data of "recently
drowned" Pleistocene-Holocene coral reefs was a significant challenge
because of very unstable borehole conditions, swells over 4 m in height,
and logging directly below the sea floor in shallow and small diameter
(HQ) boreholes without the use of a casing or mudcake.
Due to environmental constraints, no nuclear tools were deployed during
Expedition 310.
Operations
From the 37 holes drilled at 22 sites, a total of 10 boreholes were prepared for downhole geophysical measurements. After completion of the coring, the drill string was pulled and the coring bit was changed for an open shoe casing (Figure 1) to provide borehole stability in unstable sections and a smooth exit and entry of logging tools. In addition, a wiper trip was performed with fresh sea water (no drilling mud was used). Borehole conditions were extremely hostile and very often the boreholes had to be logged in intervals where the HQ drill string was used as a temporary casing.
Figure 1. An optical image (OBI40) of the open shoe casing in borehole
M0023B.
To be able to record ultra high resolution (maximum of 2mm vertical resolution for optical images) geophysical downhole logging data, the acquisition was done from the rooster box which, in the used piggy-back drilling system, is heave-compensated (Figure 2).
Figure 2: Petrophysics staff scientist in the rooster box.
The logging team consisted of two engineers, supervised and assisted by the Petrophysics Staff Scientists. All measurements were performed under open borehole conditions (no casing) with the exception of a few spectral gamma ray logs which were run through the steel pipes to obtain continuous geophysical information over the entire interval cored. Considering that, on average, the (conventionally calculated) core recovery was 57.47 %, this kind of continuous information may be essential for achieving the scientific objectives. Because of the difficult borehole conditions and time constraints it was not possible to log all tools in every borehole.
General description of logging tools
In intervals of low or disturbed core recovery, downhole geophysical
logs provide the only way to characterize the borehole section. This is
especially true when recovery is poor and when comparable measurements
or observations are obtained from core, as downhole geophysical logs allow
precise depth positioning of core pieces by visual (borehole images) or
petrophysical correlation.
The slimline suite comprised the following tools (Appendix A (link pending)
and Table 1):
Optical Borehole Televiewer (OBI 40): The OBI40 produces a mm-scale,
high resolution image of the borehole wall, similar to a subsurface endoscope.
Acoustic Borehole Televiewer (ABI 40): The ABI40 produces mm-scale, high-resolution
images of the borehole surface using acoustic pulse and echo techniques.
Hydrogeological probe (IDRONAUT): The IDRONAUT measures hydrogeological
properties of the borehole fluid only.
Spectral Natural Gamma Probe (ASGR): The ASGR allows the identification
of the individual elements that emit natural gamma rays. These include
potassium (K), uranium (U) and thorium (Th). The ASGR detector for gamma
rays is a Bismuth Germanate (BGO) scintillation crystal, and is optically
coupled to a photo-multiplier. The BGO detector has an absorption potential
eight times greater than a more classic Sodium Iodine (NaI) crystal. As
most of the spectral discrimination is performed in the high energy range,
only instruments equipped with BGO detectors prove to be reliable in the
slimline tool domain.
Induction Resistivity Probe (DIL 45): The DIL 45 provides measurements
of electrical conductivity. The output of the tool comprises two logs:
ILM (induction electrical conductivity of medium investigation depth,
0.57 m) and ILD (induction electrical conductivity of greater investigation
depth, 0.83 m). The measured electrical conductivity is finally converted
into electrical resistivity.
Full Waveform Sonic Probe (SONIC): The 2PSA-1000 sonic probe measures
compressional wave velocities of the formation. In addition, the analysis
of surface waves in the borehole (i.e. Stoneley waves) can be indicative
of formation permeability.
Caliper Probe (CAL): The 2PCA-100 is a 3 arm (mechanical) caliper tool
which measures the borehole diameter.
Data Recording, Processing and Quality
Each logging run was recorded and stored digitally. Data flow was monitored
for quality in real time using tool specific acquisition boxes and software.
Table 1 summarizes the acquisition system
for each tool. After each run, data was processed and interpreted (QA/QC)
and where necessary a repetitive section or complete new pass was acquired.
WellCAD was used for visualization and plotting the data. Sonic data was
processed using LogCruncher (Mercury Geophysics) software (Table
1). While deploying all the tools separately, a fixed zero depth position
was maintained at the top of the drill pipe in the heave-compensated rooster
box, hence no depth shifting or reprocessing based on accelerometer data
was necessary. Post-processing and repositioning of logging data to meters
below present sea level was done onshore using WellCAD.
Considering the extremely hostile borehole conditions, the shallow penetration
and the nature of the stratigraphy drilled (recent reef), the overall
quality of downhole logging data is exceptionally good. Despite the short
amount of time in between completion of coring and the start of logging,
but thanks to the wiper trips and the very permeable nature of the coral
reef succession, the optical images (OBI40) provided continuous (and color-calibrated)
images of the borehole wall at a resolution of 360 samples x 2 mm which
results in a pixel size of less than 1 (horizontal) x 2 (vertical) mm.
Where the borehole fluid was not clear (murky), the images of the borehole
wall are fuzzy. The acoustic images (acoustic hardness of the lithologies
within the coral reef sequence) of the borehole wall are not affected
by borehole fluid quality. The pixel size of the acoustic images is 1.1
(horizontal) x 4 (vertical) mm. The presented values of the acoustic caliper
were obtained by assuming a acoustic wave velocity of 1535 m/s through
borehole fluid and taking the mean value of the 288 measurements at every
sample (4 mm vertical sampling).
The quality of the Spectral Natural Gamma data is directly related to
lithology in combination with logging speed. Despite logging speeds of
1.1 m/minute and a taking a sample every 10 cm (collecting gamma ray emissions
of the formation for approximately 6 seconds for every sample) the amount
of total counts obtained are still very low. This degrades the quality
of the statistics that separates the raw counts into activity values of
naturally occurring radioactive elements such as potassium (K), uranium
(U) and thorium (Th). Negative K values are indicative of incorrect statistics.
Because there was only a short time period between the completion of coring
(including wiper trip) and logging, the IDRONAUT data should be treated
with great care. The hydrological properties of the borehole fluid measured
with this tool represent more of a mixture between fresh sea water (used
for coring and for the wiper trips) and true formation pore water.
Induction resistivity is a deep-investigation measurement and is least
sensitive to borehole conditions. Resistivity values are within the expected
range.
Measurements of compressional velocity (Vp @ 10 kHz) are also deep(er)-investigation
measurements. Unlike electrical methods, acoustic waves are highly dependent
on borehole conditions. Larger cavities causes the induced wave to scatter
and acoustic energy is lost more rapidly. Stoneley wave (surface wave
@ 1 kHz) measurements are highly dependent on borehole conditions because
the acoustic wave that propagates along the borehole wall is of interest
here. The data was filtered (frequency filter) in such a way that only
the energy around the induced frequency (source) was analyzed. Waveform
picking was done manually to ensure good quality data. All presented acoustic
data is accurate. Where no clear first arrivals in the waveform were present
in at least two receivers, a value of zero was entered in the database.
Finally, the values of the mechanical caliper are accurate. The entire
system was calibrated using rings of known diameters before every caliper
pass.
Results
Wireline logging operations in the Tiarei sites (NNE of Tahiti) produced
nearly complete downhole coverage of the post-glacial carbonate sequence
from 122 to 72 m below present day sea level. Considering the main objectives
of this expedition, it was decided to use present sea level as a datum
level instead of the sea floor as is usually done. Because of very hostile
borehole conditions around the older Pleistocene to Post-glacial carbonate
sequence boundary, it was not possible to image this boundary in every
borehole. This was only possible in borehole M0023B.
The measured geophysical parameters, including optical and acoustic borehole wall images (Figure 3a and 3b) provided the only source of continuous information of the drilled sequences in expedition 310.
Figure 3a. Imaging results of M0023B (8.1 - 9.25 mbsf) at an appropriate
scale.
Figure 3b. Imaging results of M0009D (23.75 - 25 mbsf) at an appropriate
scale.
Furthermore, by "unrolling" the images of the borehole wall (360o), into a 2d view, a cross section about 31 cm is obtained as compared to a 6.54 cm cross section of a split core obtained by HQ-diameter drill bit (Figure 4).
Figure 4. Core to logs integration at its best!
It was (therefore) possible to identify a typical stacking of Post-glacial
lithofacies from the continuous downhole geophysical logs. A typical stacking
of lithofacies is grouped into a subsequence. A subsequence consists of
three lithofacies:
The basal lithofacies of a typical subsequence consists of fragments of
branching coral colonies in a sandy/muddy matrix at proximal locations,
has elevated values in natural radioactivity, low electrical resistivity
values and lowest acoustic velocities. At distal locations, this lithofacies
is characterized by extremely open coral framework or medium sized cavities.
This lithofacies can best be observed in the interval between 22.37 and
23.00 mbsf in M0023B (Figure 5a).
Figure 5a. Logging data: Results of M0023B.
The middle lithofacies of a subsequence consists of branching coral colonies
which framework density usually increases towards the top. It is characterized
by decreasing natural radioactivity upsection, increasing resistivity
values and acoustic velocities upsection. This lithofacies is best observed
in the interval between 19.43 and 20.0 mbsf and 24.00 and 24.75 mbsf in
M0023B (Figure 5a).
The uppermost lithofacies of a typical subsequence consists of foliaceous
to tabular and encrusting coral species and minor massive corals (Porites).
It is characterized by intermediate to lower acoustic velocities, decreasing
resistivity values upsection and increasing natural radioactivity values.
It can be observed best in the intervals between 20.81-21.30 mbsf in M0023B
(more tabular), 23.00-23.62 mbsf in M0023B (more foliaceous) and 7 and
8.15 mbsf (massive Porites) in M0009D (Figures 5a
and 5b).
Figure 5b. Logging data: Results of M0009D (see Figure
5a for acronym key).
In Figure 6 borehole images and natural radioactivity logs (Total Counts only) are plotted in meters below present day sea level. In each of the logged boreholes, the boundary between the Older Pleistocene sequence and the Post-glacial sequence is indicated.
Figure 6. Composite of optical image (OBI), acoustic image (ABI),
and total gamma ray (TGR) downhole logging data for holes in the Tiarei
area.
In the Tiarei transect, the basal unit directly overlying the above mentioned
boundary consists of a semi-consolidated rubble interval having elevated
natural gamma radioactivity values in proximal locations (M0023B). Distal
locations do not show this higher gamma ray signature (with the exception
of M0021B) and, although poorly recovered in downhole logging data, it
usually consists of very open framework branching corals that are heavily
encrusted. In M0009D is it most likely that fresh(er) water ingress from
the island (Tahiti) occurs in this specific interval.
In Figure 7 borehole images and formation electrical resistivity (resistivity) are plotted in meters below present day sea level.
Figure 7. Composite of optical image (OBI), acoustic image (ABI),
and formation resistivity downhole logging data for holes in the Tiarei
area.
At this larger scale of observation and by correlating the boreholes from
the outer ridge to the inner ridge, it becomes clear that, although evolved
on a relatively steep and irregular paleo morphology, the general resistivity
pattern and absolute values of the Post-glacial sequence along this transect
are essentially the same and comparable. In each borehole the basal interval
has lowest resistivity values, values increase gradually till a maximum
value after which a sharper negative excursion to lower values can be
observed. The interval of increasing resistivity values is interrupted
once by a subtle but clear decrease in the middle of the Post-glacial
sequence. The absolute values of this decrease are higher in distal boreholes
than in the proximal boreholes.
Wireline logging operations in the Maraa sites (S of Tahiti) produced nearly complete downhole coverage of the post-glacial carbonate sequence from 102 to 41.65 m below present day sea level (Figure 8).
Figure 8. Composite of optical image (OBI), acoustic image (ABI),
and total gamma ray (TGR), and formation resistivity downhole logging
data for holes in the Maraa area.
Very hostile borehole conditions are caused by open framework coral morphologies and relatively soft microbialite encrusting along and over coral colonies. Overall, acoustic reflectivity values in the ABI-40 image logs are lower than in Tiarei. These conditions did not allow an image “recovery” as high as the one for the Tiarei transect. The boundary between the Older Pleistocene and Post-glacial sequence could not be imaged. The water depth at M0005D was 59.63 m and the Post-glacial carbonate sequence is ~27 m thick and the total depth was 102.17 mbsf (the deepest borehole in expedition 310). Geophysical wireline operations were completed in Hole M0005D from 98.01 mbsf with data coverage by all slimhole tools over the lowermost Older Pleistocene sequence cored in Hole M0005D (76.59- ~96 mbsf). A spectral gamma ray log was made through the steel drill pipes to obtain a continuous log over the entire interval comprising pre- and post-glacial deposits. From the seafloor to 32.36 mbsf (thus including the whole Post glacial carbonate sequence), the total gamma radiation (TGR) is very low (~15 cps) despite logging speeds up to a maximum of 1.1 m/min. A repetitive measurement under open borehole conditions shows exactly the same trend in total counts (Figure 9). A marked increase in TGR can be observed within the Older Pleistocene sequence in the interval between 32.36-52.0 mbsf, where Uranium contributes most to the TGR in the upper part (32.36-46.71) and Thorium, and minor Potassium, contributes most to the TGR in the lower part. In the remainder of the Older Pleistocene sequence, Uranium contributes most to the TGR values (Figure 9).
Figure 9. Logging data: a: Results of M0005D (see Figure
5a for acronym key). See text for discussion.
In Figure 8 borehole images, natural radioactivity logs (total counts only) and electrical resistivity logs are plotted in meters below present day sea level. In each of the logged boreholes the boundary between the Older Pleistocene sequence and the Post-glacial sequence is indicated. The depth below present day sea level of the Pleistocene boundary depends on paleo seafloor morphology at the time of the late glacial maximum. Although the quality and meters covered in imaging the Post-glacial sequence is less in Maraa than in Tiarei, a similar stacking of lithofacies can be identified.
Conclusions
Despite operational difficulties associated with logging a recent coral reef the geophysical downhole data is of highest quality. This continuous information quantifies the physical properties of the Tahiti Pleistocene-Holocene reefs. The results, especially imaging of the borehole wall using acoustic and optical geophysical methods, compliment the sedimentological work and allow an unambiguous correlation of cores (coral assemblages), core logs (MSCL), downhole logs and drilling parameters. The (conventionally calculated) core recovery was 57.47 %, but the image logs show that highly porous (cavities up to 0.5 m high) formations were cored (Figure 10), and suggest that the true recovery was much higher, potentially higher than 90 percent.
Figure 10. Imaging results of M0015A illustration the extremely
hostile borehole conditions.
Hendrik Braaksma: Petrophysics Staff Scientist, Laboratoire de
Géophysique et Hydrodynamique en Forage, Université Montpellier
II, ISTEEM 056 - Place Eugène Bataillon, 34095 Montpellier Cedex
5, France.
email: hendrik.braaksma@dstu.univ-montp2.fr
Jenny Inwood: ESO Petrophysicist, University of Leicester, Borehole
Research, Department of Geology, University of Leicester, University Road.
Leicester, LE1 7RH, United Kingdom.
email: ji18@le.ac.uk
Additional Expedition-related publications:
(page will open in a new window)
Scientific Prospectus
Preliminary Report
Proceedings of
the Integrated Ocean Drilling Program
