164 Preliminary Report

SITE 997
(Proposed Site BRH-1a)

Hole 997A The same BHA was run with a PDC bit and nonmagnetic drill collar. Hole 997A was spudded at 1825 hr on 6 December. The seafloor was estimated at 2770.1 mbsl, based on recovery. Nineteen APC Cores 164-997A-1H to 17H and 19H to 20H were taken from 0 to 146.9 mbsf and 147.9 to 166.9 mbsf, with a total of 165.9 m cored (104.7% recovery). A single PCS core (Core 18P) was taken from 146.9 to 147.9 mbsf, with 1.0 m cored and 0.45 m recovered. Cores were oriented using the tensor tool, starting with Core 3H and including all APC cores. Adara temperature measurements were taken at Cores 2H, 4H, 10H, 13H, 16H, and 20H. WSTP temperature and water samples were taken after Cores 6H, 10H, 13H, 16H, and 18P. The FWS was deployed before Core 15H.

Twenty-eight XCB cores and seven PCS cores were taken from 166.9 to 434.3 mbsf. Twenty eight XCB cores (Cores 21X through 54X) were obtained by coring 260.4 m (64.1% recovery). Seven PCS cores (18P, 25P, 29P, 33P, 40P, 49P, 51P, and 55P) were obtained by coring 7.0 m, with 1.47 m recovered (21.0% recovery). Massive hydrate was recovered in Core 42X (327.4 to 318.7 mbsf).

Previous PCS runs on Leg 164 were evaluated to determine the optimum conditions for recovery. The most successful PCS coring technique was to force the push-in shoe as deep as possible using a push-in force of 15-20k lb wob for 5 min, followed by 0.5 m (or 5 min) advance by rotation at 20-30 rpm. Circulation at 200 gpm was used, if required, to force the shoe to advance. The 36-3/8 in. long extension was used until the formation became hard, followed by the 18-1/8 in. extension. The basket core catcher was used while the supply lasted. The auger shoe would be used when recovery decreased below 0.20 m. The PCS cores were all obtained using the push-in shoe.

Cores 29P, 33P, and 40P were pushed in without rotation and had no recovery; however, they were followed by successful coring runs in which rotation was used. This result suggests that rotation is useful in moderately compacted clays after pushing-in. The PCS push-in shoe unscrewed from the core barrel after taking Core 55P. An attempt to recover the shoe with a modified GS overshot tool was unsuccessful. Two set screws were added to the PCS push-in and auger-type shoes to lock them on the core barrel. The PDC bit would be destroyed by rotation on the shoe; therefore, coring was terminated. The hole was filled with sepiolite mud, the pipe was pulled, and it cleared the seafloor at 0010 hr on 10 December.

Hole 997B The ship was moved 20 m northeast, and Hole 997B was spudded at 0045 hr on 10 December. The same seafloor depth of 2770.1 mbsl was used. The hole was drilled with a center bit to 318.5 mbsf at 41.1 m/hr. Three XCB cores were taken over the interval in Hole 997A where a large hydrate section was recovered. Cores 164-997B-1X to 3X were taken from 318.5 to 347.3 mbsf, with 28.8 m cored and 12.57 m recovered (43.6% recovery). The hole was drilled with a center bit from 347.3 to 414.2 mbsf at 26.7 m/hr.

Eight PCS and 35 XCB cores were taken from 414.2 to 750.7 mbsf. One PCS water sample was taken after Core 28X at 606.5 mbsf. Eight PCS cores (6P, 10P, 15P, 21P, 25P, 32P, 36P, 40P, and 44P) were taken, with 8.0 m cored and 1.62 m recovered (20.3% recovery). The practical depth limit for the PCS push-in shoe appears to be about 450 mbsf in these clays. Thirty-five XCB cores (4X to 47X) were taken from 414.2 to 750.7 mbsf, with 357.3 m cored and 239.63 m recovered (67.1% recovery).

The formation was predominately a gassy, dry clay except for a 2-m hard section at 447-449 mbsf (near the BSR at about 450 mbsf). Core was lost by extrusion from the liner top and most cores showed clear indications of gas expansion. The C1/C2 ratio in headspace gas samples declined from 16,000-20,000 near the top of the hole (36-66 mbsf) to 109 at 726 mbsf, indicating that the mostly methane gas was biogenic in origin with some migration of heavy hydrocarbons especially below 450 mbsf. A geothermal gradient of 36.9°C/km was assumed based on Hole 997A. The C1/C2 ratios declined in vacutainer samples from 108 mbsf and remained steady below 466 mbsf. Iso/normal C4-6 ratios decreased below 466 mbsf, a mild to strong petroleum odor was common, and a light yellow fluorescence was noted in sediments from 452.6 to 568.0 mbsf.

A 35-bbl sepiolite mud sweep was circulated and a wiper trip was made to 80 mbsf to condition the hole for logging. A maximum overpull of 40k lb was encountered pulling out of the hole, with a maximum drag of 40k lb at 578 mbsf running in. The hole was reamed from 565 to 660 mbsf with no fill on bottom. No sticking tendency was noted. The hole was displaced with sepiolite/seawater mud. The pipe was pulled back to 103 mbsf, and the comical side-entry sub was rigged up. Following a downhole tool failure, a DITE/SST/NGTC log was run in 13 hr to 747.4 mbsf with the CSES.

The WHOI triaxial VSP tool was run, and the pipe was run with the CSES to 690 mbsf. The tool stopped at 314 mbsf and had to be pumped out of the pipe with 800 psi. A zero-offset VSP survey was conducted in 13.25 hr, with the tool clamped at 8-m intervals from 714 mbsf. The 300 in3. water gun failed, so the survey was run with an air gun only. The dipole shear tool was run, but the tool failed and the Gearhart Owens head was replaced. The tool was run with the CSES to 660.8 mbsf and was pumped out of the drill string. A log was run in from 660.8 mbsf. The geochemical combination was run next, followed by the FMS. Following the logging runs, another zero-offset VSP was run using the WHOI triaxial VSP tool and both air and water guns. Hole 997B was then terminated, with the bit clearing the rotary table at 0730 hr on December 17. The ship was underway for Miami at 0815 hr on December 17.

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