DOWNHOLE
MEASUREMENTS
Operations
On 25 December 2000,
wireline logging operations in Hole 1189B began with the deployment of the
high-temperature/pressure telemetry gamma-ray cartridge (HTGC) with real-time
cable head temperature capabilities (MTEM). The water depth was estimated from
pipe measurements at ~1693 mbrf, and the bottom of the hole was estimated at 206
mbsf. A casing string had been set to a depth of ~32 mbsf, so the drill pipe was
placed at ~28 mbsf (see "Return to Site 1189" in "Operations
Summary" in the "Leg
193 Summary" chapter). A wiper trip to the bottom of the hole was
done ~8 hr before logging operations began, and several bridges were found in
the bottommost 40 m of the hole. After the wiper trip, the bit was released at
seafloor using the mechanical bit release (see "Return to Site 1189"
in the "Operations Summary" in the "Leg
193 Summary" chapter), and the hole was reentered to begin logging
operations.
The MTEM-HTGC tool string
was deployed, and the wireline heave compensator was engaged by circumventing
the ground fault interrupt switch as described in "Downhole
Measurements" in the "Site
1188" chapter. This tool string was deployed first because of
concerns with potentially high-temperature conditions as active venting was
clearly observed while surveying the area. The tool reached a logging total
depth (TD) of 198 mbsf, and on the way up, the tensiometer on the winch unit
registered several thousand pounds of overpull at ~189 mbsf. The MTEM sensor
recorded a high temperature of 47.8°C, therefore a WSTP deployment using the
sand line was scheduled to a depth of 132 mbsf.
The second wireline
deployment included the MTEM-hostile-environment natural gamma-ray sonde (HNGS)-accelerator
porosity sonde-hostile-environment lithodensity sonde (HLDS)-dual induction
resistivity (DIT)-tool string combination. A borehole restriction found at ~158
mbsf allowed the tool string to only reach 48 m from the bottom of the hole. The
third tool string consisted of the MTEM-natural gamma-ray tool (NGT)-dipole
sonic imager-FMS combination, and this run also achieved a logging TD of 158
mbsf. During the last two logging runs, heave measurements were recorded with
the guideline tensiometer encoder (GTE) that was installed on the wireline heave
compensator (WHC) for LWD operations. These records show <1-ft average heave
for most of the second deployment and for the entire third logging run (Fig. F140).
Borehole
Conditions and Data Quality
The average dimensions of
Hole 1189B are good for the acquisition of high-resolution logging data as the
hole diameter decreases from 14.7 in at the top to 10 in toward the bottom. The
average hole diameter measurements obtained with the two-arm FMS calipers are
12.0 and 12.5 in. The difference between measurements is most likely caused by a
slightly elliptical borehole between 40 and 80 mbsf and also between 90 and 100
mbsf (Fig. F141).
The only significant borehole enlargement (apart from cavitation below the end
of casing; 36-41 mbsf) was present at 133 mbsf, and it affects all measurements
(Fig. F141).
Electrical
Resistivity Measurements
Similar to the results
from Holes 1188B and 1188F, electrical resistivity measurements are low. The
deep resistivity values range between 0.1 and 2.8 
m,
and the medium resistivities are between 0.06 and 6.4 m.
The shallow resistivity curve shows isolated peaks with extremely high
measurements up to 101 m.
Although these peaks tend to correlate with the other resistivity measurements,
they may be erroneous. A different induction tool was used in Hole 1189B than
the one deployed in Hole 1188F that showed similar results with the medium
resistivity curve. Overall, electrical resistivity measurements show cyclic
patterns with increasing resistivity with depth between 67 and 122 mbsf (Fig. F141).
Natural
Radioactivity Measurements
Gamma-ray measurements in
Hole 1189B were obtained with HTGC, HNGS, and NGT. The gamma-ray curves show
good correlation, only the values measured with the HNGS are slightly higher
than those measured with the NGT. The major difference between these tools is
that the HNGS uses bismuth germanate scintillation crystals that provide
significantly better spectral response because of their enhanced ability to stop
gamma rays and convert their energy to full amplitude signals. In addition, the
HNGS curves are corrected for hole size during recording. These differences may
explain the discrepancy in the magnitude of the signals recorded by the
different tools. In Figure F141,
only the measurements from the HNGS are displayed for simplicity.
Total gamma-ray counts in
the open hole vary between 37 and 249 gAPI. Uranium measurements range from 2.5
to 22.5 ppm, thorium from 0.3 to 4.6 ppm, and potassium from 0.6 to 5.5 wt%.
Correlation between the total gamma-ray readings and the uranium content is much
stronger than between the total gamma-ray and potassium content. Thus, uranium
makes a large contribution to the total gamma-ray spectrum of Hole 1189B.
Neutron
Porosity and Density Measurements
Neutron porosities vary
from 12% to ~100% with an average value of 50.7%. The largest values are
observed below the casing where the tool stand off is the greatest (Fig. F141).
The neutron porosity log shows a steady downward decrease that may be related to
a decrease in hole diameter (Fig. F141).
A sharp increase in neutron porosity over a few meters is also present at 133
mbsf, where an enlargement of the borehole was observed in the caliper readings.
Density values range from
1.1 to 2.6 g/cm3 with a mean value of 2.0 g/cm3. The
density values show a slight increase with depth, which might be caused by a
decrease in hole diameter. The density log also shows a sharp decrease at the
borehole enlargement at 133 mbsf. The photoelectric factor log shows variations
between 1.8 and 10.5 barn/e-. The photoelectric factor also shows
cyclic trends between 67 and 104 mbsf that are opposite to those observed in the
electrical resistivity measurements (Fig. F141).
Sonic
Velocity Measurements
The compressional and
shear wave velocity curves were derived using a slowness time coherence
technique on the recorded waveforms. This process was performed on board with
the Schlumberger Multitask Acquisition and Imaging System (MAXIS) in near-real
time during the logging operation. The data from this hole have several invalid
intervals and should, therefore, be used as a qualitative data set. For
simplicity and clarity of presentation, the invalid intervals were excluded from
plots presented in this chapter (Fig. F141).
Compressional wave
velocity values range from 1.9 to 5.6 km/s, and shear wave velocity values show
variations from 1.7 to 3.5 km/s. Overall, compressional wave velocities show a
steady increase from below the casing to an approximate depth of 95.4 mbsf (Fig.
F141). Below
this depth, there is an abrupt decrease in compressional wave velocities
followed by a general increasing trend with depth. The abrupt decrease in
velocity correlates only with high values in uranium and may be representative
of either tool malfunction or inadequate processing because of a low energy
transmission interval. The shear wave profile shows a subtler increasing trend
with depth, with several high velocity values generally correlating with highs
in density and photoelectric factor and low values of porosity.
Temperature
Measurements
Temperature measurements
were obtained in Hole 1189B with the MTEM sensor during every wireline run (Fig.
F142). As in
Hole 1188F, all the temperature curves show similar patterns inside the drill
pipe and at the transition between casing and the open hole. The first run shows
signs of drilling disturbances as the temperature profile varies significantly
with depth. A low temperature recorded at 60 mbsf (<10°C) is followed by a
steady temperature increase down to 115 mbsf (~43°C) and alternating decreasing
and increasing trends until the bottom of the hole. The temperature measurements
from the later logging runs show significantly higher temperature patterns. The
maximum temperature recorded in Hole 1189B is 68°C, measured during the second
pass of the last tool deployment at an approximate depth of 107 mbsf. The time
elapsed between the first and the last temperature measurement was ~15 hr.
A WSTP temperature
measurement was obtained at a depth of 132 mbsf. The record shows that the tool
descended to the selected depth for a 15-min station; however, the valve opened
prematurely (Fig. F143).
A water sample was still collected as the check valve and coils contained warm
fluids (see "Geochemistry").
The short temperature record at this depth shows nearly isothermal temperatures
with an average value of 54.5°C.
Formation
Evaluation
Eight logging units were
identified in Hole 1189B (Fig. F143).
Logging Unit 1 is a transitional unit at the top of the logged interval, whereas
cyclic trends in the electrical resistivity and/or photoelectric factor curves
define logging Units 2 through 4. Several small intervals in these units show
high density and photoelectric factor values that correlate with low electrical
resistivities. These responses may be indicative of higher concentrations of
sulfides. Logging Unit 5 shows a downward increasing trend in electrical
resistivity that may indicate a change in alteration. Logging Units 6 and 8 are
characterized by uniform log responses in the electrical resistivity, neutron
porosity, density, and photoelectric factor logs. Logging Unit 7 corresponds to
the small interval with an enlarged section of the borehole causing the
electrical resistivities, gamma ray, density, and photoelectric factor log
responses to decrease and the neutron porosity to increase. This logging unit
represents a highly fractured interval or a fault zone. Apart from logging Unit
7, the logging units do not show any apparent correlation with the natural
radioactivity logs.
FMS
Images
FMS images show
significant changes with depth in the styles of alteration and fracture density.
Below casing, the images show that the upper part of the borehole (Fig. F144)
contains a higher concentration of disseminated conductive minerals than the
lower sections. These conductive minerals seem to correlate to the higher
concentrations of sulfides found in the recovered cores. Changes in alteration,
such as the transition between logging Units 2 and 3, are also prominent
features in the FMS images (Fig. F145).
The changes in resistivity of logging Unit 6 are characterized by distinctive
conductive layers that are interbedded with more resistive and foliated units
(Fig. F146).
The bottom of the logged
interval shows completely different features as the images show a more resistive
and fractured unit that tends to correspond to the less altered cores of Hole
1189B. A series of steep fractures comprising a 3-m-thick interval correlate
with logging Unit 7 (Fig. F147).
This zone may correspond to a faulted interval. Below this zone, the images are
characterized by a series of subhorizontal and subvertical fractures (Fig. F148).
Operations
LWD/RAB operations in Hole
1189C began on 28 December 2000, and were terminated on 29 December 2000, after
reaching a depth of 166 mbsf. The water depth was estimated at 1700 mbrf and
drilling operations were done with a 9.875-in bit. The RAB BHA was changed from
the previous configuration to contain one monel (nonmagnetic) sub above the tool
and two below. This was done to isolate the tool from potential changes in
magnetization of the BHA because of the presence of magnetic minerals in the
formation. A sampling rate of 10 s was used for the LWD operations.
After drilling Hole 1189C,
additional logging operations were planned to establish a direct comparison
between LWD and wireline measurements and for facilitating the interpretation of
the subsurface stratigraphy. The ultra-high-temperature multisensor memory tool
(UHT-MSM) temperature probe was deployed first because the MTEM sensor that was
previously used in other holes was not working after the last deployment in Hole
1188F. The UHT-MSM deployment found a hole obstruction at 71 mbsf and recorded a
maximum temperature of 45°C, hence, wireline deployments of the HNGS-HLDS-DIT-E
and NGT-FMS tool string combinations followed. The bottom of the drill pipe was
placed at 10 mbsf, and both tool strings logged an interval ranging from 10 to
67 mbsf.
Borehole
Conditions and Data Quality
During the two logging
runs, heave measurements were recorded with the GTE that was installed on the
WHC for LWD operations. These records show a high of ~1 m heave during the first
deployment and a high of ~1.5 m motion for the second run (Fig. F149).
The upper 70 m of Hole 1189C is irregular, and measurements from the FMS
calipers show an average diameter of 13.5 in (Fig. F150).
Below 50 mbsf, there are several small isolated zones where the FMS calipers
were close to being fully extended. Borehole sections between 44 and 49 mbsf, as
well as between 22 and 29 mbsf, are also close to reaching the maximum aperture
of FMS calipers. A section between 17.5 and 21 mbsf shows a divergence in
caliper readings because an obstruction that was encountered in every logging
run.
Electrical
Resistivity Measurements
Electrical resistivity
measurements made in Hole 1189C show the same range of low values that were
observed in other holes drilled in the Manus basin (Fig. F150).
The deep resistivity values obtained with the DIT-E range from 0.8 to 2.6 m.
Medium and shallow resistivity measurements range from 0.7 to 3.2 m
and from 0.6 to 6.0 m,
respectively. RAB measurements show similar responses over most of the
overlapped section with a few localized zones that show slightly higher maximum
values (Fig. F150).
Below 80 mbsf, the RAB measurements show a few peaks that correlate with lows in
the gamma-ray curves. The LWD maximum resistivity values are 15.1 m
for the ring measurement (RRING) and 14.3 m
for the medium button measurement (RBM).
Natural
Radioactivity Measurements
Total gamma-ray counts
obtained with the HNGS over the 10- to 50-mbsf interval range between 90 and 258
gAPI with an average value of 148 gAPI (Fig. F150).
The values obtained with the NGT over the 12- to 59-mbsf interval range from 18
to 253 gAPI with a mean value of 116 gAPI. Potassium values range from 0.6 to
4.3 wt% with an average of 2.7 wt%, whereas thorium and uranium values range
from 0.2 to 3.6 ppm (average = ~1.4 ppm) and from 5.8 to 24.3 ppm (average =
~12.0 ppm), respectively. As in previous holes, there is a good correlation
between the total gamma-ray and the uranium logs suggesting that, at least
between 20 and 55 mbsf, uranium makes the largest contribution to the total
gamma-ray spectrum.
Total gamma-ray values
recorded with the RAB between the seafloor and 161 mbsf range from 13.2 to 257
gAPI (Fig. F150).
The average value is 96.7 gAPI. Good agreement exists between the total
gamma-ray measurements from wireline and LWD measurements in the overlapping
interval. Total gamma-ray values show downward decreasing trends between 28 and
55 mbsf and between 64 and 80 mbsf and an upward decreasing trend between 86 and
105 mbsf.
Density
Measurements
Density values measured
during wireline operation between 10 and 56 mbsf are low. Values vary between
1.1 and 2.2 g/cm3 with an average value of 1.9 g/cm3 (Fig.
F150).
Measurements of the photoelectric factor range from 1.8 to 7.6 barn/e-
with a mean value of 4.0 barn/e- (Fig. F150).
The photoelectric factor log shows a downward decreasing trend between 28 and 50
mbsf similar to the one observed in the gamma-ray log.
Temperature
Measurements
The UHT-MSM temperature
probe was deployed prior to the wireline operations to determine the temperature
conditions of Hole 1188C. The probe measured a maximum temperature of 46°C at
the bottom of the hole (Fig. F151).
A steep thermal gradient of 1°-2°C/m was estimated from the up and down logs
from seafloor to ~13 mbsf. A gentler thermal gradient of 0.3°C/m was estimated
from 13 to 55 mbsf.
Formation
Evaluation
The wireline logging data
and logs from the RAB tool were used to identify 10 logging units (Fig. F150).
Although logging Units 1 and 2 can be partly characterized using the full suite
of logs obtained in Hole 1189C, the distinction of logging Units 3 through 10 is
solely based on the resistivity and gamma-ray measurements from the RAB tool.
Logging Unit 1 represents
the transition from the seafloor to a depth of 20 mbsf. This unit shows a rapid
increase in gamma-ray values followed by several prominent highs and lows (Fig. F150).
Logging Unit 2 is characterized by a decreasing trend with depth in the
gamma-ray and photoelectric effect curves. Near the top of this unit, there is a
gamma-ray high that corresponds to higher concentrations in uranium (Fig. F150)
similar to those observed in previous holes. Logging Units 3 and 5 are
represented by high gamma-ray values. The gamma-ray values measured in logging
Unit 3 are the highest values recorded in the upper 166 m of Hole 1189C, but
only slightly higher than those observed near the top of logging Unit 2 (Fig. F150).
Logging Units 4 and 6
represent opposite trends in the gamma-ray and resistivity curves. Logging Unit
4 shows a decreasing trend with depth in gamma-ray values that correlates with
an opposite trend in the electrical resistivity profiles. Logging Unit 6 shows
similar correlation between the two curves, but with downward increasing
gamma-ray counts and higher resistivity values at shallower depths (Fig. F150).
Logging Units 7 through 10 represent a sequence of alternating layers with high
and low gamma-ray values.
RAB
and FMS Images
Borehole images show that
most of Hole 1189C is characterized by subhorizontal and subvertical fracturing
as well as alternating numerous resistive and conductive features. RAB images
from 46 to ~60 mbsf show a series of conductive subhorizontal features that seem
to represent fractures (Fig. F152).
This section also displays a significant vertical to subvertical resistive
feature from 46 to 51 mbsf and a dipping resistive unit resembling a vein at 51
mbsf. Below this depth, a series of patchy resistive features are also
prominent. A RAB section from 76 to ~90 mbsf also shows similar features
resembling subhorizontal fractures and nearly vertical resistive features (Fig. F153).
A comparison between RAB and FMS images shows good agreement with some of the
larger features (Fig. F154).
Although the FMS displays higher resolution and definition of structural
features, fractures are clearly identified in both sets of images. However, the
resistive units are not as clear in the FMS logs, perhaps because of the lower
coverage of the borehole provided by the wireline tool.
