IODP Proceedings    Volume contents     Search

doi:10.2204/iodp.proc.331.101.2011

Operations

To achieve the scientific objectives of Expedition 331, drilling was planned at three main sites (proposed Sites INH-1D, INH-4D, and INH5-D, which were drilled as Sites C0016, C0013, and C0014, respectively). No less than nine additional sites were approved and considered as potential contingency sites. Of these, Proposed Sites INH-1C, INH-6B, and INH-11A were drilled as Sites C0016, C0017, and C0015, respectively.

The drilling plan was designed to penetrate the hydrothermal flow path, including the main discharge at NBC (Site C0016), a high heat flow area (Site C0013), and a low heat flow area (Site C0014). Drilling was prioritized according to potential scientific return, as well as operational feasibility. Main holes at Sites C0013 and C0014 were to be drilled through guide bases, cased with perforated casing, and supplied with corrosion caps. We carried a third guide base for a possible contingency, which was used in Hole C0016B. A complete list of sites and holes can be found in the general coring summary in Table T2.

Unusual aspects of Expedition 331 include the high heat flow encountered especially at Sites C0013–C0016. It also includes very short distances between sites, allowing the ship to reach even the most remote Site C0017 within hours of operating at the other sites. High levels of H2S gas from the cores required most of the core recovery operation to be undertaken wearing breathing apparatus at the rig floor and core cutting areas. Finally, coring systems with two different diameters were used: IODP standard systems as carried by the Chikyu (HPCS, EPCS, and ESCS) with core diameters of 2.44 inches and a commercial system taking 4 inch diameter cores.

Initial drilling at Site C0013 quickly lead to the realization that the heat flow indeed was high there, causing the plastic liners to melt, which added issues for core extraction as well as concerns about contamination. Site C0013 was approved to a total depth of 100 m, but the main Hole C0013E achieved a total depth of only 54.5 mbsf. The Chikyu eventually returned there to core with aluminum core liner and thereby retrieve cores not disturbed by thermal alteration of the liner material. Drilling at Site C0014 encountered the same issue, only slightly deeper.

Site C0014 is another discharge-recharge zone similar to Site C0013. The target depth was 200 mbsf, but drilling was terminated at 136.7 mbsf. The main objectives were reached by then, and the borehole assembly seals were starting to show damage from the heat, which was estimated to be up to 300°C, based on the composition of precipitates in the recovered rocks. Hole C0014G was used for the casing operation there.

The heat issue led to drilling at the first contingency Site C0015 to use up about half a day until it was time for crew change and the opportunity to load aluminum core liners for the IODP standard coring systems. This site is a potential methane and fluid source west of the vent axis. (All other sites are located east of the axis.)

Site C0016 targeted the active hydrothermal vent system and was drilled using the commercial BHI 4 inch coring system. Hole C0016A targeted the crest of the NBC mound but recovered no core because the core barrel severed, leaving the drill bit and full-closure core catcher of the commercial drilling assembly in the hole. The ship left the third guide base at the location for Hole C0016B before relocating to Site C0017. Hole C0016B was drilled as the last hole of the expedition, using the third guide base at the base of NBC mound. A depth of 45 mbsf was reached, but only 2 m of core was recovered. The guide base was capped with a 3 m section of casing added to function as an entry guide for downhole tools.

Site C0017 is a site of suspected cool seawater recharge to the hydrothermal system. Holes C0017A and C0017B were abandoned because the bit pulled out while retrieving the core barrel. Hole C0017C was abandoned when the core barrel came back without shoe or core catcher. The total depth of 150.7 mbsf was reached at Hole C0017D after a successful unguided reentry. This site provided a good temperature record using a combination of APCT3 and thermoseal strips, in addition to good core recovery. A happy surprise was very good recovery with the ESCS assembly of firm, sticky mud. When operations at Site C0017 were completed, the Chikyu returned to the guide base NBC mound to drill Hole C0016B.