SITE 1041

(Proposed Site CR-3)

Hole 1041A
Site 1041 is located 6.2 nmi landward of Site 1040. Hole 1041A was spudded at 0430 hr on 25 November 1996. The first APC was shot from a depth of 3315.0 m and recovered 7.4 m of core establishing a mudline of 3317.2 m. APC Core 170-1041A-2H did not fully stroke and required 40 kips overpull to extract the core from the stiff sediment beneath the seafloor. An XCB core barrel was dropped and 2 hr were required to drill down to 14.3 mbsf. Because the drill string came clear of the seabed after Core 170-1041A-2H, the interval from 0 to 14.3 mbsf had to be re drilled. The wash barrel that was in place was pulled because of the extended rotating time. XCB coring commenced with Core 170-1041A-3X and continued through Core 170-1041A-18X to a total depth of 155.1 mbsf. Coring was suspended there due to slow ROP (8.9 m/hr for last 2 cores), poor core quality (natural fractures in the clay/silt stone were highly disturbed by the drilling process), and steadily deteriorating hole conditions (fill, elevated pump pressure, and breakout torque after each connection). Gas-hydrate nodules were recovered from Cores 170 1041A-15X through 18X and fissility was observed in most cores.

The DVTP was deployed a total of three times at depths of 45.7 mbsf, 87.9 mbsf, and 126.4 mbsf. One run, at a depth of 78.2 m, recovered erroneous data. Hydrocarbon vacutainer analysis indicated that the hydrate-rich sediments had 0.8% to 1% methane.

Two 30 bbl Sepiolite mud sweeps were made in the hole at depths of 126.4 mbsf and 135.5 mbsf, respectively. The hole was displaced with 52 bbls of 10.5 lb/gal mud prior to retrieving the drill string. Once clear of the seafloor, 1.25 hr were taken to slip and cut the drilling line and the drill string was recovered back to the ship.

Hole 1041B
While the RCB BHA was being made up the vessel was offset 30 m to the east. The drill string was tripped to the seafloor and Hole 1041B was spudded at 1600 hr 27 November 1996. With a center bit in place, the bit was advanced to a depth of 155.0 mbsf. No overlap was desired between Holes A and B coring intervals as the objective of the hole was to advance to the high-amplitude reflector at about 500 mbsf as fast as possible. The drilled interval took a total of 6.75 hr and an average ROP of 37.5 m/hr was achieved. RCB coring began at that depth and continued through Core 170-1041B-25R to a total depth of 395.6 mbsf. A distinct difference in formation was identified in Core 170-1041B-15R at an approximate depth of 290 mbsf. More fill was identified between connections and higher torque was required after picking up the pipe from the dual elevator stool. Once rotation and circulation was established, however, the drilling parameters remained normal and stable. Hole conditions got progressively better during the subsequent 12 hr of coring.

After cutting Core 170-1041B-25R, the driller used a sweep of Sepiolite drilling mud to flush the hole. At this point the pipe became stuck ~12 m off bottom. Various combinations of circulation pressure, torque, and overpull were used to work the pipe free. The pipe was worked with up to 950 amps (28,000 ft-lbs) of torque, 40 kips downward drag, and 100 kips overpull, all with 500 psi back pressure. After 1 hr, the pipe came free using 200 kips of overpull. No torque was in the string at the time. With the top drive still in the string, the pipe was pulled to a depth of 3565.4 mbrf (248.3 mbsf) where all back pressure was lost and drilling parameters again became normal. At that point the hole was displaced with 74 bbls of 10.5 lb/gal mud and the drill string was pulled to clear the mudline.

During the course of drilling and coring in this hole, eight 30 bbls Sepiolite mud sweeps were pumped at depths of 203.1 mbsf, 241.6 mbsf, 270.5 mbsf, 289.9 mbsf, 309.2 mbsf, 338.0 mbsf, 366.8 mbsf, and 395.6 mbsf. The DVTP was deployed twice. The first run was made after Core 170-1041B-2R at a depth of 174.2 mbsf and did not obtain good data. A second run was made after Core 170-1041B-3R at a depth of 183.8 mbsf.

As in Hole 1041A, gas hydrate was dispersed throughout the recovered core material beginning with Core 170-1041B-1R and continuing through Core 170-1041B-25R. As the formation became more indurated and the porosity decreased, the hydrate was less plentiful but still present to some extent primarily within naturally occurring fractures. Hydrocarbon vacutainer analysis indicated that the hydrate-rich sediments had 0.7% to 1% methane. Ethane values ranged from 1214 ppm to a low of 274 ppm. Propane values ranged from 4 to 29 ppm. Only two cores registered higher hydrocarbons. Cores 170-1041B-9R and 170-1041B-11R identified 1 ppm IC4.

Hole 1041C
The ship was offset 100 m southeast of Hole 1041B and Hole 1041C was spudded with a center bit in place. Drilling advanced to a depth of 395.0 mbsf in 21.0 hr achieving a penetration rate of 28.0 m/hr. It became apparent even during the course of the drilled interval that the hole was deteriorating. The pump strokes required to displace mud sweeps continually increased above the calculated amount, indicating the hole was enlarging with time. RCB coring advanced the hole from 395.0 mbsf to a depth of 423.8 mbsf (Core 170-1041C-3R). Before Core 170-1041C-3R could be recovered the drill string again became stuck. A total of 30 min was taken to work the pipe with torque of 900 amps, 40 kips drag, 100 kips overpull, and 200-400 psi back pressure. The pipe was eventually pulled free with 225 kips of overpull. The drill string was pulled to a depth of 363.8 mbsf and the hole was displaced with 115 bbls of 10.5 lb/gal mud. The drill string was pulled clear of the seafloor ending Hole 1041C. The total depth of the hole (423.8 mbsf) was short of the desired scientific objective estimated at ~550 mbsf. Because hole deterioration continued to present a major drilling problem, an alternate site, Site CR-7, was chosen as the next location. The geologic environment was expected to be the same as that drilled at Site 1041, but the depth of the target reflector was estimated to be located at a shallower depth of 325 mbsf.

During the course of drilling and coring this hole, Sepiolite mud sweeps were pumped at depths of 280.2 mbsf (40 bbls), 309.1 mbsf (30 bbls), 337.9 mbsf (30 bbls), and 366.8 mbsf (30 bbls). There were no temperature measurements or water samples recovered from this hole.

Hydrocarbon vacutainer analysis indicated that the sediments recovered from the three cores had 71,080-10,474 ppm methane. Ethane values ranged from 83 ppm to a low of 21 ppm in Core 170-1041C-3R. Propane values ranged from 1-5 ppm. No higher hydrocarbons were present.

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