After the short 13 nmi (1.5 hr, 8.67 nmi/hr) transit to Site 978, we deployed two beacons at 1600 hr and 1840 hr, 15 June, respectively. No site survey was performed, since existing seismic data were sufficient for selecting the site and for correlation of coring results throughout this part of the east Alboran Basin. The elevation of the DES above sea level was 11.60 m for Hole 978A.
Hole 978A
We offset the ship 100 m northwest of the beacon to ensure that we were >0.5 nmi away from the known coordinates of a telecommunication cable. Hole 978 was drilled at 36°13.867'N, 2°03.424'W. The same RCB BHA was run as was used in Hole 977A, except that we added a new MBR and drill bit. To save time and reach the deep objectives as quickly as possible, the scientific party requested that the bit be lowered to the seafloor without using the VIT to determine the exact seafloor depth. The seafloor depth was determined to be at 1954 mbrf (1942.4 mbsl), based on the drill string weight indicator as the bit entered the seafloor. (N.B. The seafloor depth was later adjusted to 1941 mbrf or 1929.4 mbsl - see below.) Hole 978A was spudded at 2100 hr, 15 June.
The hole was drilled with a center bit in place to 110.3 mbsf (2064.3 mbrf), where the first RCB core was taken. The hole was then advanced (without coring) from 119.9 to 168.4 mbsf, where a second RCB core was attempted. Recovery in Cores 161-978A-1R and -2R was almost zero (0.2%). The hole was then drilled from 178.0 to 213.0 mbsf, where we started continuous RCB coring. At this depth, the formation was firm enough to be recovered without washing away. Core 161-978A-5R was advanced only 3.9 m for drilling convenience (so that the driller then could work with a full kelly).
RCB Cores 161-978A-3R to -53R were taken from 213 to 698.0 mbsf (2154 to 2629.4 mbrf, respectively). Recovery in the interval from 213.0 to 611.3 mbsf was 485.0 m (84%). Recovery was poor was in the interlayered sands and gravels. The total recovery for Hole 978A was 406.66 m (80.7%). Coring parameters were 4,000-25,000 lb WOB while rotating at 50-70 rpm with 100-200 amps torque, circulating seawater at 30-50 spm with pressures ranging from 150 to 400 psi with the core barrel installed.
A bit change was required to complete the hole, so we deployed an FFF at 0530 hr, 20 June. When pulling out of the seafloor, several attempts were required before the bit would clear the FFF without dragging the FFF up. The back-reaming buttons on the bit were apparently catching the lower lip of the FFF. We rotated the pipe about a fourth of a turn with chain tongs on the rig floor before we saw on the VIT that the FFF had dropped off. Then we withdrew the pipe from the FFF.
Upon clearing the FFF, it was noted that the drill pipe depth was significantly shallower (~17 m; 1937 mbrf) than first reported. After attaching a new bit, running back to the seafloor, and prior to reentering the FFF, we touched the seafloor with the bit while observing with the VIT. This accurate seafloor depth was 1929.4 mbsl (1941 mbrf). All previous depths were changed to reflect this correct depth.
The FFF was entered at 1530 hr, 20 June. The drill pipe was run into the hole to 33.27 mbsf (1974.27 mbrf), where it met resistance from the formation, suggesting that we probably did not reenter the previous borehole. Several attempts were made to raise and lower the drill pipe to find the same hole. We picked up the circulating head and attempted to wash down, but were met with the same amount of resistance. Finally, the top drive was picked up, and we unsuccessfully attempted to find the hole by rotating and washing to 150.21 mbsf (2090.21 mbrf). After 6.5 hr of unsuccessful attempts to reenter the same hole, we decided to terminate the hole and move on to the next site. A slug of heavy mud was placed in the hole prior to pulling out. The drill pipe cleared the seafloor at 2300 hr, 20 June. The beacons were released and recovered while the drill pipe was being pulled. The bit cleared the rotary table at 0230 hr, 21 June.
Hole 978A exhibited similar hydrocarbon patterns as Hole 977A. Core 161-978A-18R contained the maximum methane recorded for the hole of 20,710 ppm. This was accompanied by 10 ppm of C2, 77 ppm of C3, 9 ppm of IC4, and 8 ppm of IC5. Gas contents began to decrease below Core 161-978A-25. No gas trends were observed that were interpreted as consideration for abandonment.