IODP Expedition 308:
Gulf of Mexico Hydrogeology
Week 4 Report
PDF file is available for download.
June 27, 2005
Operations
Hole U1324A: Reentry in Hole U1324A was made at 1828 hr on 19 June and the bit placed at the logging depth of 54.2 mbsf. A tool string consisting of the HNGS, DSI, and GPIT was deployed first. The end
of pipe was set at 54.2 mbsf, and later moved to 49 mbsf to ease passage of the
logging string. A downlog was recorded and we encountered two obstructions at
59 mbsf and 509.6 mbsf. The shallow obstruction was passed after a few attempts
but the deeper one remained impassable. After attempting to clear the
obstruction for ~15 minutes and getting some overpull, we decided to log up
from this point. We measured gamma-ray and P&S mode at 15 Hz sampling rate
in the first pass at 275 m/hr up to 84.0 mbsf. The tool string was lowered for
a second pass to 509.6 mbsf where the same obstruction was encountered. We
began the 2nd pass from this depth recording gamma ray, P&S, low
frequency lower dipole, and Stoneley modes at 15 Hz sampling rate.
The second deployment consisted of the wireline seismic tool (WST). The
marine mammal watch began one hour prior to the WST deployment and the
operation started with putting the GI gun in the water for the beginning of the
soft start. The LFV obstructed lowering the WST into the open hole and we had
to pump seawater to ease the tool into the borehole. After this, a bad
electrical line was preventing power transfer to the main trigger box of the
shooting setup. The line was changed and the computer had to be powered down
before the configuration worked. During this time, a corroded plug connecting
the electrical leads to the GI gun was also found. This problem was also
resolved but delayed the beginning of the experiment by ~1.5 hrs. Based on LWD
caliper recordings, 16 stations were targeted at approximately 25 m intervals.
The Schlumberger equipment was rigged down by 1330 hr on 20 June. The hole was
then displaced with 44.0 barrels of 11.0 ppg neat cement forming a 145-meter
plug. The hole was successfully abandoned at 1745 hr when the bit cleared the
FFF.
Hole U1324B: The bottom hole assembly for Hole U1324B was identical to the bottom hole assembly used to core the Brazos-Trinity sites. After the VIT performed a survey of the sea floor, the driller
tagged the bottom at 1066.8 mbrf. Hole U1324B was spudded with the APC at 0250
hr. The recovery of the first core established the sea floor depth at 1067.5
mbrf. Piston coring advanced without incident to a depth of 117.8 mbsf where
the corer did not achieve a complete stroke, but did recover 9.91 m (104%). APC
coring continued to 357.9 mbsf by advancing by recovery. At this depth, one XCB
core was obtained prior to each deployment of the DVTP-P and T2P probes. Piston
coring continued to a total depth of 394.5 mbsf by advancing by recovery. A
total of 48 piston cores were taken in penetrating to this depth with an
average recovery of 101.3%. Non-magnetic core barrels were used for all piston
cores. The cores were oriented starting with U1324B-4H. The APCT was deployed
in 14 cores, and a special Fugro cutting shoe was deployed in cores 4H, 7H, and
odd numbered piston cores up to and including 49H. Coring resumed with the
extended core barrel (XCB) and deepened the hole to 608.2 mbsf. The XCB cored
portion of the hole was 223.8 m with an average recovery of 80.8%. The total
cored interval of 608.2 m was obtained with 93.7% average recovery. In
compliance with the operational protocol, heavy mud (10.5 ppg) was continuously
pumped starting at 481 mbsf and sustained until the bottom of the hole. The
DVTP-P was deployed ten times in this hole (229.1 mbsf, 362.4 mbsf, 387.9 mbsf,
464.3 mbsf, 493.1 mbsf, 522.0 mbsf, 541.2 mbsf, 560.4 mbsf, 589.2 mbsf, and
608.3 mbsf). There were also ten deployments (approximately 30-minutes each) of
the T2P probe (51.3 mbsf, 89.3 mbsf, 117.8 mbsf, 136.3 mbsf, 368.0 mbsf, 394.5
mbsf, and 593.2 mbsf). In accordance with the operating protocol, the hole was
plugged with 44 barrels of 11.0 ppg neat cement forming a plug of ~145 m. The
cement was followed by 40 barrels of 10.5 ppg mud and then chased with 50
barrels of seawater. The hole was observed by the VIT camera with the bit at
~80 mbsf and no flow was detected. The bit was pulled free of the hole at 0025
hr on 26 June. As the vessel was moved off location, a sub-sea release dart was
pumped down to swab the inside of the pipe.
Hole U1324C: The vessel was dynamically
repositioned 20 m west of Hole U1324A. After a VIT survey of the seafloor of
the new hole failed to indicate any obstructions, Hole U1324C was spudded when
the driller tagged the sea floor at 1066.5 mbrf. The hole was then drilled
ahead to 50.0 mbsf where the T2P was deployed with good results. Following the
retrieval of the probe, a single APC core was obtained for physical properties
analysis. The hole was then drilled in increments of 50 meters, alternating
between T2P and DVTP-P deployments and taking APC cores for geotechnical
analysis. As of 2400 hr on June 26, drilling operations were down to 200 mbsf,
coring Core U1324C-4H.
Preliminary Science Results
Results from the vertical
seismic profile (VSP) experiment show velocity variations in the shallower
section. This data will be useful for interpretation of the seismic data. Sonic
logs obtained by wireline logging were of good quality but show little
variations, whereas gamma ray data confirmed the trends already observed with
the MWD/LWD string.
The lithologies encountered
in Hole U1324B are mainly greenish gray clay with silt. Silt increases with
depth, and between Cores U1324B-52X and U1324B-54X (396.8 to 419 mbsf), the
major lithology is a sandy silt. From Core U1324B-54X to Core U1324B-59X, the
lithology is again a greenish gray mud with clay. One of the conclusions drawn
from core description is that the intervals of coarser lithologies detected in
MWD/LWD and wireline logs are characterized by silty sediments, and not
necessarily sand.
Results from
micropaleontological analysis indicate that sedimentation rates were very high,
and establishing a biostratigraphic framework for Site U1324 has proved to be
difficult. Based on the lithologies encountered in Hole U1324B, we interpret
the following geologic history. In the Pleistocene, the Mississippi River
brought sediment to the Ursa region. These sediments were deposited as
hemipelagic mud, interbedded with silt and sand transported by turbidity
currents. Sometime in the Upper (?) Pleistocene, sediments became progressively
finer. Finally during the Holocene the location of the Mississippi delta
changed and a drape of hemipelagic mud was deposited at the location of Site
U1324.
Coring operations in Hole
U1324B gave us the opportunity to deploy the two temperature and pressure
probes available on board, the T2P and DVTP-P. Abnormal readings of below
hydrostatic pressure in the formation puzzled us. Considering the degree of
deformation present in APC cores, we inferred that suction on the formation
deformed the soil. However, multiple testing on XCB cores and further analysis
of the tool proved that the problem resided with the tools leaking, and not
with the coring technique.
Obtaining in-situ pressure
data is crucial for the complete success of Expedition 308; hence every effort
was made to solve this problem. Hole U1324C was dedicated to geotechnical
sampling and deployment of the T2P and DVTP-P. Deployments of both probes in
Hole U1324C proved that the tools were now working well. The pressures recorded
in-situ in Hole U1324C are in between lithostatic and hydrostatic, which is in
agreement with the model of an over pressured basin tested by Expedition 308.
Further measurements and data analysis is needed, but results obtained at Site
U1324 are very promising.
Technical Support Activities
A supply boat arrived on the
morning of June 20 to take off the LWD tools and 2 engineers. IODP received a
shipment of microsphere tracers, 4 racks of argon gas and spares for the T2P
tool. The LWD hole U1324A was wire line logged and a VSP was successfully run
on June 20 after the supply boat departed. Hole 1324B was then core and the
shipboard labs were busy processing the cores throughout the week. The week
ended with Hole 1324C and multiple down hole tool runs with the T2P and WSTP-P
tools.
HSE Activities
The Marine Emergency Team (MET) was walked through their part in the upcoming fire drill by the First Mate.
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