BACKGROUND

Geological Overview: The Gulf of Mexico

The Gulf of Mexico is a type location for a shallow drilling campaign aimed at understanding how sedimentation drives compaction and fluid flow (Fig. F4). Sedimentation, deformation, hydrodynamics, slope stability, and biological communities are interwoven in the Pleistocene strata of the Gulf of Mexico. 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) describe the physiographic and bathymetric characteristics of this continental slope (Fig. F4). In the region of offshore Texas and western Louisiana, individual slope minibasins are surrounded by elevated salt highs (Pratson and Ryan, 1994) producing a remarkable hummocky topography. This morphology is obscured in the eastern Gulf, where sedimentation has been very rapid and more recent than the region of offshore Texas and Louisiana. To evaluate the impacts of different depositional settings and rates on sediment properties and fluid flow, Pleistocene sediments will be drilled in the Brazos-Trinity Basin #4 and the Ursa Basin (Fig. F4).

The Brazos-Trinity Basin #4 is 200 km due south of Galveston, Texas (USA) in ~1400 m water depth (Figs. F4, F5). The basin is one of a chain of five basins that are connected by interbasinal highs. It is a classic area for analysis of turbidite depositional environments because it is used as a modern analog to describe the formation of deepwater turbidite deposits (Anderson and Fillon, 2004; Badalini et al., 2000; Beaubouef and Friedmann, 2000; Fraticelli, 2003; Morton and Sutter, 1996; Satterfield and Behrens, 1990; Suter and Berryhill, 1985; Winker, 1996; Winker and Booth, 2000).

The primary data set used to evaluate the well locations is a high-resolution two-dimensional (2-D) seismic survey shot by Shell Exploration and Production Company to image the turbidite stratigraphy (Fig. F6). The line spacing is ~300 m. The four proposed drilling locations are shown on dip seismic Line 3020 (Fig. F7). A strike line through proposed Site BT4-2A is also illustrated (Fig. F8).

Proposed Site BT4-2A (Figs. F5, F6, F7, F8) is located where the turbidite deposits are thickest, whereas proposed Site BT4-4A (Figs. F5, F6, F7) is along the southern flank of the basin where there are almost no turbidite deposits. Shell drilled the Brazos-Trinity Basin #2 and encountered basinal turbidites composed of interbedded sands and mudstones and an underlying hemipelagic mudstone. Recovery may be difficult in the poorly consolidated turbidite sands at Site BT4-2A. Alternate sites BT4-3A and BT4-4A will have drilling conditions similar to BT4-1B, as they are located on the basin flank.

Ursa Basin (~150 km due south of New Orleans, Louisiana [USA]) lies in ~1000 m of water (Figs. F1, F9). The region is of economic interest because of its prolific oilfields that lie at depths greater than 4,000 meters below seafloor (mbsf). Mahaffie (1994) described the geological character of the Mars oilfield. The Ursa field is in Mississippi Canyon Blocks 855, 897, and 899 and is 11.9 km east of the Mars tension leg platform (TLP).

We are interested in the sediments from 0 to 1000 mbsf. Four extraordinary three-dimensional (3-D) seismic data sets are available for the Ursa Basin (Fig. F9). Shell and industry partners shot the Ursa exploration survey for exploration purposes. The high-resolution surveys were shot by Shell for shallow hazards analysis.

Winker and Booth (2000) described deposition of Pleistocene and Holocene sediments in the Ursa region. The Mississippi Canyon Blue Unit is a late Pleistocene, sand-dominated, "ponded fan" that was deposited in a broad topographic low 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 was mud dominated and had dramatic along-strike variation in thickness. Pulham (1993) described a similar facies assemblage for this region.

Seismic Line A–A' (Fig. F10) illustrates the proposed boreholes. The sedimentary section is composed of a 300 m thick overburden that is predominantly mudstone. Beneath the overburden lies the first significant sand: 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 mudstone (Figs. F10, F11). A leveed-channel facies overlies the Blue Unit: it has a sand-cored channel that is flanked by mud-prone levee deposits. A mudstone package that thickens to the west overlies this sand assemblage. This mudstone package has numerous detachment surfaces that record slumping. The overlying mudstone is the eastern margin of a larger levee channel system formed to the west.

Shell made downhole pressure measurements with a pore-pressure penetrometer (piezoprobe) at the Ursa platform (Eaton, 1999; Ostermeier et al., 2000; Pelletier et al., 1999) (Fig. F11). They also acquired whole-core samples and performed consolidation experiments to evaluate preconsolidation stress and estimated 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 the hydrostatic (P_h) and the lithostatic (_v) (Fig. F11).