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Site C0002 summary

Scientific objectives

The primary drilling plan for Expedition 348 was to extend the riser hole at Site C0002 from 860 mbsf to a target depth between 3600 and 4400 mbsf. Because of the unknown condition of the open hole below 860 mbsf left after Expedition 338, a plan was made to sidetrack below the 20 inch casing shoe at 860.3 mbsf and redrill the interval parallel to, but 10–20 m laterally away from, Hole C0002F and then set 13⅜ inch casing and 11¾ inch liner successively.

From previous drilling (Fig. F3), it was known that the Kumano forearc basin sedimentary package comprises the interval from 0–940 mbsf and is underlain by deformed sediment of the inner accretionary wedge (e.g., Ashi et al., 2009). The entire zone from ~940 mbsf to the plate boundary reflector at ~5200 mbsf exhibits few coherent seismic reflections that would indicate intact stratal packages, in contrast to the outer accretionary wedge seaward of the Kumano Basin region (Figs. F2, F5; see also Moore et al., 2009). This seismic character was thought to indicate complex deformation within the inner wedge, perhaps best characterized as a subduction mélange or protomélange.

The main research objectives for this interval were to drill and sample the interior of the accretionary complex in the midslope region beneath the Kumano forearc basin with collection of cores, drill cuttings, and mud gas; collect an extensive suite of LWD logs to characterize the formation; and utilize leak-off test (LOT) and downhole pressure and mud-weight data to constrain in situ stress state. Sampling and recording downhole data from this previously unsampled interval was driven by the following specific questions:

  1. What is the thermal, diagenetic, and metamorphic history of the sedimentary rock below the Kumano Basin?
  2. What is the dehydration budget for hydrous minerals (e.g., smectite group clays) and the extent of dehydration reaction progress as a function of depth?
  3. What is the mechanical and structural evolution of the inner wedge?
  4. Are there indicators of zones of low effective stress and/or high pore fluid pressure?
  5. How do the properties of the inner wedge sediments compare with the Shikoku Basin and trench sediments that are input to the wedge?
  6. What are the horizontal stress orientations and magnitude within the deep interior of the inner wedge? How does the stress orientation relate to the current state of the earthquake cycle?
  7. What is the mechanical state and behavior of the formation and how does it relate to the current state of the upper plate above the seismogenic plate boundary thrust?
  8. What are the dominant faulting processes and deformation mechanisms, and how do they vary with depth?

Planned operational strategy

The interval from 860.3 mbsf to the target depth of 3600 mbsf (or as deep as 4400 mbsf, depending on drilling conditions and available time) was to be drilled in riser mode with collection of continuous LWD resistivity, sonic, gamma ray, and annular fluid pressure logs. During riser drilling, mud return would allow for a comprehensive analysis of drill cuttings and mud gas, as was performed at Site C0009 and Hole C0002F and described by Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists (2010) and Moore et al. (2013), respectively. Coring (100 m total) was also planned to sample the inner wedge but was restricted to one interval from 2300 to 2400 mbsf, just below the intended 13⅜ inch casing shoe. After installing the 13⅜ inch casing string to 2400 mbsf, planned operations were to conduct a LOT and then drill to 3600 mbsf or more and install 11¾ inch liner. At the end of Expedition 348, the borehole was to be suspended for reentry and further deepening to the planned plate boundary target during later riser drilling operations.

Additionally, as a test of the in-development small-diameter rotary core barrel (SD-RCB) system, a separate riserless hole (C0002M) was to be spudded and drilled to 475 mbsf and cored to 512.5 mbsf (see “Operations summary”; Tables T1, T2). These cores were planned as an engineering test and to compare the core quality to standard RCB cores previously cut from the same interval in Hole C0002B (Ashi et al., 2009); they were to be described and subjected to standard shipboard analysis by the Expedition 348 Scientific Party.

Operational results

During Expedition 348, this plan was carried out with overall success, overcoming numerous borehole stability challenges. Hole C0002N was successfully sidetracked from Hole C0002F at 860.3 mbsf and drilled and cased with 13⅜ inch casing to 2009 mbsf, short of the original 2300 mbsf objective. A stuck and severed bottom-hole assembly (BHA) in the 13⅜ inch casing shoe (see “Operations summary”) necessitated a second sidetrack operation, this time through casing, which was successful. The borehole was then extended to 3058.5 mbsf, including a coring interval of 55.5 m (reduced from 100 m because of time constraints) and cased with 11¾ inch liner to a depth of 2922.5 mbsf, 677.5 m shallower than the originally planned target. A full suite of LWD logs were collected, as well as cuttings at ~5 m intervals and cores from a 55.5 m interval with overall recovery of 56.7%.