After a 439 nmi transit from Site 975 (36 hr; 12 nmi/hr), we conducted a seismic survey at Site 976 (52 nmi; 9.75 hr). We then positioned over the site and deployed two beacons (at 0100 and 0330 hr, 21 May, respectively). Both beacons were left on due to heavy seas and strong currents. The strong surface current flowing in from the Atlantic was mostly between 2.0 and 2.5 nmi/hr and varied from 250° to 280°. Site 976 was in the general shipping lane for all traffic entering and exiting the Mediterranean Sea. At times, more than 10 ships could be seen on the radar within a 12 nmi radius. This level of ship traffic continued throughout the time at Site 976. The elevation of the DES above sea level was 11.05 m for Holes 976A and 976B, 11.35 m for Hole 976C, and 11.38 m for Holes 976D and 976E.
Hole 976A
A reentry cone installation was planned at Site 976, so the first hole was used to establish the mud line and conduct a jet-in test. We spudded Hole 976A at 0615 hr, 21 May. Core 161-976A-1H was taken with the bit at 1115 mbrf and recovered 5.92 m; therefore, the seafloor was defined to be at 1107.5 mbsl. We then continued lowering the bit into the seafloor by circulating water only to determine the amount of 16-in. conductor casing that could be safely washed into the seafloor with the reentry cone. An XCB core barrel with a center bit was used during the jet-in test. The jet-in test took 1.5 hr and penetrated to 65 mbsf, using up to 45 strokes per min (spm).
Hole 976B
We offset the ship 15 m to the southeast (i.e., up-current to the deep Mediterranean outflow) and spudded Hole 976B at 1000 hr, 21 May. The hole was vertically offset 2 m from Hole 976A.
Core 161-976B-1H was taken with the bit at 1113 mbrf and recovered 3.45 m of sediment; therefore, the seafloor was defined to be at 1108 mbsl. Sea conditions at the time the mud-line core was taken may have contributed to the difference in the mud-line core recovery between Holes 976A and 976B. APC Cores 161-976B-1H through -14H were taken to 127 mbsf and recovered 128.04 m (101%). Cores 161-976B-3H to -14H were oriented with the Tensor tool. Temperature measurements were attempted during Cores 161-976B-3H and -6H, but good data were not recovered due to tool movement, caused by the sea conditions and the short, rigid drill string.
The failure of one of the flappers in the core catcher resulted in part of Core 161-976B-3H to be extruded onto the drill floor. The recovered cores experienced significant gas-induced expansion, causing parts of the cores to extrude from the liner. To reduce core disturbance and expansion, holes were drilled in the core liners prior to cutting them into sections to allow the gas to escape. In the APC cores, a C1 maximum of 66,555 ppm was measured in a headspace sample from Core 161-976B-12H; however, no trends that might affect safety were observed.
XCB Cores 161-976B-15X to -74X were taken from 127.0 to 677.3 mbsf and recovered
358.34 m (65%). Three WSTP measurements were attempted prior to Cores 161-976B-18X, -20X, and -26X (1284.4, 1303.7, and 1351.7 mbrf, respectively). Gas was present in the
XCB cores, with the maximum C1 being recorded as 14,662 ppm from headspace analyses. No significant trends were seen in either C1 or C1/C2 ratios.
Fine sands(?) encountered from 367 to about 518 mbsf resulted in a very low core recovery. Less than 10 min was required to cut some of the cores in this interval. While coring through the sands, the borehole was completely circulated from the bottom up to the seafloor before recovering the core barrel each time. Despite the loose nature of the formation, no fill or significant hole problems were encountered while coring this interval. Firmer clays were then encountered below 518.3 mbsf (1637 mbrf). Additional sandy intervals were also seen from 621.6 mbsf down to about 670 mbsf, where basement was reached. Overall recovery for both the APC and XCB portion of the hole was 71.8%.
The hole was conditioned for logging with a wiper trip. Overpull of about 30,000 lb was observed in two zones while pulling the pipe up the hole. We had to use the top drive and rotate the drill
pipe to get it to pass through numerous indurated layers throughout the sand interval (357.7 to 518.3 mbsf). After passing down through the sand, we found 36 m of fill in the bottom of the hole. We then reamed to the bottom of the hole and swept it with 30 bbl of high viscosity mud. We positioned the bit at 72 mbsf (1191 mbrf) to log. Since the first tool was unable to exit the bit, we dropped and retrieved an XCB core barrel. This cleared the LFV (lockable float valve) and allowed the tool to pass through the bit on the second attempt. The tool, however, only made it to 350 mbsf before encountering a bridge, and the hole was logged from 350 to 36 mbsf. The quad-combo caliper data indicated that most of the hole had washed out to larger than the caliper limit of 18.5 in. Therefore, we decided to terminate logging at 0930 hr, 26 May.
We then deployed a free-fall funnel (FFF) so that we could continue Hole 976B using RCB-coring. The BHA for the RCB was assembled with (1) a 9-7/8-in. RBI C-4 bit, (2) a mechanical bit release (MBR), (3) a head sub (HS), (4) an outer core barrel (OCB), (5) a top sub, (6) a head sub (HS), (8) eight 8-1/4-in. drill collars (DC), (9) jars, (10) two 8-1/4-in. drill collars (DC), (11) a crossover sub (XO), (12) a 7-1/4-in. DC, (13) a crossover sub (XO), (14) five joints of 5-1/2-in. drill pipe (DP), and (15) a crossover sub (XO).
We reentered the FFF at 2330 hr, 26 May. The drill string was run into the hole, and approximately 37 m of fill was found at the bottom of the hole. The hole was washed and reamed to the bottom. RCB coring began at 0830 hr, 27 May. RCB Cores 161-976B-75R to -106R were taken from 677.2 to 928.7 mbsf (1796.3 to 2047.7 mbrf) and recovered 49.12 m (19.5%). RCB drilling parameters were 15,000-23,000 lb WOB (weight on bit), 50-70 spm, and 40-70 rpm. Pump pressures with the core barrel in ranged from 350 to 700 psi.
Core jamming was noted in the first two cores. Short cores were attempted as a means to increase core recovery, but did not appear to make significant difference. The material being cored appeared to be a mixture of hard rocks and soft, unconsolidated, fine-grained fault gouge (only minimally recovered), resulting in a high rate of penetration (5 m/hr) but poor recovery. We believe that the fault gouge was being washed away and that only the rocks were recovered. In addition, we attempted taking cores both with, and without, a core liner. We thought that coring without a core liner might reduce the chances that a piece of core might jam off and prevent more core from entering the core barrel.
Prior to completing the hole, we made two wiper trips when the bit had reached a penetration of 1869.6 and 1961.1 mbrf (750 and 842 mbsf, respectively). These wiper trips were conducted to relieve an increase in pressure and as a preventive measure. During each wiper trip, the bit was pulled up to 659 mbsf (1778 mbrf). We terminated coring in Hole 976B after a final penetration to 928.7 mbsf.
In preparation for logging, we made a final wiper trip up to 535.9 mbsf (1655 mbrf) to condition the hole. We had some difficulty releasing the bit using the releasing tool to trigger the MBR. At first we thought that, since the MBR would not release, we might have to pull the string to the rig floor and then reenter the hole with a logging bit. Finally, after numerous attempts over a 2 hr period to jar off the overshot, the MBR sleeve shifted and released the bit. The releasing tool was rerun back down the pipe to confirm that the bit was released and to shift the sleeve into the logging position.
We used a conservative two-step approach to log, ensuring that we obtained good log data in the basement sections first. For the first log, we placed the bit just deeper than the top of basement at 698 mbsf. We were uncertain if, once we pulled above basement, we could get back into basement after dropping the bit. We ran the logging tools in the following order: quad-combo, FMS, BHTV, and GLT. The logs were run to 916.7, 914.7, 914.7, and 908.7 mbsf (2035, 2033, 2033, and 2027 mbrf, respectively). The drill pipe weight indicator during the first run suggested that material may have been starting to tighten around the drill pipe. The top drive was used between runs to circulate and make sure the pipe remained free. Logging was allowed to continue, since this weight increase did not appear to be a serious threat to the drill string or the BHA.
The logs indicated a >5° hole angle. This deviation is not surprising, based on the range of structural dips observed in the core and the variability in hardness of the formation. The hole diameter was relatively constant from 781 to 928 mbsf (1900 to 2047 mbrf, respectively). Hole diameters were considerably more variable above 1900 mbrf. The log data, however, were still considered to be very good. The basement logs were finished at 0600 hr, 1 June.
We then pulled the bit up to 536 mbsf (1655 mbrf) to attempt a second series of logs from 536 mbsf to the top of basement. Results from the quad-combo caliper data during the first run revealed that this portion of the hole was seriously washed out. We decided to terminate logging operations and attempt to log this interval in one of the next holes to be drilled at this site. Logging operations were completed at 1100 hr, 1 June. We displaced the hole with 80 bbl of 10.5 lb/gal barite mud before pulling out of the hole. The bottom of the drill pipe cleared the seafloor at 1245 hr, 1 June.
Hole 976C
The ship was offset 50 m east (upstream to the deep Mediterranean outflow) of Hole 976B,
and Hole 976C was spudded at 2000 hr, 1 June. Core 161-976C-1H was taken with the bit at 1116.0 mbrf and recovered 5.98 m; therefore, the seafloor was defined to be at 1108.2 mbsl. APC Cores 161-976C-1H to -14H were taken to 129.5 mbsf and recovered 135.88 m (105%). Cores were oriented beginning with Core 161-976C-3H. ADARA temperature measurements were taken during Cores 161-976C-3H, -6H, -9H, and -12H. The maximum gas detected was 32543 ppm C1.
XCB Cores 161-976C-15X to -40X were taken from 129.5 to 379.7 mbsf (1249.0 to 1499.2 mbrf, respectively) and recovered 204.28 m (81.65%). Several cores with very low recovery occurred in similar depth intervals as at Hole 976B. Some sediment residue was observed on the inside of the liner, suggesting that sediment may have been inside the liners at one time. There were a couple of these low recovery cores (Cores 161-976C-20X and -25X) which had holes through the middle of the material that remained inside the cutting shoe. One possibility is that the sediment in the core barrel may have been forced out by gas expansion(?) as the cores were being withdrawn from the seafloor. Penetration rates of about 8-12 m/hr were achieved with circulation rates of 30-40 spm, 10,000 lb WOB, and 50-60 rpm. Overall APC/XCB recovery for Hole 976C was 89.58%. Prior to pulling the bit out of the hole, 100 bbl of 10.5 ppg heavy weight mud was displaced into the hole. The bit cleared the seafloor at 1145 hr, 3 June.
Hole 976D
Hole 976D was cored primarily to take high-resolution interstitial water samples in the upper 30 mbsf. The ship was offset 25 m northwest of Hole 976C (as we could not move farther to the east because of our distance from the beacon) and spudded Hole 976D at 1230 hr, 3 June. Core 161-976D-1H was taken with the bit at 1111.0 mbrf and recovered 1.54 m of sediment; therefore, the seafloor was defined to be at 1107.6 mbsl. APC Cores 161-976D-1H to -4H were taken from 0 to 30 mbsf (1119 to 1149 mbrf, respectively) and recovered 30.76 m (102.5%). After taking Core 161-976D-4H, the APC/XCB BHA was pulled out of the hole, clearing the seafloor at 1445 hr, and the rig floor at 1645 hr, 3 June.
Hole 976E
Hole 976E was drilled and cored so that additional samples could be taken across the sediment/ basement contact and so this same interval could be logged. Therefore, an MBR was added to the RCB BHA. Hole 976E was spudded at 2130 hr, 3 June. We washed and drilled with a center bit to 543.81 mbsf (1662.81 mbrf), where we began RCB coring. RCB Cores 161-976E-1R to -28R were taken from 543.8 to 736.3 mbsf, coring 192.5 m and recovering 64.85 m (33.69%). We advanced only 5.0 or 4.6 m on 16 of these 28 cores. We did this to enhance our chances of good core recovery because we thought taking short cores might reduce the possibility of core pieces jamming. The other 12 cores advanced the standard 9.5 m.
Recovery in Hole 976E was better than in Hole 976B across, what was initially believed to be, the same transition zone of breccia, fault gouge, and metamorphic basement. The basement, however, was encountered approximately 10 m higher than in Hole 976B. A significant portion of the metamorphic basement exhibited a vertical foliation, which may have resulted in less core jamming and enhanced the core recovery.
We used a combination 8-finger with a 4-petal core catcher for the first four cores in the hard clay. The core catchers were then switched to a 10-finger placed in front of the 4-petal core catcher for the remainder of the sediment coring. Once basement was reached, we switched to two, rotatable 8-finger core catchers.
Drilling parameters while coring with the RCB were 35-45 spm, 15,000-25,000 lb WOB, and 50-70 rpm. Attempts to keep annular velocity low to reduce the hole erosion (to enhance chances for good logging conditions) resulted in slow penetration. The pump strokes were increased slightly to 45 spm on the sixth core, which helped the ROP and decreased the pressure (cleaned the hole of cuttings better).
We had hoped that the smaller diameter RCB hole would result in better hole conditions across the sediment/basement transition so that logs could be used to help fill in the gaps created by the poor recovery in this interval.
A short wiper trip was made to 570 mbsf (1689 mbrf). When we moved the bit back to the
bottom, approximately 20 m of fill was encountered. We reamed the fill out of the hole, released the bit, and shifted the MBR sleeve back into the logging position. The pipe was raised to 570 mbsf (1689 mbrf), and logging operations were begun. The standard quad-combo log was run first. Some difficulty was encountered in getting the tool through the sediment/basement transition. Once logging was started, approximately 9000 lb of tension was experienced while pulling the log through basement section. This tension, combined with the severely washed-out borehole above 631 mbsf, caused us to terminate logging. The quad-combo tool was able to obtain logs from 713 to 570 mbsf.
Erratic signals with the two seafloor dynamic positioning beacons occurred almost daily. Signals from both beacons were temporarily lost around 1500 hr, 7 June. We deployed a backup beacon, but it failed to respond, so we deployed a second backup beacon. In the meantime, the signals from the first two beacons were reacquired.
The bottom of the drill pipe cleared the seafloor at 0545 hr, and the rig floor at 0800 hr, 8 June. Three of the four beacons deployed were recovered at the conclusion of operations at Hole 976E. We began the transit to Site 977 at 0830 hr, 8 June.