162 Preliminary Report

Site 986

The vessel covered the 685 nmi to Site 986 (SVAL-1) at an average speed of 11.9 kt and encountered no difficulties while underway. At 0642 hr on 10 August 1995 the vessel slowed to survey speed (5.3 kt) and a seismic survey was carried out near the drill site. The main purpose of the survey was to offset slightly from the proposed drill site (SVAL-1) to avoid drilling through the entire thickness of a major debris flow identified in the pre-cruise data. Once the survey was completed, the ship returned to the site coordinates using GPS data.

The operational plan for this site called for two APC holes drilled to refusal or to approximately 200 mbsf. One hole was to be deepened to XCB refusal, which was estimated to occur at about 500 mbsf. The third hole was to be cored just short of the XCB hole TD and then continuously RCB cored to a depth of 900 mbsf. This hole would not penetrate basement, but drill through the entire glacial sediment package, which was the major site objective.

Core orientation was not taken due to the high latitude of the site. A minimum number of ADARA temperature measurements were taken in the upper (APC) portion of the hole in order to establish a temperature gradient for the site. A full suite of logging tools was to be deployed on the deepest site, although intermediate logging of the XCB hole was to be considered if the hole TD reached an adequate depth.

The positioning beacon was deployed at 1208 hr on 10 August, initiating Hole 986A. A standard APC/XCB BHA was madeup, but the nonmagnetic drill collar was not required at this site. Hole 986A was spudded, and routine piston coring continued through Core 986A-14H, at which point the core pressure failed to bleed off, indicating incomplete stroke. Since APC refusal occurred at such a shallow depth it was decided to continue coring operations with the XCB system to approximately 200 mbsf, if recovery and core quality remained acceptable. XCB coring was initiated with Core 986A-15X. Coring continued with excellent recovery, averaging 81%, except for two cores with zero and 0.75 m recovery. Coring was halted after Core 986A-24X when the scientific target was reached.

Methane was encountered beginning with Core 986A-2H and continued through Core 24X at 206.0 mbsf. Headspace data for the hole indicated methane (C1) concentrations ranging from 955 to 57852 ppm, and ethane (C2) concentrations ranging from 2 to 65 ppm. Propane (C3) was detected beginning with Core 12H and ranged from 1 to 7 ppm. The methane/ethane ratios varied from 356-11570. Higher molecular weight hydrocarbons were not detected.

Hole 986B was spudded 15 m south of Hole 986A. We suspected that the first core barrel impacted a dropstone, which would have affected the total recovery. In addition, if this were true the mudline established from this core would be erroneous, and this was not desirable on a hole with a projected deep penetration. It was therefore decided that a new hole should be spudded.

The spudding of Hole 986C took place at the same location as Hole 986B (no vessel offset), and the bit was positioned at the same level (2061.0 m) prior to spudding. Routine piston coring continued until Cores 986C-6H through 8H failed to bleed off pressure, indicating incomplete stroke. Three successful ADARA temperature measurements were taken on Cores 986C-3H, 5H, and 7H, and these data indicated a high temperature gradient of 152°C/km.

XCB coring was initiated with Core 986C-9X and continued until coring was halted after Core 986C-44X, when a core barrel became stuck inside the drill pipe. After several attempts at jarring the barrel loose, it came free. After recovering the core barrel, we found that the XCB cutting shoe thread connection had over-torqued, leading to a swollen box and subsequent mechanical failure. Continued coring with the XCB system was deemed unwise and would risk losing the opportunity to log the existing hole; therefore, coring operations were suspended.

Small quantities of hydrocarbons were present throughout the core until TD with Core 986C-44H at 408.0 mbsf. Headspace data for the hole indicated methane (C1) ranged from 3334 to 49887 ppm, and ethane (C2) from 10 to 117 ppm. Propane (C3) ranged from 0 to 16 ppm. The methane/ethane ratios varied from 256 to 1672. Higher molecular weight hydrocarbons were not detected.

As done on the previous holes, two annular-hole volumes of seawater were circulated. A wiper trip with the drill string was then made to 2150.4 m (87.4 mbsf). No overpull was experienced during the wiper trip, but 30,000 lb of drag was noted at 2304 m during the trip back to bottom. In addition, 15 m of fill were identified on bottom. The go-devil for locking open the LFV was pumped downhole while another two hole volumes of fluid were pumped. The pipe was then pulled to a logging depth of 102.6 mbsf. Drilling mud was not required during the drilling operation, and the hole was considered to be in excellent condition; therefore, the hole was not displaced with mud for the logging run. Three logging runs were successfully made on this hole. The Quad combo, FMS, and GHMT/NGTC tool strings were deployed to within 21.0, 36.0, and 41.0 m, respectively, of the bottom of the hole. After the final suite of tools was recovered, the logging sheaves were rigged down, the bit was run to bottom, and the hole was filled with heavy mud.

The vessel then moved 50 m to the south in DP mode in preparation to core Hole 986D. This was the first hole on Leg 162 where the Rotary Core Barrel (RCB) coring system was used, which required a change in the bottom-hole assembly and the type of rotary core bit used. The new BHA was tripped back to bottom and Hole 986D was spudded at 0137 hr on 15 August. Because several hundred meters of the upper formation were to be drilled rather than cored, the seafloor depth from Hole 986C, 2063.0 m, was used. This was more accurate than the depth that would have been obtained by the driller "feeling" for bottom with the RCB drilling assembly.

Drilling ahead continued with a center bit in place to a depth of 2450.8 m or 387.8 mbsf. The center bit was recovered via wireline. Hole deviation measurements were taken on bottom and at 100 m increments on the way out of the hole. A hole deviation of approximately 3.6° was determined based on these measurements.

Continuous RCB coring commenced with Core 986-1R. Core recovery was highly variable, ranging from 0% to an excellent 100%. Beginning with Core 986D-22R a string of five straight empty core barrels was retrieved through an apparent coarse-grained sandy debris-flow zone, but flows of this nature were not unexpected in this geological environment. Core 986D-27R recovered a mere 0.33 m and then core recovery improved significantly. With the exception of two zero-recovery cores all remaining cores recovered between 11% and 102%. Small quantities of hydrocarbons were present throughout the coring of the hole, just as in the other holes cored at this site. Headspace data were monitored closely throughout the coring process.

Coring was terminated after Core 986D-60R, which reached a depth of 3027.6 m (964.6 mbsf). The termination depth was in excess of the original scientific target depth of 900 mbsf, but did not reach the desired 1000 mbsf. Coring was halted due to several low-recovery cores and to the bit plugging, which occurred on three successive core barrels. In addition, primarily because of the bit plugging problems, the rate of forward progress decreased substantially. The bit deplugger was deployed several times during the coring process when we suspected dropstones were plugging the bit throat. One final run with the bit deplugger was made after recovering Core 986D-60R, and pump pressures were once again restored to normal.

Upon completion of the coring program, the hole was swept with two mud sweeps of 60 barrels each. A wiper trip with the drill string was then made to 2163.0 m (100.0 mbsf). While running back in the hole, a hard bridge (45,000 lb of bit weight) was tagged at a depth of 2557.0 m (494.0 mbsf). The top drive was picked up and a center bit was dropped at that point. The hole continued to cause problems from that point on and required a total of 6.5 hr of reaming to get back to the original hole TD. Several additional gel mud sweeps were pumped during the reaming operation.

After again sweeping the hole with two gel mud pills, the center bit was recovered, and two additional wireline runs were made to release the bit and reverse shift the mechanical bit release (MBR) sleeve back down again. Because of the difficulty experienced in reaching bottom with the top drive and a working drill bit, it was considered unlikely that the feat could be duplicated upon completion of logging with open-ended pipe. Therefore, the hole was displaced with heavy mud to avoid the need to get back to bottom and to enhance the chances of the logging program succeeding in the quickly deteriorating hole.

After filling the hole with mud, the pipe was tripped to a logging depth of 2385.7 m (322.7 mbsf). A maximum of 15,000 lb of overpull was experienced throughout the trip. The logging equipment was rigged and the first run, with the Quad combo suite of tools, was deployed to a depth of 2468.0 m (405.0 mbsf). This was only 82.3 m below the open-ended pipe. The logging tools were removed and the pipe was lowered to a depth of 2501.1 m (438.1 mbsf). A second run with the Quad combo tools reached a depth of 2514 m and was eventually worked to a depth of 2550 m (487.0 mbsf) or 48.9 m below open-ended pipe. Further logging attempts were thought to be futile, and subsequent logging was abandoned. The tools were recovered, the logging sheaves rigged down, and the pipe tripped back to the surface. The beacon was recovered while a BHA inspection took place, and the ship departed the site at 2100 hr on 20 August 95.

TO Site 987

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