168 Prospectus


Although virtually all individual holes planned for Leg 168 are separated by greater distances than normal for single-site specifications (i.e., as defined by the range for single-beacon acoustic navigation), the holes have been grouped into three operational "super sites" according to geographic area and primary objectives: (1) the Hydrothermal Transition site, (2) the Permeable Penetrators site, and (3) the Lithospheric Heat Flow site.

The specific drilling strategy to be adopted during the leg involves a mixture of conventional drilling, coring, and reentry operations arranged in a sequence of single holes at each site (Table 1). It represents a departure from traditional ODP operations, but provides an efficient means by which the primary objectives, including the safe establishment of four cased reentry holes sealed with CORK hydrologic observatory installations, can be met during the 56 days allotted to the leg. There is a considerable amount of excellent work that cannot be fitted into the time frame of the leg, given the estimated operations schedule. This work has been included in Table 2 as a suite of second priority holes. Any time gained (e.g., through more efficient casing and CORK operations at reentry holes) could allow additional drilling, coring, and downhole measurements at one or more of the second priority sites, although the likelihood of this possibility is unclear until the program is well underway.

Leg 168 will begin by coring two holes with the Advanced Hydraulic Piston Corer (APC) and Extended Core Barrel (XCB) at proposed Sites HT-1A and HT-2A. These holes will be cored only to the top of basement; coring will end with the first recovery of basalt (or with any other reliable indication that basement has been reached if there is no recovery). Although it would be scientifically desirable to collect additional basalt cores within upper basement at these sites, such operations could jeopardize the hydrologic experiments planned for nearby proposed Sites HT-3A and HT-4A if the hydrologic seal overlying the most permeable upper volcanic basement is breached. Frequent measurements of in-situ temperature and frequent whole-round sampling for geochemistry and physical properties will accompany all APC/XCB coring, as described in the next section. Drilling will be completed at Sites HT-1A and HT-2A without tripping the drill string back to the rig floor; the ship will be dynamically positioned from site to site. Acoustic beacons will be dropped for all HT holes under Global Positioning System (GPS) navigational control prior to the deployment of the drill string at Site HT-1A. No underway geophysical data will be required at any of the sites.

Upon completion of operations at Site HT-2A, the ship will be dynamically positioned toward Site HT-4A and a jet-in test will be conducted in preparation for reentry and casing operations (preferably as far from Site HT-4A as possible). The drill string will then be returned to the surface, and a reentry cone and 16" conductor casing assembled. This assembly will be lowered to the seafloor at Site HT-4A along with a bottomhole assembly (BHA) for the Rotary Core Barrel (RCB). The casing will be jetted in and released, and RCB coring will continue to basement. Coring will be continued into basement only deep enough to find a suitable casing and cementing point, probably about 10 m below the sediment/basement interface. This will constitute the pilot hole. We will follow this nested approach to eliminate the high risk of hydrologic disturbance from a separate pilot hole. Following completion of the RCB operations, the hole will be enlarged to accept a casing string, which will then be assembled and run into the hole. After the casing is cemented into place, RCB coring will continue into upper basement for an additional ~30 m into the permeable and hydrologically active part of the upper igneous crust. Additional drilling may be required to reach this "hydrologic basement." The pipe will then be tripped back to the surface, and a packer BHA assembled for quantification of the bulk permeability of upper basement. A flowmeter log (Becker et al., 1994) will be run as part of packer operations, should initial testing with the packer alone indicate the presence of significant bulk permeability. The flowmeter log should give an indication of the distribution of permeability in uppermost basement (although more complete quantification of this distribution must await deeper basement drilling). It will also provide a check on the quality of the hydrologic seal around the cemented casing shoe. After packer operations have been completed, the pipe will be returned to the surface and a CORK assembled and deployed. The CORK body will be lowered into the reentry cone and casing, followed by the data logger and the sensor/sampling cable, which will span the cased section and extend into the open hole below. The cable will include 10 thermistors distributed along its length, with a concentration of thermistors at the bottom (for monitoring of open basement), and an osmotic fluid sampler located in the open section of the hole. The CORK instrumentation will provide a continuous record of formation pressure, temperature, and basement-fluid chemistry for a period of up to several years following deployment and permit observations to be made of the transient recovery from disturbances caused by drilling of equilibrium formation conditions and of any natural hydrologic variations.

Operations at Site HT-3A will be postponed until near the end of the leg. This will provide optimum flexibility if contingency plans need to be reviewed. Site HT-3A is considered the lowest priority of those to be instrumented with CORKs.

Following operations at Site HT-4A, the ship will transit to the Permeable Penetrator Site PP-1A and operations will continue with a single APC/XCB hole to the top of basement. Logging will be conducted after the completion of coring operations. Another APC/XCB hole will be drilled at Site PP-3A, and a jet-in test conducted for Site PP-4A. A complete reentry system, casing, and CORK will then be deployed at Site PP-4A, with RCB coring completed through the reentry cone, as at Site HT-4A. This hole may also be logged, depending on results of logging at Site PP-1A and on time constraints. A single-hole coring, casing, and CORK operating sequence will then be completed at Sites PP-5A and HT-3A, again following the sequence of operations at Site HT-4A, including pre-CORK packer tests. Total penetration into basement will also be ~40 m (30 m below the casing shoe) as an initial target.

If, at the end of packer operations at Site PP-5A, it is anticipated that insufficient time is available for drilling, reentry system installation, packer work, and CORK deployment at Site HT-3A, the fourth CORK installment of Leg 168 will be abandoned. The time will be used for additional basement penetration and/or work at the second priority sites listed in Table 2.

Specifically, additional work could include (1) continued drilling deeper into basement and downhole measurements at Site PP-5A (prior to CORK installation), (2) a third hole ( Site PP-10A) drilled into the basement ridge, located along strike from Sites PP-3A and PP-5A, (3) a hole (Site PP-2A) into sediments between the buried ridge ( Site PP-3A) and trough (Site PP-1A) , (4) one or more holes drilled at Site PP-6A (RCB) and/or Site PP-7A-PP-9A (APC/XCB) at the basement outcrop, (5) APC/XCB coring at Site LH-2A, or (6) APC/XCB coring at Site HT-3A. The time required for each of these options is listed in Table 2. The estimate given for the deepening of Site PP-5A is for 100 m (one bit) of advance. In the event the fourth CORK hole cannot be drilled, the decision about what additional work will be completed will be based on the results of the leg at that point, and will be made by the Co-Chief Scientists, in consultation with the Operations Superintendent, Staff Scientist, and the Leg 168 scientific party.

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