Next Section | Table of Contents
doi:10.2204/iodp.sp.301.2004
Planned operations during Expedition 301 are summarized in Table T2. The expedition will begin with a jet-in test at Site SR-1, followed by emplacement of a reentry cone and 20 inch conductor casing. This cone and casing system will be used to establish the deep basement hole, Hole SR-1A. A more traditional strategy would begin with sediment coring, but this approach is not planned for Expedition 301 for several reasons. First, we already have a good understanding of the sediment thickness and properties on the basis of extensive site survey data and prior work at nearby Site 1026 during ODP Leg 168, which was cored continuously. In addition, sediment cores recovered during Expedition 301 will be subjected to extensive microbiological sampling and analyses, but we will begin at-sea operations less than 24 h after leaving port and it will be difficult to establish protocols and procedures for sediment sampling, handling, and analysis so quickly after the start of the expedition (particularly since this is the first expedition of IODP). Third, we want to save sediment coring options for times later in the expedition, when packer testing or observatory installation operations cannot be completed because of weather or sea state. We also would like to wait to dedicate time to sediment coring until we have greater confidence of achieving high-priority basement and observatory operations. Finally, geochemists and microbiologists may find it more scientifically desirable to complete coring work at First Ridge, rather than at Second Ridge. The sediment layer above shallow basement at First Ridge is much thinner, fluid is known to be seeping upward from basement, piston coring to basement will be easier and recovery more complete, and we would have time to core several FR holes in the time required to drill a single SR hole. Therefore, determining where coring will take place will require discussions with the shipboard geochemists and microbiologists to ensure the science return is maximized.
After installation of the cone and casing in Hole SR-1A, we will drill through ~260 m of sediments and into uppermost basement, and then set and cement 16 inch casing. Drilling will continue with a 143/4 inch tricone bit to penetrate quickly through the most unstable zone in upper basement, estimated to be ~50–100 m thick. When drilling conditions indicate that basement rocks are more stable, 103/4 inch casing will be run and cemented into place. Coring will commence using a standard 97/8 inch bit and RCB system. Based on precruise time estimates, we anticipate 470 m of basement penetration (Table T2), but we may core somewhat more or less basement in Hole SR-1A during this first stage of operations, depending on rates of penetration, drilling conditions, and the nature of the rocks recovered. Hole SR-1A may be deepened later in the expedition, depending on the timing of observatory installation at this and other sites.
Once total depth is achieved in Hole SR-1A, the hole will be logged with conventional tools (triple combination [triple combo] and Formation MicroScanner/Dipole Sonic Imager [FMS/DSI] tool strings), a borehole televiewer (Ultrasonic Borehole Imager [UBI]), and a VSP (three-component Well Seismic Tool [WST-3]) using a single generator injector (GI) gun source. Packer experiments will be conducted in straddle-packer mode in the open hole. It is likely that we will move to other holes for drilling or Cork observatory operations prior to installation of the Cork observatory in Hole SR-1A because this is to be the most complex observatory installation. Depending on drilling conditions and timing, we may also wait to complete logging and other downhole work until after we return to this hole later in the expedition. Downhole logging is important for geophysical and hydrogeologic characterization and evaluation of alteration and fracturing and will guide positioning of packers for both short-term and longer-term experiments.
The ship will be offset ~20 m north-northeast from Hole SR-1A, and Hole SR-1B will be established in the same way, with a cone and 20 inch conductor casing. Once again, we will drill into uppermost basement and install and cement 16 inch casing. At this point, we have two options: (1) drill into upper basement using a 143/4 inch tricone bit or (2) attempt to core uppermost basement using a standard 97/8 inch RCB system. We consider the first option to be safer, since our primary objective is to emplace a Cork observatory to monitor conditions in uppermost basement. Also, we know from experience in Hole 1026B that drilling conditions in upper basement may be difficult and core recovery is likely to be low. It is unlikely that we would be able to core deeper than 20–40 m into basement in any case. If we do core this section, we will have to ream out the hole to make it large enough to receive 103/4 inch casing, which is needed to keep the hole open for the long-term observatory. Holes in unstable formations are more likely to remain open if they are drilled quickly and not reamed. We will be in a better position to evaluate the desirability of coring uppermost basement in Hole SR-1B when we see how well we achieve our drilling, casing, coring, and depth objectives in Hole SR-1A. As a practical matter, any time spent coring shallow basement in Hole SR-1B would have to come out of that for depth penetration in Hole SR-1A and/or sediment coring.
Once casing is in place in Hole SR-1B, we will conduct packer tests by setting the packer in casing and testing the open interval below. This open interval will be too short and unstable for wireline logging. We will then deploy the first Cork-II observatory of the expedition, which will be configured to isolate a single interval of upper basement (Fig. F10). Operations will include deployment of downhole temperature sensors, fluid samplers, and microbiological incubation substrate. Pressure sensors, data loggers, and an additional fluid sampler will be deployed and attached to the Cork-head plumbing during a later expedition by ROV.
We will move to Site 1026, recover the Cork system currently in the reentry cone in Hole 1026B, and deploy a new Cork-II system that will isolate a single interval of uppermost basement (Fig. F11). Cork operations will include deployment of downhole temperature sensors, fluid samplers, and microbiological incubation substrate. Pressure sensors, data loggers, and an additional fluid sampler will be deployed and attached to the Cork-head plumbing during a later expedition by ROV.
We have reserved sufficient time during the expedition to APC core the sedimentary section at Site SR-1. This would likely result in partial penetration of APC core barrels, since the sedimentary section is dominated by sandy turbidites, followed by "overdrilling" to successive coring depths. There is some risk in this approach, involving APC coring in unstable sands (as we encountered during ODP Leg 168), and the available time might be better used to piston core at the First Ridge or to wash partway to basement prior to the start of coring. A final decision will be reached regarding the location and extent of Expedition 301 sediment coring operations on the basis of experience gained during the first part of the expedition and discussion with shipboard scientists regarding the scientific merits of each area. Sediment coring will be accompanied by measurements of sediment temperatures using the Davis-Villinger Temperature Probe (DVTP) and/or the APC Temperature (APCT) tool. If sediment coring is completed near the middle of the expedition, we will still need to save 36 h of contingency time for the end of the program, in case difficulties are encountered when deploying the final Cork-II system.
The ship will return to Hole SR-1A and either continue coring (if this is desired, time allows, and depth goals are not achieved), complete downhole measurements (if these were not already completed), and/or deploy the Cork-II system. The Cork-II in Hole SR-1A will isolate three distinct basement intervals (Fig. F12). Downhole temperature sensors, fluid samplers, and microbiological incubation substrate will be placed in the lowermost isolated interval, and additional temperature sensors may be deployed on the line above the bottom plug of in the inner (4Pressure sensors, data loggers, and additional fluid samplers will be deployed and attached to the Cork-head plumbing during a later expedition by ROV. However, there will be one seafloor fluid sampler attached to the Cork-II head and left running when the Cork-II is deployed by the drillship. Having a fluid sampler attached and running immediately after Cork installation will allow us to observe the early stages of borehole recovery.
The ship will move to Site 1027, recover the Cork system currently in the reentry cone in Hole 1027C, and deploy a new Cork-II system. We will check the total depth of Hole 1027C prior to Cork-II deployment using a cleanout bit on the drill string. The new Cork-II system will isolate two intervals in upper basement (Fig. F13). Downhole temperature sensors, fluid samplers, and microbiological incubation substrate will be placed in the lowermost isolated interval. Pressure sensors, data loggers, and additional fluid samplers will be deployed during a later expedition by ROV. If contingency time is left at the end of the expedition (allocated in case of difficulties with the final Cork-II deployment), it will be available for coring, temperature measurements in sediments, or other operations.