OPERATIONS SYNOPSISTransit to Site 1186
The crew spudded Hole 1186A with the RCB at 2130 hr on 22 October. We drilled ahead with a wash barrel in place to a depth of 697.4 mbsf, where we initiated coring. The formation was unexpectedly soft, and drilling through this interval required only 9.45 hr. The average rate of penetration was 60 m/hr.
We rotary cored the sediment portion of Hole 1186A from 697.4 to 968.6 mbsf (271.2 m interval). The low average recovery of 18% was caused by very soft sediment with abundant chert stringers that left fragments in every core to a depth of 957 mbsf. Below that depth, recovery began to improve. We contacted basaltic basement at 968.6 mbsf. This depth is based on the recovery of 1.4 m of basalt from a cored interval (Core 192-1186A-30R) that ended at 970.0 mbsf. This is slightly deeper than the value of 966.8 mbsf calculated using standard ODP conventions for curated positions of core material. We continued coring in basaltic basement until we reached 1034.0 mbsf. The average recovery in basement was 59% at an average penetration rate of 1.8 m/hr. The only problem we encountered during coring operations was when a fresh core barrel, dropped after retrieval of Core 192-1186A-31R (970.0-976.2 mbsf), failed to land properly in the BHA. This problem was cleared by deploying a bit deplugger. We surmised that some basalt may have slipped out of the previous core barrel during recovery and partially obstructed the landing of the new core barrel.
After reaching 1034.0 mbsf (67.2 m into basement), we decided to end coring operations for several reasons: (1) slow penetration rates, (2) concerns that a bit change and reentry with a FFF would be risky because of ~970 m of open hole in chert-rich sediment, (3) chemical analyses of the basalt indicated that it was closely similar to basalt at Site 1183, and (4) we wanted to leave enough time to drill an additional basement site.
In preparation for logging, we circulated a 50-bbl sepiolite mud flush in the hole from a depth of 1033 mbsf. After filling the borehole with 340 bbl of sepiolite mud, we released the bit at the bottom of the hole. The crew assembled the Schlumberger logging equipment and placaed the bottom end of the BHA at 123 mbsf. Hole 1186A was logged with a single pass of the triple combination (triple combo) tool string and two passes of the Formation MicroScanner logging tool (FMS)/sonic tool string. During the first logging run, we lowered the triple combo from the bottom of the pipe at 123 mbsf to 1030 mbsf, coming within 4 m of the bottom of the hole without difficulty. We logged upward to 675 mbsf, just above the top of the cored interval. During the second logging run, we lowered the FMS/sonic tool string to 1032 mbsf (within 2 m of bottom) without difficulty, and logging proceeded upward. The first pass was interrupted by a loss of telemetry from the tool string, requiring reinitialization of the instrument. Logging then continued up to 696 mbsf. We then lowered the tool back to the bottom of the hole and made a second pass up to 686 mbsf.
After rigging down the logging equipment, we recovered the drill string and dismantled the BHA in preparation for the transit to Site 1187. The vessel departed for Site 1187 at 2115 hr on 28 October.
Site 1187 | Table of Contents