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doi:10.2204/iodp.proc.308.215.2012

Regional studies and prior drilling in the Ursa Basin

The Mars-Ursa salt-withdrawal basin (Ursa Basin) is 210 km south-southeast of New Orleans, Louisiana (USA), on the northeastern Gulf of Mexico continental slope (Fig. F1A). Late Pleistocene deposition from the ancestral Mississippi River is recorded by a southward bulge in the 500 and 1000 m bathymetric contours. Mars Ridge, a prominent north-south–trending bathymetric high, bounds the study area to the west (Fig. F1B).

The prolific Mars and Ursa oil fields lie at depths >4000 meters below seafloor (mbsf) in this region. Mahaffie (1994) described the geological character of the Mars oil field. The Ursa field is in Mississippi Canyon Blocks 855, 897, and 899 and is 11.9 km east of the Mars tension-leg platform.

Late Pleistocene strata in the Mars-Ursa region have received attention since the 1990s, when overpressured and unconsolidated sandstones in the shallow section plagued drilling operations (Ostermeier et al., 2002; Winker and Shipp, 2002). This drove the acquisition of high-resolution 3-D seismic data and geotechnical cores to study the geological and geotechnical framework in the region (Winker and Shipp, 2002).

Late Pleistocene shelf, shelf-margin, and turbidite deposits sourced from the Mississippi River are termed the eastern depositional complex. Eastern depositional complex strata in Ursa Basin accumulated outboard of the shelf break during marine isotope Stages (MIS) 2–4 in response to late Wisconsinan continental glaciation (Sawyer et al., 2007b; Winker and Booth, 2000; Winker and Shipp, 2002). Sawyer et al. (2007b) integrated 3-D seismic data with industry log data to characterize the shallow stratigraphy in this region. These strata are divided into the Blue Unit and overlying mudstone-prone leveed-channel deposition. The Blue Unit is a sandstone-dominated turbidite deposit that was formed in a broad topographic low that extended 200 km to the east and west and 100 km to the north and south (Pulham, 1993; Sawyer et al., 2007a; Winker and Booth, 2000). The overlying leveed-channel systems contain both channels and bounding levees.

Shell Oil Co. made downhole pressure measurements with a pore pressure penetrometer (piezoprobe) at the Ursa platform (Eaton, 1999; Ostermeier et al., 2002; Ostermeier, 2001). They also acquired whole-core samples and performed consolidation experiments to evaluate preconsolidation stress and estimate overpressure. Their work indicated that near the Ursa platform, overpressure begins near the seafloor and that pore pressure is ~50% between the hydrostatic pressure (u_h) and the overburden stress (σ_v).

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