165 Preliminary Report

SITE 165

The ODP Operations and Engineering personnel aboard JOIDES Resolution for Leg 165 were:
Operations Superintendent: Michael Storms
Schlumberger Engineer: Raymond Faust

Site 998

The 585 nmi transit from Miami was uneventful with calm seas and favorable currents. The ship proceeded at an average speed of 11.0 kt, dropping to a slightly lower speed of 8.25 kt while heading into the Gulf Stream current.

At Hole 998A, core recovery with the Advanced Piston Coring (APC) system was excellent (100%-107%). The Extended Core Barrel (XCB) coring system achieved good recovery (typically 60%-100%) and rate of penetration until the cutting shoe began to jam. Core quality also deteriorated with depth, though hole conditions remained excellent throughout the drilling.

Drilling at Hole 998B was initiated 20 m to the northeast of Hole 998A. The hole was drilled down to 558.5 mbsf with the Rotary Core Barrel (RCB) and center-bit combination before attempting core recovery. This depth was selected to allow about 80 m overlap of the lower, poorly recovered section of the formation from Hole 998A. Core recovery with the RCB was exceptionally good, averaging 83.1% for the hole.

Four suites of logging tools were deployed in the hole: The first two runs, with the Quad Combination tool and the Geochemical Logging tool, reached the bottom of the hole without incident and logged from 904 to 172 mbsf. Owing to an obstruction in the hole, the third and fourth runs, with the Formation MicroScanner and Geological High-Sensitivity Magnetic tool, only reached a total depth of 430 mbsf and logged up to 172 mbsf.

Site 999

The 468-nmi transit from Site 998 was slower than anticipated owing to the failure of a propulsion motor; this required that two motors be taken offline, which reduced average speed to about 10.7 kt.

Hole 999A was cored with the APC (average recovery 104.6%) and the XCB (89.0%) systems. The latter suffered from core biscuiting (a phenomenon in which the core is broken into many short cylindrical chunks that are surrounded by fine-grained drill cuttings and paste). Coring operations were terminated on this hole when the formation became indurated enough to provide an acceptable casing seat, which was then used in drill plans for Hole 999B, the reentry hole. Logging activities in Hole 999A were abandoned after the initial logging tool encountered an obstruction and then experienced repeated electrical failures.

The reentry cone and casing were installed in Hole 999B without incident. A CC-4 tungsten carbide roller cone insert bit was selected for RCB coring because the formation was expected to get progressively harder, eventually grading into silicified limestone and chert. The bit repeatedly clogged with aluminum debris from the cementing dart used in the casing installation, resulting in no core recovery for the first four cores. The debris was removed after tripping the drill string back to the surface, which resulted in a delay of ~14 hr. The rate of penetration (ROP) rapidly deteriorated with depth; the ROP for the last two cores was 3.2 m/hr and 2.4 m/hr, respectively, compared with 4.7 m/hr for the previous 100-m interval. At these rates we would not have been able to reach the depth objectives, and so a decision was made to switch to a PDC anti-whirl core bit. The bit change required another trip to the surface with the drill string, another reentry into the hole, and another ~15 hr delay.

The new PDC bit proved to be worth the extra time, as there was a slight improvement in core quality and a significant increase in the ROP, which averaged 8.9 m/hr over the first 100-m interval, or 2-4 times that achieved by the CC-4 bit. Even when the formation became a well-indurated limestone, the ROP only dropped to 6.9 m/hr for the PDC bit.

The ROP only began to decrease significantly at a depth of about 1046 mbsf, during coring of 165- 999B-59R, which was also marked by an increase in drill pump pressure from 500 psi to about 750 psi. Core 165-999B-59R was thought to be the core that would contain the K/T boundary interval. Our primary concern was that the increased pump pressures would wash away part or all of the critical interval, and thus a joint decision was made to retrieve the core after advancing only 4.6 m. The ROP dropped even farther, to about 1.5 m/hr, during coring of the next three cores (165-999B-60R through 62R), which indicated bit degradation. Perhaps due to luck, or to the extra care taken while drilling through the critical interval, the overall recovery (92.9%) for the last four cores was over 10% higher than achieved for the other part of the hole cored with the PDC bit.

Having determined that we were about 15 m into the Upper Cretaceous section and realizing that further coring at the very slow ROP would gain us little given the time constraints, we ended coring operations at a depth of 1066.4 mbsf. The hole was then prepared for logging with sepiolite mud sweeps without using a wiper trip. Hole conditions were excellent for logging, which resulted in smooth logging operations and high quality Quad Combo, Geochemical Logging Tool, and Formation Microscanner logs.

Site 1000

Before heading to Site 1000 from Site 999 we docked in Kingston, Jamaica, to take aboard a Honduran observer and a scientist from the JOIDES office in Cardiff, to load propulsion motors and freight, and to disembark the Colombian observer/scientist and a scientist (who left for medical reasons). The 550-nmi transit from Kingston to Site 1000 was covered at an average speed of 9.6 kt.

Hole 1000A was cored with the APC and XCB systems. Owing to the thick section of unconsolidated and poorly indurated sediments, we were able to recover 34 APC cores, which gave a 103.5% recovery for the interval from 0.0 to 312.9 mbsf. Two cores (165-1000A-10H and 13H) were highly disturbed upon recovery, possibly because the flapper on the core catcher may not have fully opened, thus forcing the core material to flow past the flapper. These intervals were later successfully recovered with the RCB system in Hole 1000B. The XCB system performed with high rates of penetration (ROP) and high recovery (86.5 m/hr with 100% recovery to 400 mbsf, 72.2 m/hr with 100.3% recovery to 500 mbsf, and 49.4 m/hr with 98.6% recovery to 525 mbsf) until a depth of 524 mbsf (Core 56X). Owing to poor recovery and a slow ROP on the next three cores, drilling was halted in Hole 1000A.

Hole 1000B was spudded about 30 m to the southwest of Hole 1000A with the RCB system. The hole was drilled ahead to 79.3 mbsf and then Core 165-1R was recovered in roughly the same interval as the disturbed Core 165-1000A-10H. It was decided to drill ahead to 117.3 mbsf and take Core 165-1000B-2R in roughly the same interval as the disturbed Core 165-1000A-13H. The hole was then drilled ahead to 503.5 mbsf where continuous RCB coring was initiated. Overall core recovery for Hole 1000B averaged 67.6%. Coring ceased at a depth of 695.9 mbsf due to time constraints, and the hole was prepared for logging operations. Quad Combo, Geological High-Sensitivity Magnetic, and Formation MicroScanner logs were acquired in Hole 1000B.

Site 1001

Even with all propulsion motors back on line, the brisk trade winds slowed the vessel to an average speed of 10.1 kt on the 291-nmi transit from Site 1000. We arrived on site early Friday morning, 2 February 1996, to begin drilling operations.

Hole 1001A was spudded and cored with the RCB system, because the primary objectives was to acquire lithified rocks below a suspected abbreviated Neogene section and because there was insufficient time to piston core the unlithified sediments and then spud another hole with the RCB system. We used the subsea video camera as an aid to observe the bit contact with the seafloor and to obtain a mudline core. This accurately defined the water/seafloor interface, though only a small piece of ooze was recovered in Core 165-1001A-1R. Core recovery varied between 0.5% and 22% for the first six cores. Recovery for Cores 11R through 18R ranged from 80% to 102%, but dropped off rapidly after a major unconformity was encountered at 165.68 mbsf (at Section 165-1001A-18R-4, 88 cm), where middle Miocene ooze overlies middle and lower Eocene chalk and limestones with numerous, thin chert layers. Recovery through the hard limestones and chert ranged from 0% to 57%, but improved significantly below 303 mbsf where the chert layers were absent to very sparse. Some of the highest recovery intervals were at the K/T boundary (Core 165-1001A-38R) and at the basement contact (Core 165-1001A-52R), the latter of which was of exceptionally good quality.

Coring ceased in Hole 1001A after cutting four cores below the basement contact. The hole was circulated clean and a wiper trip was made to 112 mbsf in preparation for logging operations. Quad Combo, Formation MicroScanner, Geological High-Sensitivity Magnetic logs were acquired in Hole 1001A.

Hole 1001B is located approximately 15 nmi to the south of Hole 1001A. Two spot RCB cores were collected, at 25.3 and 150.3 mbsf, as the hole was being drilled down to 206.5 mbsf, where continuous coring operations began. In general, core recovery matched that from Hole 1001A for similar depth intervals. The interval cored, however, was offset by 2 m in depth from that of Hole 1001A in hope of recovering the intervals missed in Hole 1001A, and to position the K/T boundary interval within the core liner, rather than in the core catcher. This strategy was successful as we not only collected the K/T boundary where planned, but also managed to recover another Paleocene/Eocene boundary section and another basement/sediment contact. Coring ceased at a depth of 488.3 mbsf (2 m into basement) due to time constraints.

Site 1002

The 635-nmi transit from Site 1001 was made at an average speed of 10.5 kt. While the ship was on route to the site, two Venezuelan observers arrived by helicopter, and the Honduran observer and the JOIDES scientist departed. A short site survey was conducted with the 3.5-Hz echo-sounder on approach to the site to verify that the site was properly located on the proposed local topographic high. Five holes were drilled, with each hole being offset by about 15 m east of the previous hole.

Hole 1002A was spudded with the APC system at 0.7 mbsf. The core liner was full (102.7% recovery), but did not contain a mudline, which necessitated spudding Hole 1002B, because the shipboard geochemists needed a mudline core for their proposed interstitial water studies. Hole 1002B recovered 6.2 m of sediment, including the mudline.

The APC system performed well, with 109.1% average recovery for the hole. Cores 12H-18H suffered from flow-in disturbance in the lower 2 to 3 m of each core. The liner for Core 165-1002C-18H was severely damaged and the core had to be pumped out of the core barrel, though 9.92 m of sediment was still recovered. Core 165-1002C-19H also returned to the surface shattered, but this time only 0.15 m of sediment and hard dolomite was recovered.

The APC/XCB coring systems were used for Holes 1002D and E. Hole 1002D was spudded with the drill string 3 m higher than that used at Hole 1002C, and Hole 1002E was offset another 3 m, to offset core breaks. The XCB system was used after Core 12H for Hole 1002D and after Core 13H for 1002E to avoid the flow-in problem and to penetrate the dolomite lenses more easily. Recovery was very good for both holes, averaging over 105% for the APC portions and over 70% for the XCB portions. Coring operations for both holes were halted just above the hard dolomite encountered in Core 165-1002C-19H to avoid penetrating any potentially dangerous gas pockets.

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