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doi:10.2204/iodp.proc.327.103.2011

Background and objectives

Site U1362 (prospectus Site SR-2) is located at the eastern end of the Ocean Drilling Program (ODP) Leg 168 drilling and observatory transect and in the same area where work was completed during Expedition 301 (Davis, Fisher, Firth, et al., 1997; Fisher, Urabe, Klaus, and the Expedition 301 Scientists, 2005). Site U1362 is positioned above a basement high covered by ~240 m of turbidites and hemipelagic mud, south-southwest of Hole 1026B and north-northeast of Holes U1301A and U1301B (Fig. F1). Hole U1362A (prospectus Hole SR-2A), the deepest of the two new holes, was intended to be drilled and cased through the sedimentary section and the uppermost 100 m of basement, with coring and sampling planned for ~100–260 m into basement and final hole depth to be determined by hole conditions and available time. The operations plan included wireline logging with a single string (to assess lithologic layering and properties and identify suitable locations for setting packers), testing for permeability using the drill string packer, and instrumenting the borehole with a CORK.

Hole U1362B (prospectus Hole SR-2B) was designed to be drilled and cased through sediment and the uppermost ~30 m of basement, followed by ~50 m of basement drilling with no planned coring or logging. A 24 h pumping and tracer injection experiment was to be completed before the borehole was instrumented with a CORK to monitor a single basement interval. Both of the CORKs to be deployed at Site U1362 were designed to include instruments to monitor formation fluid pressure and temperature, sample fluids (using downhole and wellhead OsmoSamplers), and provide growth substrate for microbes inhabiting the basement aquifer.

The original plan was to have these two basement holes separated by only 40–50 m, 200 m south of Hole 1026B and 800 m north of Holes U1301A and U1301B. Shortly before the start of Expedition 327, the position of Hole U1362B was moved ~300 m south of Hole U1362A (Fig. F1B) because of conditions in Hole U1301A and consideration of cross-hole experimental goals.

Like the Site U1362 CORKs, the two CORKs installed at Site U1301 have 16 inch casing through the sediment section and 10¾ inch casing across upper basement. The annulus between these two casing strings in Holes U1301A and U1301B was supposed to contain a rubber casing seal near the seafloor to help isolate the underlying formation from the overlying ocean, but the seals were not available for use as intended during Expedition 301. As a result, both holes remained unsealed and functioned as hydrothermal siphons following Expedition 301, with cold bottom water flowing rapidly into upper basement (Fisher et al., 2008). Hole U1301A “turned around” spontaneously in fall 2007, after which it discharged warm shimmering hydrothermal basement fluids at the seafloor (Wheat et al., 2010).

Multiple unsuccessful cementing attempts were made to seal the 10¾ inch casing strings against basement rocks at depth (with the R/V JOIDES Resolution during Expedition 301) and to seal Hole U1301B between the 16 and 10¾ inch casing strings at the seafloor (with the JOIDES Resolution during Expedition 301 and with the DSRV Alvin during expeditions in summer 2006 and 2007). An additional attempt to cement the cones using the JOIDES Resolution was scheduled at the start of IODP Expedition 312 in fall 2005, but these operations were canceled because of poor weather. Cementing of both cones was finally accomplished with the JOIDES Resolution in summer 2009 during IODP Expedition 321T. However, examination of the reentry cones and wellheads 3 weeks after that operation and collection of borehole pressure data by submersible and remotely operated vehicle (ROV) in summer 2009 and 2010 (just prior to the start of Expedition 327) indicated that only Hole U1301B was sealed. Hole U1301A continued to discharge warm basement fluids as of the start of Expedition 327.

We originally intended to make a final cementing attempt in Hole U1301A during Expedition 327, but just before the expedition we determined that cementing during Expedition 321T may have fouled two of the pressure gauge monitoring lines installed in Hole U1301B (although the shallowest gauge continues to work normally and indicates conditions returning toward an undisturbed state). In addition, we realized that leaving Hole U1301A open and flowing could be advantageous for the cross-hole tracer experiment planned for Expedition 327 because it might induce some of the tracers pumped into Hole U1362B to flow to the south (toward Hole U1301A), a direction opposite to the inferred regional flow direction in basement. Comparison of tracer transport north and south of Hole U1362B could then help researchers assess the net rate of transport in basement through comparison with the flow rate up Hole U1301A. In addition, Hole U1362B will be used for a 1–2 y cross-hole experiment, beginning when a large ball valve in the wellhead is opened during an ROV servicing expedition in summer 2011. Having Hole U1362A offset 300 m from Hole U1362B and completed in uppermost basement will provide an additional monitoring point for this experiment.

Reflection seismic data were collected across locations for Holes U1362A and U1362B as part of the 2000 ImageFlux expedition (Zühlsdorff et al., 2005; Hutnak et al., 2006) (Fig. F2). These seismic lines show features common to others collected in this area: layered sediments, the sediment/basalt interface, and a series of high-angle normal faults that help to define regional abyssal hill topography running subparallel to the active spreading center 100 km to the west. Holes U1362A and U1362B were placed near the peak of the buried basement ridge that underlies this site, just to the east of a high-angle normal fault in the volcanic crust.

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