DRILLING STRATEGYThe complete Leg 205 sequence of operations and time estimate is shown in Table 1 and Table 2.
sampling line, running from the instrument hanger head all the way down to the sealed-off zone, will be installed for future pressure/fluid sampling purposes.
Sites 1040R-A, 1040R-B, and 1040R-C
Each of these sites will be completed with a modified CORK identical to that described for Site 1039R-A above. Each installation will be designed to monitor a single zone. The first two, 1040R-B and 1040R-C, will both monitor the décollement (Figure 8B). The ~500 m separation between these two sites minimizes the chances that seal failure or other difficulties at one site will affect the other installation. They are close enough, however, that any seismically-induced pressure transients may be expected to propagate from the down-dip to the up dip location. At each of these sites, we will install a reentry cone with 16-1/2-in casing and then drill to total depth. Although the uppermost section will be drilled without coring, the rest of the section will continuously cored with the RCB (300-365 mbsf at Site 1040R-B; 220-279 mbsf at Site 1040R-C) to determine the exact location of the décollement as well as the best location to set the 10-3/4-in casing shoe and the 4-1/2-in casing packer. While drilling/coring at Site 1040R-B only, a number of Davis-Villinger temperature probe (DVTP) temperature measurements will be made, as formation conditions permit, to complement existing data. Following the coring, the next step will be to reenter the hole with a larger bit and open the hole up to 14-3/4-in in preparation to install the 10-3/4-in casing. Finally, the 10-3/4-in casing will be cemented in place (with the casing shoe just above the décollement) and the osmo sampler will be emplaced with the packer seated below the 10-3/4-in casing shoe in the uppermost décollement sequence.
Site 1040R-A, the last site planned for Leg 205, will monitor the uppermost underthrust sediments (Figure 8C). After installing the reentry cone and 16-1/2-in casing, the hole will be opened to 14 3/4-in and the 10-3/4-in casing installed with the casing shoe below the decollement. The osmo sampler will be emplaced with the packer seated below the 10-3/4-in casing shoe in the uppermost underthrust sequence. Currently there is no time available to core any of the section penetrated at Site 1040R-A. If time permits, limited coring would include the decollement and the upper underthrust sequence (~320-410 mbsf).
No logging is planned for any of these sites (Sites 1040R-B, 1040R-C, or 1040R-A) based on the
experience with LWD and drilling at Site 1040 during Leg 170.
Alternate sites for each of the primary sites are presented in Table 2 and in the "Site Summaries" section.
Logging Plan | Table of Contents