The objective at this site is to re-enter the cone at Hole 801C (Leg 129) and penetrate through the upper, oxidative alteration zone in basaltic basement, deepening Hole 801C at least another 250 m, to >850 mbsf or ~400 m sub-basement. A drill-string length of 6530 m will be deployed, which would be one of the longest drill strings that the JOIDES Resolution will have deployed, and drilling will be in basaltic rocks. Expected rates of penetration with the rotary core barrel (RCB) may be as low as 1.5 m/hr (as experienced during Leg 129) and trip times will be long. It should be possible with 20 days on site to achieve >250 m of penetration into basement, and conduct three logging runs. We will actively monitor the chemical composition of the cores on board the ship to determine the changes in the alteration zones.
The most likely drilling problems to be encountered will be due to drilling materials (junk) left in the hole. In such an event some time will be spent fishing the junk, to continue the hole to >250 m depth into basement. A decision will be made on site as to the time to be dedicated to fishing and continued drilling, given the requirement for at least 15-20 days operations at Site BON-8A. If a new hole is needed for technological reasons, our first recourse will be to move a few 100 ft and wash to basement and start again. In the case where drilling problems are geological (an unfavorable formation) an alternate site has been designated (PIG-3B), ~75 km east of Site 801 (Fig. 2). Given the amount of time involved in washing down to the last depth of penetration at Hole 801C (~600 m) or starting a new hole at PIG-3B, it is likely that if such problems are encountered after the first week of operation at Hole 801C, further efforts will be shifted to Site BON-8A.
The objectives for Site BON-8A are to continuously core the sedimentary section (470 m) and the upper pillow alteration zone in the basement section (as deeply as possible). Drilling conditions and hole stability in the sedimentary section are predicted to be good at this site, but recovery will be moderate to poor in the anticipated chert-rich sediments. In order to optimize recovery, a combination of the advanced hydraulic piston/extended core barrel/motor-driven core barrel (APC/XCB/MDCB) corers will be used, ideally, to core the entire sedimentary section. Recovery within chert sequences, especially of soft, chalky sediments, was still a problem on Leg 129 because of the need to pump heavily during chert penetration to keep the hole clean. The dramatically increased recovery of hydrothermal sediments in the Trans-Atlantic Geotraverse (TAG) area on Leg 158 with the newly designed MDCB raises the encouraging possibility of similar enhanced recovery in chert/chalk sequences. Four logging runs will follow APC/XCB/MDCB drilling in the pilot hole.
As stated above, if the drilling operation in Hole 801C fails, and there is insufficient time to achieve our stated objectives at this site, operations will be moved to Site BON-8A. Complete operations at this site involve coring of the sedimentary section, logging the sediments, installation of a reentry cone and casing and cementing into basement (470 m), and drilling with a combination of RCB, MDCB, and diamond coring system (DCS) to at least 200 m below the sediment/basement interface. The basement will be logged following drilling operations. The entire operation involving the establishment, coring, and logging of a legacy hole will take 26-28 days.
Depending on the time spent in Hole 801C, a series of scenarios are envisaged for coring the sediments and penetrating at least 50 m into basement, and logging the section at Site BON-8A. We stress the fact that it is essential that the sedimentary section at this site be logged to define the proportions of representative lithologies and compute the mass flux into the subduction zone. This operation could involve a minimum program, involving drilling a single hole using a single RCB bit, through the sediments and basement, dropping the bit, and logging through the hole through the drill-string (10-12) days. A more complete drilling operation (using APC/XCB/RCB/MDCB combinations) at this site would involve reentry of the hole by use of a Free-Fall Funnel (FFF). This would allow deeper coring into basement and complete logging of the section (15 to 22 days), depending on depth of penetration into basement and the number of logging operations. The logging operations will involve, either (1) a logging run after the sediment section is cored and a further run after basement is cored, thus creating maximum hole quality for logging the sediments, or (2) a single logging operation at the end of coring the sediments and basement. If hole quality is not good enough for logging, the possibility exists for drilling a separate hole using a tricone bit, followed by logging.
In the case of loss of the hole at Site BON-8A (again this would most likely be because of drilling problems involving junk in the hole in the basement section), we propose to drill a hole adjacent to the lost hole. For this hole, we would wash down through the sedimentary section, install a second reentry cone, and drill as deep as possible within the time constraints; a minimum drilling operation as described above would be essential to the success of the project. In the case where the drilling problems are geological (an unfavorable formation) an alternate site has been designated (proposed Site BON-9), ~10 km east of BON-8A (Figs. 6, 7).
To 185 Logging Plan
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