Site 1148 (Proposed Site SCS-5C)

The beacon was deployed on the GPS coordinates of Site 1148 (proposed site SCS-5C) at 2015 hr on 30 March.

Hole 1148A

Hole 1148A was spudded with the APC at 2215 hr on 30 March. The seafloor depth calculated from the recovery of the first core was 3308.7 m. Piston coring advanced with 16 APC cores to 143.3 mbsf with an average recovery of 101.1%. The cores were oriented starting with Core 4H. During piston coring, downhole temperature measurements with the APCT were obtained at 29, 58, and 105 mbsf. Based upon these measurements, a temperature gradient of 83°C/km was calculated.

Coring was resumed with the XCB and advanced to 450 mbsf with 100% recovery. At 152.9 mbsf, the DVTP was deployed. The data obtained at 153 mbsf confirmed the temperature gradient calculated from the three APCT measurements. At ~460 mbsf, a sharp transition from clayey chalk to an underconsolidated clayey formation was encountered, and recovery dropped off sharply. From 460 to ~600 mbsf, the average recovery was 47%, with several cores around 500 mbsf having almost no recovery. In an effort to increase recovery, some core barrels were recovered after advancing only 5 m instead of the nominal 9.6 m. Between 600 mbsf and the bottom of the hole at 704 mbsf, average recovery returned to almost 100%. The XCB average recovery for Hole 1148A was 86.9%; the average recovery for this hole was 89.8%.

Headspace analysis indicated that the concentration of methane was below 10 ppm down to 430 mbsf. It then increased gradually to a maximum concentration of 569 ppm at 580 mbsf. More complex hydrocarbon molecules were not detected above 468 mbsf. Below this depth, the abundance of both ethane and propane followed a trend similar to that of methane, peaking at 570-580 mbsf (25 and 10 ppm, respectively). Butane and pentane were detected from samples taken below 570 mbsf, with maximum readings of 28 and 32 ppm, respectively, from the last core (77X).

A wiper trip was made up to 107 mbsf and then back to bottom. Approximately 19 m of soft fill was found at the bottom of the hole. The hole was washed and reamed from 685 to 704 mbsf. After the hole was flushed with a 30-bbl mud treatment, the bore was displaced with an additional 215 bbl of sepiolite. The bit was pulled back and positioned at 107 mbsf for logging. Wireline logging in Hole 1148A was completed with three successful runs: triple combo (107-704 mbsf), FMS-sonic (180-704 mbsf), and GHMT (166-704 mbsf). With the exception of a washed-out interval above 200 mbsf, the hole was in the best condition of any of those logged on this leg. The drill string was lowered after the first and second logging runs because of several zones of reduced hole diameter that were attributed to swelling clays. After logging, the hole was abandoned with 30 bbl of 10.5-lb/gal mud. The bit cleared the seafloor at 1600 hr on 5 April.

Hole 1148B

The vessel was offset 20 m east of Hole 1148A. Before spudding the new hole, a near-bottom-water temperature was obtained with the APCT. Hole 1148B began at 2045 hr from a seafloor depth of 3303.4 m, and piston coring advanced to 145.1 mbsf with 96.2% recovery. Several liners split during the piston coring, which resulted in less than expected recovery. Cores were oriented starting with Core 3H. A 4-m interval was washed ahead from 36.6 to 40.6 mbsf to ensure a stratigraphic overlap with Hole 1148B.

While piston coring in Hole 1148B, we received permission from ODP to deepen Hole 1148B to basement or to a maximum of 850 mbsf, as long as rigorous safety monitoring did not preclude further drilling. After piston coring in Hole 1148B, it was decided to drill ahead with a center bit in place through the interval 145-440 mbsf that was well recovered in Hole 1148A in order to save time for deepening the site. The XCB recovery from 460 to 600 mbsf was 44%, slightly less than in the same interval in Hole 1148A. Recovery in the interval 600-646 mbsf improved again to 92%.

To save further time for coring the deep objectives of this site, the interval 646-700 mbsf was drilled ahead with the center bit in place. At midnight on 8 April, XCB coring resumed and advanced from 700 to 809 mbsf with 83.5% average recovery at an average rate of penetration of 15 m/hr. Results of the headspace analysis were closely monitored to observe any trends suggesting that we were approaching an accumulation of hydrocarbons. The drillers had instructions that if any sudden change in drilling rate or torque was observed, the core barrel would be recovered and a headspace analysis performed before coring resumed. While coring from 774 to 775 mbsf, the drillers experienced higher than normal torque with an apparent increase in bit weight of ~5,000 lb. These symptoms suggested that there was some hydraulic levering of the bit off bottom due to a packed-off annulus. The core barrel was recovered early, and the core-catcher sample was subjected to hydrocarbon analysis. While we waited for the results, a 30-bbl sepiolite mud sweep was circulated through the annulus. After the headspace analysis results confirmed that there was no change in hydrocarbon concentration, coring resumed.

The interval 809-853 mbsf was cored at a low rate of penetration of 6 m/hr with very low recovery (22%) because of frequent jamming of the brittle claystone in the core catcher. At 1300 hr on 10 April, the last core was recovered from the target depth of 853 mbsf. The total core recovered in this hole was 364.4 m, representing 72.8% of the cored interval of 500.6 m. The drilled-down interval was 352.6 m, with a total penetration of 853.2 m. The hole was displaced with 30 bbl of 10.5-lb/gal mud, and the bit cleared the seafloor at 1535 hr on 10 April. The drill string was recovered, and the beacon was retrieved before the drilling equipment, hydrophones, and thrusters were secured. At 2215 hr on 10 April, the vessel began the 267-nmi voyage to the Hong Kong pilot station.

Table of Contents