We list six specific scientific objectives of Expedition 308.1. Document how pressure, stress, and geology couple to control fluid migration on passive margins.
We hypothesize that flow-focusing is present at Ursa (Fig. F3). This results in a characteristic spatial distribution of fluid pressure and rock properties (e.g., consolidation, permeability, and shear strength) in the mudstone overlying the Blue Unit. By measuring fluid pressure, logging, and coring in the mudstone above the Blue Unit, we will establish the vertical and lateral variation in pressure and rock properties above the Blue Unit. This will provide a first-order test of the flow-focusing model and will image the flow system within the shale bounding the permeable sand unit.2. Establish reference properties at Brazos-Trinity.
A critical goal of the research is to establish a reference log and core properties where overpressure is not present at a range of effective stresses. These data will serve as a baseline against which the properties measured at Ursa can be compared, allowing us to establish the deviation in sediment and fluid properties caused by flow-focusing, fluid overpressure, and low effective stress.3. Illuminate the controls on slope stability.
Massive paleolandslides are present at Ursa. Determination of pore pressure, rock properties, and overburden stress will allow prediction of the potential for slope failure in the present and will allow estimation of the conditions that drove previous slope failures. The measured geotechnical properties are critical inputs of numerical models used to estimate what drives slope failure and when slope failure occurs.4. Understand timing of sedimentation and slumping.
A growing issue of contention is just how rapid the sedimentation rate was in the Ursa Basin, what the age of slumping was, and how this ties to the sea level cycle. A successful drilling campaign integrated with existing well and seismic data will allow these issues to be evaluated with complete data sets.5. Establish geotechnical and petrophysical properties of shallow sediments.
We desire to understand the state and evolution of geotechnical and petrophysical properties of mudstone at effective stresses encountered from the seafloor to 600 mbsf. We will derive a complete logging suite, in situ measurements of permeability and pressure, and core samples, which will allow us to understand the compaction process near the seafloor. These data will provide unparalleled insight into mudstone permeability. Permeability, compressibility, and sedimentation rate are the key parameters to understand the generation of overpressures. This study will illuminate how permeability of fine-grained mudstones evolves through time and changes in effective stress.6. Provide extraordinary data set to observe ponded and channelized turbidite systems.
Brazos-Trinity and Ursa are foci of study for turbidite depositional systems.
IODP Expedition 308 is an abbreviated form of Proposal 589-Full3 entitled "Overpressure and Fluid Flow Processes in the Deepwater Gulf of Mexico: Slope Stability, Seeps, and Shallow Water Flow." The abbreviated program was originally described in addendum 589-Add, submitted to IODP on 22 October 2004. Two components of 589-Full3 will not be accomplished: (1) measurement of in situ pressure within the Blue Unit and (2) long-term monitoring of pressure using CORKS in the mudstone above the Blue Unit and in the Blue Unit. Subsequent to submission of 589-Add, the number of operational days was reduced to 21 from the 30 days envisioned in the addendum. As a result, double-coring at each site will not be possible and there will be only limited wireline logging.
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