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doi:10.2204/iodp.pr.308.2005
OVERVIEW OF EXPEDITION ACHIEVEMENTS AND PRELIMINARY SCIENTIFIC ASSESSMENT
We address the expedition achievements in light of the six original scientific objectives of Expedition 308 (see Introduction).
1. Document how pressure, stress, and geology couple to control fluid migration on passive margins.
Our goal was to establish the vertical and lateral variation in pressure and rock properties above the Blue Unit to test the flow-focusing model and image the flow system within the mudstone capping the Blue Unit.
A fundamental achievement of the expedition is that we established the overpressure profile as a function of depth at two key locations in Ursa basin: Site U1322 and U1324 (Fig. F44). These measurements were difficult and we had a high failure rate (see "Challenges of Measuring Pressure"); however, ultimately we acquired enough data to constrain the overpressure field above the Blue Unit. To our knowledge, this is the first time in DSDP/ODP/IODP history that the spatial variation of the pressure field has been documented at this resolution. Previous deployments of the DVTPP generally yielded only single measurements in boreholes. We also acquired an extraordinary temperature data set that documented striking differences in temperature gradient between Sites U1322 and U1324 (Fig. F44). We also took an extensive amount of whole core for geotechnical analysis. Geotechnical experiments on these cores will further constrain the in situ pressure through analysis of the preconsolidation stresses. Modeling of the DVTPP and T2P pressure dissipation profiles will also further constrain the pressure field.
Our initial observations suggest both lateral and vertical flow are present within the Blue Unit. A fundamental result is that the pressure gradient at the two sites (U1322 and U1324) are similar despite the large difference in sedimentation rate at these locations. We infer that the hydrodynamic flow field within the Blue Unit is more complicated than originally envisioned (Fig. F46) (see "Preliminary Interpretation of Overpressure and Hydrodynamics in Ursa Basin"). Ultimately, to understand the flow field within and around the Blue Unit, it will be necessary to sample the pressures within the Blue Unit sands.
We acquired whole core at Sites U1322 and U1324, LWD/MWD logs at Site U1322, U1323, and U1324, and wireline logs at Site U1324. These data will be used to accurately constrain the variation in rock properties across Ursa Basin. All objectives were met at Sites U1322 and U1324. Because we encountered shallow-water flow at Site U1323, we were unable to core at this location. LWD/MWD, wireline logging, and coring proceeded with extraordinary efficiency despite the fact we were drilling in zones of significant overpressure.
2. Establish reference properties at Brazos-Trinity Basin #4.
We wanted to establish reference logging and core properties where overpressure is not present at a range of effective stresses in the Brazos-Trinity Basin. Coring and logging were successful at the Brazos-Trinity Basin #4 locations. However, only a limited number of pressures were measured due to early struggles with the DVTPP and T2P (see "Challenges of Measuring Pressure"). An intriguing result is that the mudstones beneath Brazos-Trinity Basin #4 (Site U1319) may be overpressured (Fig. F36). If Site U1319 is overpressured, then Sites U1319 and U1320 will provide important examples of a normally pressured location (Site U1319) and overpressured location (Site U1321) in the same location.
3. Illuminate the controls on slope stability.
We wanted to determine pore pressure, rock properties, and overburden stress to predict the potential for slope failure and estimate the conditions that drove previous slope failures. We gained a beautiful suite of data (whole core for geotechnical analysis, in situ pressures, and logs) across the failure surfaces. A striking result is the high degree of consolidation that is present within the slumped units (Fig. F37). A major component of the shore-based science will study the geometry, physical properties, timing, and pressures associated with these slumps.
4. Understand timing of sedimentation and slumping.
We wanted to establish the age of sediments in Brazos Trinity Basin #4 and Ursa Basin. Our preliminary results (Figs. F28, F32) suggest that the Brazos Trinity Basin #4 sediments span MIS VI to present, whereas the Ursa mudstones are <70 ka. At Brazos Trinity Basin #4, slumping, turbidite deposition, and sea level change were tightly linked. Dramatically high sedimentation rates were documented in Ursa Basin. Shore-based research will further constrain the chronostratigraphy of these systems.
5. Establish geotechnical and petrophysical properties of shallow sediments.
We wanted to break new ground in understanding geotechnical and petrophysical properties of shallow sediments (0–1000 mbsf). To support and complement core observations and laboratory measurements, we derived a complete logging suite, in situ measurements of pressure, and whole-core geotechnical samples. We will use these data to understand the compaction process near the seafloor and the evolution of overpressure during sedimentation. The ultimate scientific impact of acquiring these data will unfold in the years ahead. However, the Expedition 308 data set represents a linked data set that has the potential to provide unparalleled insight into mudstone permeability and rheology.
6. Provide an extraordinary data set to observe ponded and channelized turbidite systems.
Expedition 308 sampled the ponded turbidite system at Brazos Trinity Basin #4 and the channelized systems present in Ursa Basin. These data are of great interest to both academic and industry researchers and will be deeply studied in postcruise research.
Expedition 308 was the first time where downhole pressure and lithology were monitored in real time (MWD) and it was the first time that weighted mud has been used as a tool to drill through overpressured regimes (see "Challenges of Drilling in Overpressured Basins"). Real time monitoring allowed us to observe shallow-water flow and to respond to this incident by raising the mud weight in order to hold back flow into the borehole. At both Sites U1323 and U1324 we showed that weighted mud and real-time monitoring can be used to safely drill and complete operations. Future expeditions in a variety of settings might benefit from the planned use of weighted mud to stabilize the borehole.
We expect research on the cores and data generated during Expedition 308 to break new ground, especially in the field of geotechnical and hydrogeological analysis of continental slope sediment successions, be it at passive or active continental margins. Despite initial setbacks, we have shown that programs of in situ measurement of pore pressure in fine-grained sediments can be done with overall success. We have demonstrated that drilling into overpressured formations with riserless technology can be managed using heavy mud, fluid flow into the borehole can be controlled, and operations can be safely concluded without risk to the seafloor environment.