SITE 856

Hole 856H

The ship was moved in DP mode to Hole 856H, where a Drill-in-Casing (DIC) had been set with 12 m of 11-3/4" casing during Leg 139. The hole had been cored to 93.8 mbsf, making it the deepest oceanic hole in sediment-covered massive sulfides. Operations were terminated on Leg 139 due to unstable hole problems; however, there was a possibility that several years of water flow in the open hole had cemented or stabilized some of the upper hole section. The plan was to obtain a temperature profile and fluid sample and attempt to deepen the hole. The water depth was 2434.5 mbrf. The RCBBHA was run as follows: 9-7/8" RBI CC-7 bit, MBR, HS, OCB, TS, HS, 7 X 8-1/4" DC,TDC, 2 stands of 5-1/2" transition DP, XO. The hole was relocated and reentered in 45 min. The bit encountered fill at 73.8 m, and the hole was washed 20 m in soft fill to TD at 93.8 m with moderate torque and packing-off on the last connection. RCB wash barrel Core 169-856H-18W was pulled with 3 pieces of massive sulfide recovered. RCB Cores 169-856H-19R to 65R were taken from 93.8 to 500.0 m with 12.1% recovery. Coring 406.2 m required 88.5 operating hours including two precautionary short trips. The rate of penetration averaged 5.0 m/hr in massive sulfides and 38.4 m/hr in thermally altered mudstones. Recovery was reduced by brittle mudstone fracturing and jamming in the 8 finger and 8 finger combination core catchers and core liner. The massive sulfide zone extended to 110 m, with a sulfide stringer zone of hydrothermally altered mudstone with sulfide veins and pyrite banding to 202 m. Below the sulfides were brittle mudstones to 432 m, basalt sills and sediment to 472 m, and pillow basalts and flows to 500 m. Hole conditions while coring on Leg 169 were good with only occasional erratic torque and fill on connections. Sepiolite mud sweeps were circulated near the end of each core. Coring parameters were 10-25 Kips WOB and 50 rpm at 100-200 amps torque, circulating 250 gpm at 350 psi.

After conditioning the hole for logs, the core bit was dropped on the seafloor to leave the hole clean for future drilling. The hole was reentered with the open end drill string, which was parked at 22.5 m for logging. The main logging tools had relatively low temperature ratings (Triple Combo and Sonic/FMS to 175°C and Geochem to 130°C); therefore, a temperature run was planned first to check bottom hole temperatures and thermal rebound rates. The temperature tool from the Bureau de Recherche Geologique et Miniere in Orleans (BRGM)was chosen for the first run because it was rated to 250°C. The 2.5-m-long BRGM temperature tool weighed only about 50 kg; therefore, the loggers were advised that it was not heavy enough to meet ODP third party tool guidelines (227 kg minimum). A 1-1/2" slotted sinker bar (30 kg) was added to the line but the tool weight was still not adequate to permit the winch operator to "see" the tool. The logging line was run to an apparent depth of 471 m but the BRGM tool was pulled because of erratic readings. The logging tool had evidently stopped on a shallow ledge or bridge at about 61 m (without the winch operator's knowledge) while the logging line continued to be run into the hole. The new Schlumberger high-temperature logging line was twisted, kinked, and knotted and 300 m were cut off. An attempt was made to clean out the ledges with the open end pipe. It was worked past ledges from 50 to 63 m, but could not be worked deeper and was pulled. The bit was not dropped in the hole because the intention was to leave the hole clear for future deepening.

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