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doi:10.2204/iodp.proc.308.215.2012

Introduction

Integrated Ocean Drilling Program (IODP) Expedition 308 was dedicated to the study of shallow overpressure, flow focusing, and slope stability (see the “Expedition 308 summary” chapter [Expedition 308 Scientists, 2006a]). Operations during Expedition 308 included drilling three locations in Ursa Basin (Sites U1322–U1324) on the continental slope of the Gulf of Mexico, offshore Louisiana in ~1000 m water depth (Fig. F1). At Sites U1322 and U1324, a logging-while-drilling (LWD) hole and a coring hole were completed; at Site U1323, only a LWD hole was completed. Drilling was performed at a second location (Brazos-Trinity Basin IV) to compare observations made in Ursa Basin.

This synthesis paper summarizes operations and scientific studies performed during and after Expedition 308 with an emphasis on the study of fluid flow, overpressure, consolidation, and slope stability in continental margin sediments. We focus our discussion on the results within Ursa Basin. These studies have helped advance the high-priority goals of Expedition 308. Other research papers since the end of Expedition 308 include studies of sedimentology (Mallarino et al., 2006; Pirmez et al., in press), microbiology (Nunoura et al., 2009), rock magnetics (Fu et al., 2008), and pore pressure at Brazos-Trinity Basin IV (Schneider et al., 2009). A comprehensive listing of publications associated with Expedition 308 can be found at publications.iodp.org/proceedings/308/308bib.htm.

Expedition 308 was motivated by observations and models that suggested lateral flow could cause slope instability (Dugan and Flemings, 2000; Flemings et al., 2002). Rapid sediment loading drives overpressure (u*; pressure in excess of hydrostatic) in basins around the world (Rubey and Hubbert, 1959). In mudstone-rich environments, sedimentation can be rapid, permeability is low, and compressibility is high. In these environments, fluid flow cannot keep up with the sediment load, the fluids bear some of the sediment load, and pore pressures are greater than hydrostatic. Recent work has focused on how sedimentation and common stratigraphic architectures couple to produce two- and three-dimensional (3-D) flow fields (Flemings et al., 2002). For example, if a permeable sandstone body is rapidly loaded by low-permeability mudstone of varying thickness, fluids flow laterally within the aquifer to regions of low overburden before they are expelled into the bounding mudstone. This will create characteristic distributions of rock properties, fluid pressure, effective stress, temperature, and fluid chemistry in the aquifers and bounding mudstones. This process drives slope instability near the seafloor (Dugan and Flemings, 2000; Flemings et al., 2002); in the deeper subsurface, it drives fluids through low-permeability strata to ultimately vent at the seafloor (Boehm and Moore, 2002; Reilly and Flemings, 2010; Rubey and Hubbert, 1959; Seldon and Flemings, 2005). Expedition 308 documents the spatial variation in pressure, vertical stress, and rock properties in a flow-focusing environment.

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