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doi:10.2204/iodp.pr.336.2012

Operations plan

The initial operations plan for Expedition 336 anticipated installation of subseafloor borehole observatories (CORKs) at three sites (DSDP Hole 395A and prospectus Sites NP-1 and NP-2), with basement recovery at Sites NP-1 and NP-2 and sediment coring at all three. The initial plan (detailed in the Expedition 336 Scientific Prospectus; Edwards et al., 2010) had to be modified because of unforeseen events. In adjusting the operations plan, the overall strategy was tailored to achieve three objectives (ranked in order of decreasing priority): (1) install CORK observatories, (2) recover and log basement, and (3) recover sediments.

Operations began in Hole 395A, where the old-style CORK and thermistor string were removed before logging and depth-checking to 610 mbsf, introducing minimal amounts of surface seawater. A summary of logging results in Hole 395A is shown in Figure F3. We then attempted to install a new multilevel CORK with downhole and surface experiments, as illustrated in Figure F4. The final installation step failed when the wellhead was broken off as we tried to unlatch the CORK running tool, so a new hole (U1382A) was set up and drilled 50 m west of Hole 395A to install a shallow, single-level CORK observatory. Hole U1382A was drilled to 210 mbsf, penetrating 90 m of sediment and 120 m of basement. After coring was completed, 105.6 m of open borehole was logged (Fig. F5), and the hole was sealed with a 189 m long CORK completion string (Fig. F6).

We then planned to install the deep (500 msb) CORK observatory in the northern part of North Pond at prospectus Site NP-2. Our strategy was to set up the hole with 20 inch casing in the sediment cover, followed by 16 inch casing in upper basement. We then planned to deepen the hole with a 14.75 inch tricone bit to install 10.75 inch casing to ~140 msb. This strategy had to be changed, however, when Hole U1383B, located 6 km north-northeast of Hole U1382A, was lost as a result of the destruction of the tricone bit. The configuration of Hole U1383B is shown in Figure F7. Although this hole could not be deepened, the seafloor infrastructure and open-hole basement section are entirely available for an observatory installation. A new hole, U1383C, was spudded into basement and cased with 10.75 inch casing to 60.4 msb. This hole was then RCB cored to 331.5 m, logged (Fig. F8), and installed with a three-level CORK (Fig. F9).

With our basement objectives largely achieved, we cored sediment with the advanced piston corer (APC) in the vicinity of the CORK observatories, drilling two holes at Site U1383, one at Site U1382, and one at Site U1384 (in the prospectus Site NP-1 area). We moved between locations without retrieving the drill string, APC cored to basement, and then extended core barrel (XCB) cored into basement for ~1 h. These cores were the focus of intensive microbiological and geochemical sampling.

Hydrologic (drill string packer) testing

In situ hydrogeologic testing is essential for quantifying crustal properties (transmissive and storage) that control ridge-flank hydrothermal circulation. We planned to use a drill string packer for hydrologic testing, which is reliable and relatively easy to integrate as part of a comprehensive program of basement drilling, sampling, and experiments. Tests in multiple depth intervals were to be conducted to discern the difference between test results at two depths and to quantify differences in hydrologic properties in discrete depth intervals.

The drill string packer was part of a bottom-hole assembly (BHA) that is compatible with logging so that a separate pipe trip was not required. Wireline logs were run before packer testing to assist with identifying zones suitable for packer element inflation (massive and in gauge). Tests followed the standard approach: (1) inflate and set the packer at the deepest setting point, (2) complete testing at that depth, and (3) deflate the packer, raise it to a shallower depth, and repeat the testing. The difference between test results at two depths can be used to quantify differences in hydrologic properties in discrete depth intervals. Tests are generally repeated at each testing depth, using two or more different pumping rates, to verify test response and formation properties.

Pressure data were generally collected using autonomous downhole pressure gauges that were suspended below a go-devil that was dropped into the packer when it was positioned at depth in the borehole. These gauges were recovered after the complete testing sequence had been run. The shipboard Rig Instrumentation System was used to record key pumping parameters: time, stroke rate, total number of strokes, BHA depth, and standpipe pressure, which is a backup for downhole pressure records.

Logging/Downhole measurements

Downhole measurements during Expedition 336 focused on characterizing crustal physical properties and defining structural and lithologic boundaries as a function of depth. In addition, wireline logging data were compared to results of laboratory analyses of discrete samples to help delineate alteration patterns, fracture densities, and structural orientations and determine how these correlate with fluid flow. These measurements complement core measurements by determining the thickness and structure of lithologic units in intervals where core recovery is poor. These logs were also critical for both shipboard hydrologic (packer) tests as well as for the precise depth placement of the CORK experiments.

Wireline tool strings were deployed in all basement holes and provided measurements including temperature, natural gamma ray, density, porosity, resistivity, sonic velocity, and microresistivity. Descriptions of the wireline tools and their applications are available at iodp.ldeo.columbia.edu/​TOOLS_LABS/​index.html.

We deployed adapted combinations of tool strings, including probes to measure borehole caliper, natural gamma ray (Hostile Environment Natural Gamma Ray Sonde), temperature (Modular Temperature Tool), and resistivity (Formation MicroScanner) (the latter not in Hole 395A). Additionally, we employed the Deep Exploration Biosphere Investigative tool (DEBI-t) in a tool string. This tool was specifically designed and built for Expedition 336 to image the natural fluorescence of microbial communities exposed on the borehole wall.

Summary of operational achievements

Our achievements in meeting our operational goals are summarized below:

1. Drill a basement hole to ~565 mbsf at prospectus Site NP-1, core the lowermost 200 m, log the hole, and install a multilevel CORK.

We drilled a 331.5 m deep hole (U1383C) in the NP-2 area and cored/logged the lowermost ~260 m. A summary of the coring and logging results for Hole U1383C is presented in Figures F10 and F11. Further details will be provided in the “Site U1383” chapter of the Expedition 336 Proceedings volume. A three-level CORK observatory was installed in Hole U1383C, with packers centered at 56, 142, and 196 mbsf (Fig. F9). Hole U1383B was penetrated to 89 mbsf and was equipped with a reentry cone and remotely operated vehicle (ROV) landing platform for future observatory science objectives.

2. Drill a basement hole to ~175 mbsf at prospectus Site NP-2, core ~70 m of the basaltic crust, conduct hydrologic tests and wireline logging, and install a single-level CORK.

A shallow, single-level CORK was installed in Hole U1382A, and this CORK will sample/monitor upper basement between 101 and 210 mbsf. A summary of the coring and logging results for Hole U1382A is presented in Figure F12. Further details will be provided in the “Site U1382” chapter of the Expedition 336 Proceedings volume.

3. Recover the existing CORK in Hole 395A, conduct downhole wireline logging, and install a multilevel CORK.

The CORK installed in Hole 395A during ODP Leg 174B, including the entire thermistor string, was successfully recovered. The hole was successfully logged with the new in situ deep ultraviolet fluorescence tool for detecting microbial life in ocean floor borehole (the DEBI-t); other logging data obtained include spectral gamma ray and temperature. A new three-level observatory was lowered into the seafloor; however, the CORK wellhead broke off, which will preclude our ability to conduct any downstream wellhead operations. Nevertheless, there is no conclusive evidence that the downhole portion of the observatory operations was significantly compromised, so a 4 y experimental run followed by downhole experiment/instrument recovery is still planned. Options for future use of the instrumentation in Hole 395A will be explored during upcoming ROV dives.

4. APC core the thin sediment cover in single holes at four sites (prospectus Sites NP-1 and NP-2, Hole 395A, and Site 1074).

ACP coring of sediments at Sites U1383 (prospectus Site NP-2; two holes), U1382 (near Hole 395A), and U1384 (prospectus Site NP-1) was achieved, and XCB coring retrieved short sections of the sediment/basement interface from all locations. The sediments comprise nannofossil ooze, foraminiferal sand, and brown clay; Site U1382 also features layers with basalt and peridotite debris. The sediments were heavily sampled for microbiological and pore water analyses.

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