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doi:10.2204/iodp.sp.317.2009

Coring and drilling strategy

General operations plan

Leg 174A drilling on the New Jersey shelf illustrated the difficulty of shelf drilling with a dynamically positioned drilling vessel (Austin, Christie-Blick, Malone, et al., 1998). The main problem was the collapse of loose sands trapping the drill string, although stationkeeping in shallow water was also occasionally difficult. Therefore, the proposed drilling strategy for Expedition 317 has been heavily influenced by Leg 174A experience. The choice of site locations is partly a compromise between the need to sample as many of the interpreted sequence boundaries as possible and to avoid deep penetrations in shallow water on the shelf. In particular, we have tried to minimize penetration depth at the shallowest water shelf sites (Sites CB-01A and CB-02A).

The overall operations plan and time estimates are summarized in Table T1. Time estimates are based on formation lithologies and depths inferred from seismic and regional geological interpretations including prior drilling in this area (Site 1119). After departing from Townsville, Australia, we will transit for ~10 days to Canterbury Basin and prepare for drilling operations.

In order to determine the impact of global sea level change on deposition cyclicity, it is important to drill the same sedimentary sequences at critical locations (e.g., landward of clinoform breakpoints where paleoenvironmental data are critical for eustatic amplitude estimates and on paleoslopes where increased microfossil abundance provides optimal age control). For example, Leg 174A drilling recovered valuable paleoenvironmental information diagnostic of a shallow-water, lagoonal environment landward of a breakpoint. Therefore, it is essential to conduct operations on the modern shelf (Sites CB-03, CB-01, and CB-02) to reach sequence paleoshelves landward of their breakpoints, as well as upper paleoslopes of the oldest sequences. In addition, paleoslopes of many sequences can be penetrated at moderately shallow slope Site CB-04.

The proposed drilling strategy is to begin by drilling at shelf Site CB-03B at the deepwater end (121 m) of the shelf transect (Fig. F7; Table T1) if weather conditions allow (see below). This will provide experience in shelf sediment drilling before moving to sites in even shallower water. Actual penetration depths may vary from (but will not exceed) the proposed target depth (1249 mbsf), depending on drilling progress (time consumption) and bit wear out.

We will then move to the primary slope Site CB-04B, which should provide good age control for sequences drilled on the shelf (at Site CB-03B). An additional target at this site is penetration and recovery of the Marshall Paraconformity, estimated to lie at ~1600 mbsf (Fig. F9). If hole conditions and/or time constraints do not allow for such deep penetration at this site, actual penetration depth need not exceed the depth of the lowermost unconformity recovered at shelf Site CB-03B.

On completion of slope drilling, we will move back to the shelf and drill the two additional shelf Sites CB-01A and CB-02A (Fig. F7; Table T1) to provide spatial control of facies within sequences and to recover the lowermost unconformities landward of their clinoform breakpoints.

Drilling operations in shallow water require special operational precautions to ensure safety for crew and equipment. Weather conditions (particularly sea state and resulting heave behavior of the vessel) are critical and will therefore greatly influence this drilling plan (see "Risk and contingency strategy").

Coring strategy

The first hole at each site (Hole A) will be cored with advanced piston corer (APC)/extended core barrel (XCB) to refusal depth (estimated to be ~250–350 mbsf). The second hole (Hole B) at each site will be drilled without coring to a depth slightly above the refusal depth of Hole A (e.g., ~300 mbsf), and rotary core barrel (RCB) coring will extend from this depth to the target depth.

After coring is completed in the B holes, holes will be conditioned, loaded with mud, and the bit released in the hole. We will then run a series of wireline logs, including a check shot vertical seismic profile (VSP) in open hole (see "Logging/downhole measurements strategy"). It is assumed that borehole walls in the B holes will increasingly deteriorate during the course the drilling operations. Therefore, we plan to log the two deep-penetration Sites CB-03B and CB-04B in two stages (upper and lower parts of the holes) to maximize the quality of the logging results (see "Logging/downhole measurements strategy"). This includes drilling a dedicated logging Hole C at Site CB-04B to a depth of ~800 mbsf (covering the upper deteriorated section of Hole B at this site).

During Leg 174A, it was found that RCB drilling was the most effective approach at shelf sites with a high proportion of unconsolidated sand. If APC/XCB coring results prove to be unsatisfying at Canterbury shelf sites (e.g., during operations at the first shelf Site CB-03B), subsequent shelf sites (Sites CB-01A and CB02A) could be approached by RCB drilling alone.

We assume single penetrations at all sites. Double penetrations would be valuable; however, it is essential to obtain information from multiple sites in order to calibrate the sequence stratigraphy. Given the time limitations, therefore, double penetrations are not feasible.

Primary sites

Shelf sites

Site CB-03B

This site (~121 m water depth) is located at the deeper (outer) end of the shelf transect (Fig. F7). It was chosen as primary site in response to an Environmental Protection and Safety Panel (EPSP) request (December 2005 meeting) to avoid high seismic amplitudes observed at Site CB-03A. Site CB-03B is located updip from Site CB-03A where there is no EW00-01 crossing strike profile. Site CB-03B is, however, located on commercial crossing strike profile CB-82-25. If cored to the anticipated target depth (1249 mbsf), it will penetrate sequence boundaries of Unconformities U19–U4. Because of anticipated deterioration of the upper part of the hole during drilling of this deep-penetration site, it is also planned to log the "A" hole (after completion of APC/XCB coring).

Sites CB-01A and CB-02A

These two sites are located at the landward end of the shelf transect at ultra-shallow water depths (85 and 111 m). Target penetration depths are 780 and 800 mbsf (penetrating Unconformities U19-U4 and U19-U6, respectively, at different palaeodepth locations). The location of Site CB-01A enables us to reach Unconformity U4 with minimum penetration, and this is the only site that can sample shelf facies near paleoshelf edges of Unconformities U4 and U5. Likewise, Site CB-02A samples shelf facies near paleoshelf edges of Unconformities U7 and U8. If time is limited at the end of the expedition, reduced penetration or, alternatively, merging of these two sites into one intermediate site, alternate Site CB-01B (located between Sites CB-01A and CB-02A), may be considered (see "Risk and contingency strategy"). Sites CB-03B and CB-04B are designed to sample the corresponding clinoform fronts.

Slope site

Site CB-04B

This site (~346 m water depth) serves as primary site for drilling the slope of the Canterbury Basin (Fig. F9). Drilling would penetrate Unconformities U6–U9, U11, and U13–U19. Drilling time estimates presume reaching the Marshall Paraconformity, estimated to lie above the target depth of 1700 mbsf. If reaching the Marshall Paraconformity is abandoned as an objective for this site, the penetration depth need not exceed the depth of the lowermost unconformity recovered at the shelf sites. To maximize logging quality, the upper part (~800 m) of the hole will be logged in a dedicated logging hole "C."

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