Leg 174B Scientific Report
OBJECTIVES AND METHODS
By leaving Hole 395A open for over 20 yr, with revisits for discrete data sampling roughly every 5 yr, we have learned only that the downhole flow has apparently continued at a significant rate. We have no resolution as to possible variations in downhole flow rates with time (as has been documented in Hole 504B), let alone the constancy or variability of the driving forces responsible for the downhole flow. Furthermore, we still do not understand exactly where the downhole flow is directed in the formation, other than the general statement that it is directed into the upper 300 m or so of basement.
The Leg 174B program was designed to address these important issues by providing essential information about the in situ physical properties, permeability structure, and formation pressure, which are keys to understanding the crustal hydrogeology at Site 395. With about five days to be spent at Hole 395A during Leg 174B, the operational program was scheduled to begin with about three days of logging, followed by about two days for installation of a CORK. These were planned in a sequence requiring two trips of the drill string, as follows:
- 1. Logs: After initial reentry with a logging bottom-hole assembly (BHA), a temperature log with the Davis-Villinger Temperature Probe (DVTP) was run, followed by three Schlumberger logs to delineate the fine-scale permeability structure of the open-hole section penetrated by Hole 395A. The three Schlumberger logs included two advanced sondes run for the first time in an ODP hole, the Azimuthal Resistivity Imager (ARI) and Dipole Shear Imager (DSI). If time allowed and formation conditions warranted, a flowmeter was also prepared for possible deployment.
- 2. CORK: Deployment of a fully configured CORK to seal the hole, instrumented with a 595 m-long, 10-thermistor cable, pressure sensor in the sealed section, and a reference pressure sensor at seafloor depth.
The CORK installation will provide a long-term (5 yr or longer) record of (1) the rebound of temperatures and pressures toward formation conditions after the emplacement of the seal, (2) possible temporal variations in temperatures because of lateral flow in discrete zones, and (3) pressure variations, which in a sealed hole would be the primary manifestation of changes in the forces that drive the natural circulation system. The first installment of data from the CORK experiment is scheduled to be collected during February 1998, utilizing the French submersible Nautile, with support from the National Science Foundation.
The primary purpose of the CORK experiment is not necessarily to assess the equilibrium pre drilling thermal regime (which we can estimate from detailed heat-flow surveys as in Fig. 2), but instead to monitor how the hydrologic system varies with time as natural hydrogeological conditions are re-established. Full thermal re-equilibration could require many tens or hundreds of years if it occurs only by conductive processes, but could also occur in much less time if the Langseth et al. (1984, 1992) model of active lateral circulation is correct. We are interested primarily in exploring the causes of the hydrogeological state and any possible temporal variations, with the simplest goal to determine how these are associated with and controlled by formation pressure and/or permeability structure. It is impossible to model or predict all of the possible outcomes of the experiment, but considering two possible end-member results might be instructive.
- 1. If the model of active lateral circulation is basically incorrect and downhole flow is indeed simply an artifact of drilling, then sealing the hole should remove the driving force for the downhole flow, and temperatures and pressures will slowly and smoothly trend toward values consistent with conductive, hydrostatic processes.
- 2. If there is some element of truth to the model of active lateral circulation in basement, with this circulation providing the driving pressure differential for the downhole flow, then sealing the hole will not change the driving force, and lateral circulation should continue even though the seal has stopped the downhole flow. Pressures in the sealed hole should approach a nonhydrostatic value in an irregular fashion that reflects variability in the natural hydrogeologic processes. Similarly, temperatures will rebound toward values consistent with the circulation system, also in an irregular fashion that reflects natural hydrogeologic variability. In addition, differences in the behavior of the temperature sensors should reflect vertical variations in the lateral flow regime because of fine-scale permeability variations. We understand so little about crustal hydrogeology that simply defining the natural time and space scales of such variability will be a very important result.
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To Scientific Results - Site 1074
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