LOGGING AND DOWNHOLE MEASUREMENTS PLAN
The main objectives of the downhole measurements program will be to assess how pressure, stress, and geology control fluid migration on a passive margin; establish reference geotechnical and petrophysical properties at a site where overpressures are not present as well as in an overpressure zone; learn about factors controlling slope stability; determine major depositional events and timing of landslides; and provide information about turbiditic processes that occur along the continental slope. In addition, the downhole measurements plan will attempt to define structural and lithologic boundaries as a function of depth, establish site-to-site correlations to seismic and lateral lithostratigraphic variations, produce direct correlations with discrete laboratory data, and identify potential conduits that may serve as pathways for fluid migration. Finally, downhole measurements will complement core measurements by filling gaps in downhole stratigraphy, determining the thickness of lithological units in intervals where poor core recovery is prevalent, and provide the means for potential correlation with the extensive seismic data grid that is available over these areas. Both MWD and wireline logging tool deployments will be used to obtain the downhole measurements proposed for this expedition; the operational plans are described below.Measurement While Drilling
At the conclusion of coring activities at the Brazos-Trinity sites, the operational plan will shift to logging activities with MWD capabilities at the two main sites, BT4-2A and BT4-4A (Table T2). These sites will serve as a reference location for physical and chemical properties. Site BT4-2A is located in the section of greatest overburden thickness above the hemipelagic shales. Site BT4-4A is located along the southern flank of the basin where there is almost no turbidite overburden above the hemipelagic shales. MWD operations will then follow at the three Ursa sites (URS-3C, URS-2C, and URS-1B). At the conclusion of the MWD operations, the tools and the MWD engineer will be offloaded to a transfer boat (Table T5). Overall, MWD data will provide information throughout the drilling depth on physical properties that will be used to test spatial variation in rock properties associated with the flow-focusing model. The MWD tools and measurements will include Resistivity-at-the-Bit GeoVision resistivity (GVR), Azimuthal Density Neutron (ADN), and measurements of pressure (annulus pressure while drilling; APWD). The GVR provides azimuthal resistivity images of the borehole and gamma ray measurements; the ADN provides azimuthal borehole compensated formation density, neutron porosity, and photoelectric factor measurements; and the APWD will provide measurements of annulus pressure for identifying potential shallow flow and overpressure conditions.Wireline Logging and Vertical Seismic Profile
A series of three tool string deployments are planned for the Brazos-Trinity and Ursa sites. These tool strings include the triple combination (triple combo), the Formation MicroScanner (FMS)-Dipole Sonic Imager (DSI), and a zero-offset VSP. Detailed descriptions of all wireline tools and applications are provided at iodp.ldeo.columbia.edu/TOOLS_LABS/index.html. A detailed time estimate for the wireline logging can be seen in Table T3.
The triple combo with caliper measurements will be used to assess the initial postdrilling borehole conditions such as hole size and postdrilling fluid temperatures. In addition, this tool string will obtain potassium, uranium, and thorium concentrations as well as formation density, photoelectric effect, electrical resistivity, and porosity in situ profiles as a function of depth. The FMS will provide high-resolution borehole images of lithostratigraphic sequences and boundaries, oriented fracture patterns, and information regarding hole stability. The DSI will produce a full set of compressional and shear waveforms, cross-dipole shear wave velocities and amplitudes measured at different azimuths, and Stoneley waveforms. The zero-offset VSP will provide the shallow sediment velocity gradient information and interval velocity that will be necessary for potential core-log-seismic correlations.
These measurements will be utilized for characterization of stratigraphic sequences; determination of potential geological factors that may influence fluid migration; establishing geotechnical and petrophysical properties at these sites; and providing information about the depositional events, timing of landslides, and turbiditic processes along the continental slope. These types of measurements can be used to determine preferred fracture orientations and fracture densities, paleostress directions, and permeability estimates, all required to accurately model the hydrological characteristics of this passive margin system. The velocity gradient, sonic velocities, and densities could be used for the calculation of synthetic seismogram models and a direct correlation with high-resolution seismic data that have been obtained for these sites.In Situ Measurements
Pressure, hydraulic conductivity, and temperature will be measured in the mudstones with the T2P designed by the Massachusetts Institute of Technology (MIT; USA), the Pennsylvania State University (USA), and IODP-Texas A&M University (TAMU; USA). These data will help constrain the rock properties and flow field in the sediments. The tapered probe measures pressure and temperature at the narrow tip of the probe; a second pressure measurement is collected slightly up-probe from the sensors at the tip (Fig. F12). The design allows for rapid measurement (~190 min total operational time) (Table T6) of high-quality pressure and temperature in low-permeability sediments. During penetration of the probe (<5 min), we will not circulate. However, during the pressure measurement, circulation is possible.
The T2P delivery and drive assembly interfaces with the drill string and is integrated into the IODP operational protocols. We will use the existing colleted delivery system (CDS) now used to deploy the DVTPP, to deploy the T2P. The drill string is first raised several meters off the bottom of the borehole. The probe is then lowered down the drill string in the extended configuration (stroke = ~3.3 m) and engages the bottom-hole assembly (BHA). The drill string is then lowered to insert the probe into the foundation. Once the force of penetration exceeds the weight of the telescoping section of the CDS, the section retracts with little change in force until it engages the collet. With the section fully retracted, the weight of the drill string is used to force the probe into the foundation. The CDS has 53 kN (12,000 lb) driving force capacity controlled by a safety mechanism. Following penetration (controlled by lowering the string a specified distance), the drill string is raised 2 m to extend the telescope and decouple the probe from the drill string. This prevents drill string movement (due to ship heave) from disturbing the probe during dissipation.
A second component of in situ measurements is routine temperature measurements at each site. Temperature measurements will be made during APC operations using the APCT tool. We have allocated time to make three APCT measurements and one DVTPP measurement per coring hole (Table T5). Each measurement will take ~15 min.
Next Section | Table of Contents