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doi:10.2204/iodp.proc.308.101.2006

Introduction

Rapid sediment loading (>1 mm/y) drives overpressure (P*; pressure in excess of hydrostatic) in basins around the world (Rubey and Hubbert, 1959; Fertl, 1976). Sedimentation is so rapid that fluids cannot escape, the fluids bear some of the overlying sediment load, and pore pressures become greater than hydrostatic (Fig. F1).

Recent work has focused on how sedimentation and common stratigraphic architectures couple to produce two- and three-dimensional flow fields. For example, if a permeable sand is rapidly buried by low-permeability mud of laterally varying thickness, fluids flow laterally along the sand to regions of low overburden before they are expelled into the overlying sediment (Fig. F2A). This creates characteristic distributions of rock properties, fluid pressure, effective stress, temperature, and fluid chemistry in the aquifers and bounding mud (Fig. F2B). This flow-focusing process can cause slope instability near the seafloor (Fig. F3A) (Dugan and Flemings, 2000; Flemings et al., 2002); in the deeper subsurface, this process can drive fluids through low-permeability strata to ultimately vent them at the seafloor (Fig. F3B) (Boehm and Moore, 2002; Davies et al., 2002; Seldon and Flemings, 2005).

Expedition 308 documented the spatial variation in pressure, vertical stress, and rock properties in a flow-focusing environment. We first established rock and fluid properties along a transect at a reference location (Brazos-Trinity Basin IV). We then drilled multiple holes along a transect in the overpressured Ursa region to characterize spatial variation in rock properties, temperature, pressure, and chemistry.

Geological overview: Gulf of Mexico

Sedimentation, deformation, hydrodynamics, slope stability, and biological communities are interwoven in the Pleistocene strata of the Gulf of Mexico (Fig. F4). Rapid sedimentation upon a mobile salt substrate is the driving force behind many of the active processes present (Worrall and Snelson, 1989). Bryant et al. (1990) described the physiographic and bathymetric characteristics of this continental slope (Fig. F4). In the region offshore Texas and western Louisiana, individual slope basins are surrounded by elevated salt highs (Pratson and Ryan, 1994), producing a remarkable hummocky topography. This morphology is obscured in the eastern Gulf of Mexico downslope of the present Mississippi River, where sedimentation was focused during the Quaternary, whereas the region offshore Texas and Louisiana was relatively starved of sediment during the same time period.

Late Pleistocene sediments were drilled in Brazos-Trinity Basin IV and Ursa Basin (Fig. F4). The sedimentation rate in Brazos-Trinity Basin IV was relatively low, whereas the sedimentation rate in Ursa Basin was envisioned to reach rates of at least 1 cm/y. We anticipated hydrostatic pore fluid pressures in Brazos-Trinity Basin IV and overpressured pore fluids in Ursa Basin.

Geological setting

Brazos-Trinity Basin IV

Brazos-Trinity Basin IV is 200 km due south of Galveston, Texas (USA) in ~1400 m of water (Figs. F4, F5). The basin is one of a chain of five basins that are separated by interbasinal highs. It is a classic area for analysis of turbidite depositional environments and it is used as a modern analog to describe the formation of deepwater turbidite deposits (Winker, 1996; Badalini et al., 2000; Winker and Booth, 2000).

The primary data set used to evaluate the well locations is a high-resolution two-dimensional (2-D) seismic survey acquired by Shell Exploration and Production Company to image the turbidite stratigraphy (Fig. F5). The line spacing is ~300 m. Dip seismic Line 3020 shows the three drilled locations (Fig. F6). A strike line through Site U1320 is also illustrated (Fig. F7). Site U1320 (Figs. F5, F6, F7) is located where the turbidite deposits are thickest, whereas Site U1319 (Figs. F5, F6) is on the southern flank of the basin where turbidite deposits are more condensed. Site U1321 was scheduled for logging-while-drilling/measurement-while-drilling (LWD/MWD) activities only.

Ursa Basin

Ursa Basin (~150 km southeast of New Orleans, Louisiana [USA]) lies in ~1000 m of water (Figs. F4, F8). The region is of economic interest because of its prolific oilfields that lie at depths >4000 meters below seafloor (mbsf). The Ursa field is in Mississippi Canyon Blocks 855, 897, and 899 and is 11.9 km east of the Mars tension leg platform.

Six extraordinary three-dimensional (3-D) seismic data sets are available within Ursa Basin (Fig. F9). Shell and industry partners shot the Ursa 96 exploration survey for exploration purposes. The high-resolution surveys were shot by Shell for the purpose of shallow hazards analysis.

Winker and Booth (2000) described deposition of Quaternary sediments. The Blue Unit is a late Pleistocene, sand-dominated, “ponded fan” that was deposited in a broad topographic depression that extended in an east-west direction for as much as 200 km and a north-south direction for as much as 100 km. The Blue Unit is overlain by a leveed-channel assemblage that is mud dominated and has dramatic along-strike variation in thickness. Pulham (1993) described a similar facies assemblage for this region.

Shell made downhole pressure measurements with a pore-pressure penetrometer (piezoprobe) at the Ursa platform (Eaton, 1999; Pelletier et al., 1999; Ostermeier et al., 2000, 2001) (Fig. F10). They also acquired whole-core samples and performed consolidation experiments to evaluate preconsolidation stress and estimate overpressure. Piezoprobe measurements (circles) and maximum past effective stresses interpreted from consolidation experiments (triangles) indicate that (1) overpressure begins near the seafloor and (2) the pore pressure is ~50% of the way between hydrostatic (P_h) and lithostatic (σ_v) (Fig. F10). Pressures (both hydrostatic and lithostatic) are calculated from below seafloor and not from sea level.

Seismic Line A–A′ (Fig. F11) illustrates the Ursa drill sites. The sedimentary section is composed of a 300 m thick overburden that is predominantly mudstone. Beneath the overburden lies the first significant sand of the Blue Unit. The Blue Unit has a relatively flat base. Its upper boundary has relief, which most likely reflects postdepositional erosion. The Blue Unit is composed of interbedded sand and mud (Figs. F10, F11). A levee-channel facies overlies the Blue Unit; it has a sand-cored channel that is flanked by mud-prone levee deposits. A mud package that thickens to the west overlies these deposits. This package contains numerous mass transport deposits (MTDs). The uppermost mud contains distal deposits of a larger levee-channel system, formed to the west, and hemipelagic drape.

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