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doi:10.2204/iodp.sp.344.2012

Drilling and coring strategy

Drilling strategy

The overall operations plan and time estimates for Expedition 344 are summarized in Table T1. Alternate sites have been selected and are presented in Table T2. Time estimates are based on formation lithologies and depths inferred from seismic and regional geological interpretations, including prior drilling in this area (Legs 84, 170, and 205 and Expedition 334). After departing from Balboa, Panama, we will transit for ~1.6 days to the Costa Rica sites and prepare for drilling operations.

On 19 January 2012, the Co-Chief Scientists, seismic experts, and IODP representatives met in College Station, Texas, to discuss moving primary sites from the Expedition 334 transect into the 3-D seismic data volume. At the time of the meeting only preliminary seismic analysis was available. However, seismic data showed that sediments within the 3-D seismic volume northwest of the Expedition 334 transect are significantly thicker. The thickness of sediments substantially increases drilling times and precludes meeting many of the CRISP Program A primary science objectives. Therefore it was decided to retain the sites along the Expedition 334 transect as primary drilling sites, with the exception of Site U1379. At Site U1379 primary science objectives were met during Expedition 334. However, this site was also intended to be the pilot hole for riser drilling that intersects the seismogenic zone. The water depth at this site is too shallow for riser drilling, and as discussed later, this site has been moved into the 3-D seismic volume.

The proposed drilling strategy is to begin by drilling at reference Site U1381 on the incoming Cocos plate (Table T1; alternate Site CRIS-19A, Table T2). This site has primary objectives of characterizing the incoming sediment section and oceanic crust and understanding the nature of the hydrologic system within the upper oceanic crust. During Expedition 334, two holes were drilled at this site, Holes U1381A and U1381B. Hole U1381A was drilled with the rotary core barrel (RCB) coring system to recover as much sediment and basement as possible within the specified time window. Hole U1381B was RCB drilled to retrieve the uppermost 30 m of sediment for detailed geochemical sampling. During Expedition 344, we will use the advanced piston corer (APC) to basement to collect a high-quality sediment core to supplement the RCB sediment core collected during Expedition 334.

Following drilling at Site U1381 we will move to mid-slope Site U1380 (Table T1; alternate Site U1378 and proposed Site CRIS-12B, Table T2). During Expedition 334, Sites U1378 and U1380 were drilled into the middle slope of the Costa Rica margin but the primary objective of penetrating basement was not achieved because of poor hole conditions. Extra time has been planned for installing casing and cementing through the problematic depth interval.

After the mid-slope site we will drill proposed toe Site CRIS-9A (Table T1). This site is located at the frontal sedimentary prism and represents the base of the slope. This site provides sampling of the plate boundary at shallow depth through the sedimentary frontal prism to define the fault state, composition, and fluid system. Proposed Site CRIS-9A will be drilled through 780 m thick sediments and continue for 200 m into the oceanic basement. Drilling at proposed Site CRIS-9A will allow us to constrain the décollement geometry and deformation at a shallow depth, define fluid pathways, and link these to the seismic cycle.

Finally, we will drill proposed upper slope Site CRIS-13B (Table T1; alternate Sites U1379, CRIS-14A, and CRIS-15B, Table T2). Site CRIS-13B is located above the locked portion of the plate boundary. Drilling here will allow us to characterize fluid flow and the basement beneath the upper slope, which is necessary for understanding the drilling conditions for deep holes. Proposed Site CRIS-13B will be drilled through 1330 m thick sediments and continue for 100 m into basement.

Coring strategy

The first hole at proposed Sites CRIS-9A and CRIS-13B will be cored with the APC/​extended core barrel (XCB) to refusal depth (estimated to be ~500 mbsf). A deeper hole at those sites and at Site U1380 will be drilled without coring by installing a center bit to a depth slightly above the refusal depth of Hole A (e.g., ~490 mbsf), and RCB coring will extend from this depth to the target depth. See the “Site summaries” for exact depths. APC cores will be taken with nonmagnetic core barrels until overpull limits are exceeded.

After completing coring at each site, the holes will be conditioned, displaced with logging mud, and logged as per the logging plan (see “Downhole measurements strategy”). As per policy, all holes in continental margins into consolidated sediments will be plugged and abandoned with cement plugs.

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