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doi:10.14379/iodp.sp.352.2013

Drilling and coring strategy

As already noted, we plan to achieve our goal of sampling the full volcanic stratigraphy of the Bonin fore arc by drilling two offset sites (BON-1A and BON-2A; see “Site summaries”), each made up of ~750 m of lava overlain by at least 100 m of sediment (Fig. F14). The precise location of the sites was constrained by the presence of sediment ponds. Site BON-1A (Line IBr11n, common depth point [CDP] 5364; 28°27.01′N, 142°45.35′E; 4778 m water depth) is designed to first encounter FAB and reach the sheeted dikes, thus drilling the oldest rocks in the sequence. Site BON-2A (Line IBr11, CDP 66252; 28°24.46′N 142°36.55′E; 3137 m water depth) will start in boninites and finish in FAB, completing the sequence. We expect this to enable us to obtain a full section in a single expedition, something that could not be guaranteed with a single 1750 m hole. In the event of either of these being unsuccessful or giving unexpected results, we have identified four contingency sites close to Sites BON-1A and BON-2A and one at Site 459 in the Mariana fore arc (see “Risks and contingency”).

Previous experience indicates that engineering conditions at IBM fore-arc sites are likely to be favorable (Fig. F15). Drilling at Site 459 penetrated sediments and then basalts similar to those expected at Site BON-1A at a rate of ~700 m in ~6 days. Drilling at Site 458 achieved similar penetration rates to ~450 meters below seafloor (mbsf). Drilling in Hole 786B, which penetrated sediments and then boninites similar to those expected in the upper part of Site BON-2A, drilled to >800 mbsf with a single drill bit in 11 days. This was a particularly stable hole, probably because fluid circulation filled veins and healed fractures. This experience leads us to conclude that drilling without a riser will not face drilling, safety, or environmental problems.

The temperature at the bottoms of the holes should not present a problem: the temperature gradients in outer fore arcs are the lowest on the planet. Temperature measurements to 110 mbsf at Leg 125, Site 792 (about halfway between the trench and the magmatic arc) define a heat flow of 56 mW/m², which gives a thermal gradient of 34°/km for the basement rocks. The thermal gradient farther from the arc at Sites BON-1A and BON-2A should be significantly lower, indicating that the temperature at the bottom of a 1000 m deep hole should be <50°C.

In detail, Hole 786B drilling indicates formation hardness changes at 400 and 690 mbsf and implies that an average penetration rate of 46.3 m/day can be achieved. This is equivalent to 1.92 m/h, compared to a typical average of 1.8 m/h, the faster penetration perhaps due to the better cementation of rocks resulting from a long history of fluid circulation. Extrapolation of the penetration curve gives an estimated drilling time for a 1000 m hole of 14–15 days for linear extrapolation and 16–20 days for the more likely nonlinear extrapolation. Of course, unexpected issues such as bit failure or hole instabilities could slow progress.

The scientific goals of this expedition are best achieved by obtaining a full lava stratigraphy. Moreover, for purposes of testing hypotheses and economic constraints, we need to be able to achieve this in a single ~60 day expedition. Ophiolite studies, coupled with the seismic studies reported here, show that the lava thickness is likely to be 1.25 ± 0.25 km, probably beyond that likely to be drilled at one site in a single expedition, especially if casing is needed for hole stability and to provide a legacy. For these reasons, we choose instead to use the offset drilling approach, drilling at both Sites BON-1A and BON-2A. Casing will be used to improve hole stability.

We aim to run the standard set of wireline logs for crustal sections (see “Downhole measurements strategy”). The Formation MicroScanner and borehole televiewer are essential for understanding the history of fracturing of, and hence fluid flow in, the fore-arc crust. They are also useful for preparing a complete lithostratigraphic log in the inevitable event of incomplete core recovery and will also help us to understand the structure of the fore arc at the drilled sites. Physical properties tools are needed for synthetic seismograms and ground-truthing seismic images.

The proposed drilling and coring strategy for the primary and alternate sites are presented in Tables T1 and T2, respectively. The time estimates used are based on formation lithologies, depths inferred from seismic and regional geological interpretations, and prior drilling in this area (Legs 60 and 125).

Seismic profiles for all sites are included in “Site summaries.” Sites BON-1A and BON-2A are expected to recover the lower and upper volcanic stratigraphy, respectively.

Hole A at each site will consist of a jet-in test, the purpose of which is to determine the length of the initial casing string.

Hole B at each site will be cored with the advanced piston corer (APC)/extended core barrel system to the sediment/basement interface (100–250 mbsf). APC cores will be taken with nonmagnetic core barrels until overpull limits are exceeded.

Hole C at each site will start with casing operations: a reentry cone with the initial 16 inch casing string will be installed to the depth determined by the jet-in test, followed by a second 10¾ inch casing string. We will use a two-casing string strategy to save time and also based on what we know from previous drilling in this area. Rotary core barrel coring will extend from the bottom of the casing to the target depth, requiring multiple drill bit changes.

After completing coring in Hole C at each site, the holes will be conditioned, displaced with logging mud, and logged as described in the logging plan (see “Downhole measurements strategy”).

For logistical reasons, we aim to drill Holes A and B at Site BON-2A before drilling the full sequence of Holes at Site BON-1A. We then plan to return to Site BON-2A to complete drilling.

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