Site 1146 (Proposed Site SCS-4)

The 78-nmi transit to Site 1146 (proposed site SCS-4 ) was accomplished in 7 hr at an average speed of 11.1 kt. The beacon was dropped on precise GPS coordinates at 2345 hr on 21 March. The PDR depth referenced to the DES was 2108.4 m.

Hole 1146A

Hole 1146A was spudded with the APC at 0500 hr on 22 March. The seafloor depth was defined at 2102.6 m from the recovery of the first core. Piston coring advanced with 21 APC cores to 193.9 mbsf. The cores were oriented starting with Core 3H and then continuously from 6H through 21H. During piston coring, downhole temperature measurements were obtained at 32, 61, 99, and 146 mbsf. A temperature gradient of 62°C/km was calculated from the data obtained by the last three measurements. The hole was deepened with the XCB to the target depth of 607.0 mbsf. The average recovery for this hole was 99.5%. Methane concentration increased with depth and peaked at 563 mbsf with 87,000 ppm, then dropped to 5,000 ppm at the bottom of the hole. The maximum ethane concentration of 200 ppm (and <10 ppm propane) was found in samples at the bottom of the hole.

The hole was flushed with 30 bbl of mud and displaced with an additional 190 bbl of sepiolite. No restrictions in hole size were observed by the driller when the pipe was raised to the logging depth of 88 mbsf. Wireline logging began at 1615 hr on 24 March and was completed with three successful runs: triple combo (85-600 mbsf), FMS-sonic (239-600 mbsf), and GHMT (239-600 mbsf). The drill string was lowered to move the bit from 88 to 239 mbsf after the first run because of numerous washouts and ledges in the upper part of the hole. After logging, the hole was abandoned with 30 bbl of 10.5-lb/gal heavy mud. The drill string was pulled back, and the bit cleared the seafloor at 1330 hr on 25 March.

Floats associated with a long-line fishing array were observed drifting near the ship as Hole 1146A was being displaced with sepiolite mud. As the drill string was pulled up to logging depth, a small amount of monofilament fishing line (30 m) replete with hooks and some live bait was removed from two stands. Two small fishing boats were observed in the area. The drill crew also retrieved two mahi-mahi (3-4 ft long) at the rig floor. The fish had safely passed up through the guide horn and out the DES and ended up in our barbecue grills. A diver inspection of the thrusters and main propeller shafts was planned for the Hong Kong port call.

Hole 1146B

The ship was offset 20 m east of Hole 1146A, and Hole 1146B was spudded with the APC at 1500 hr on 25 March. The seafloor depth inferred from the recovery of the first core was 2103.2 m. Piston coring advanced to 216.3 mbsf, and the cores were oriented starting with Core 3H. The hole was deepened to 245.1 mbsf with three XCB cores. The average recovery for the hole was 98.7%. The bit was pulled clear of the seafloor at 1415 hr on 26 March.

Hole 1146C

The vessel was offset 10 m east of Hole 1146B. To obtain a stratigraphic overlap with data from the previous holes, the initial APC core of Hole 1146C was shot after placing the bit one meter below the mudline depth of Hole 1146B. Hole 1146C was spudded at 1540 hr on 26 March, and piston coring advanced to 162.5 mbsf. The cores were oriented starting with Core 3H. The hole was deepened to the maximum target depth of 603.5 mbsf with 46 XCB cores. During XCB coring, the interval from 220.1 to 224.1 mbsf was drilled ahead to maintain an overlap with Hole 1146A. The average recovery for the hole was 101.4%.

The total core recovered at this site was 1452.2 m, representing 100.1% of the cored interval of 1450.6 m. The bit cleared the seafloor at 0100 hr on 29 March as the drill string was recovered. Before leaving the site, the components of the bottom-hole assembly were subjected to the routine end-of-leg magnetic particle inspection. At 0615 hr on 29 March, the vessel began the short voyage to Site 1147.

Operations-Site 1147

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