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Preliminary scientific assessment

Because the primary scientific target of the Site C0002 riser drilling is the plate boundary fault zone at depth, the most important objectives of Expedition 348 were a hybrid of (1) technical milestones in the form of achieving a depth and casing target for future deepening and (2) scientific goals of investigating the structure, lithology, physical properties, fluid content, and state of stress in the middle of the inner wedge.

Technical objectives

The highest priority requirement for this expedition was to deepen the hole and set the stage so that the main target reflector at ~4600–5200 mbsf could be reached in future riser drilling. Toward this end, we had a goal to extend the cased borehole from 860 to 3600 mbsf, bypassing the uncased zone from Expedition 338 drilling of Hole C0002F (2006 mbsf total depth) through a sidetrack below the 20 inch casing shoe (Fig. F5), and install 13⅜ inch casing and 11¾ inch liner to reach that intermediate total depth with a stabilized borehole. Despite multiple operational difficulties and, at times, unstable borehole conditions, the 13⅜ inch casing was successfully cemented into place at 2008.9 mbsf, slightly shallower than the planned 2300 mbsf target. A stuck and severed 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 extended to 3058.5 mbsf and cased with 11¾ inch liner at 2922.5 mbsf, 677.5 m shallower than the planned target. Cementing of this liner was a challenge and required a cement squeeze job, but indications are that the liner was successfully cemented into place.

While it was a disappointment that the full depth objective was not met, it is a substantial achievement that the main borehole was extended and cased more than 2060 m beyond its previous depth. Holes C0002F, C0002N, and C0002P now constitute the deepest hole ever drilled (and cased) in scientific ocean drilling. With the 11¾ inch shoe shallower than 3000 mbsf depth, reanalysis of the casing plan to reach total depth will be required, but we believe the 5200 mbsf target can still be met in this hole. For these well engineering aspects, the expedition can therefore be deemed a strong partial success.

Scientific objectives

Our goals included characterizing the materials, architecture, and ambient conditions (stress, pore pressure, and chemistry) of the interior of the inner wedge with a suite of LWD logs including images, cuttings analysis, and ~100 m of core samples. In reality, a comprehensive LWD suite was obtained, and high-quality logs were recorded that have yielded a wealth of information on bedding and fracture orientation, formation seismic velocity, resistivity, natural gamma radiation, and borehole shape via the novel continuous ultrasonic caliper. These data sets have been (or are being) used to address all planned goals. Cuttings analysis was carried out in greater detail, with refined techniques relative to all previous scientific ocean drilling efforts and has yielded lithologic, structural, and physical properties information at ~5–10 m resolution. This effort succeeded beyond initial expectations; in particular, the analysis of physical properties such as porosity and P-wave velocity on cuttings yielded better than expected results owing to refined techniques used with care by a skilled team. The detailed analysis of structural fabrics in cuttings by optical and scanning electron microscopy methods was also a somewhat unexpected bright spot. Overall, experience has greatly improved the handling of cuttings since Expeditions 319 and 338. Collection of annular pressure-while-drilling data LOTs at the upper casing shoe and just below the 13⅜ inch casing sidetrack and analysis of drilling mud data from riser operations provided additional valuable constraints on in situ stress magnitudes and rock properties and, with additional analyses, may potentially constrain in situ pore fluid pressure.

Because of time lost to operational delays, coring was limited to ~60 m and six cores. More cores would have been preferable to provide material needed for high-priority postexpedition studies of rock properties to best address both scientific and well engineering objectives, but the cores still afforded a wealth of information on the geology of the inner wedge. It was hoped that the cores would yield usable pore water for geochemical analysis; however, the sediment proved to be too strong and low in porosity to yield water. The GRIND method produced mixed results (see “Geochemistry”) and overall was clearly inferior to direct pore water analysis. This was somewhat disappointing and suggests that the utility and methods of pore water analysis will need to be reevaluated for future deep drilling.

In summary, during Expedition 348, we met our goals of characterizing the deep interior of the inner accretionary wedge in full over the (somewhat reduced) interval that was drilled and sampled. The operational goal of advancing the borehole and leaving it in a stable cased state ready for further deepening also was completed, albeit ~677 m shallower than initially planned.