Seafloor and near-surface faults can be a significant geohazard in many of deepwater basins that are being exploited and de-veloped today. Occasionally, careful assessment of geome-tries and geomechanical properties of these fault networks can prove to be critical input to development system decisions, from the initial phase of feasibility studies to the final design and layout decisions. In this study, high-resolution 3D seismic interpretation, empirical methods, and geomechanical model-ing were used to predict future seafloor fault activation and resulting fault geometries. This methodology was applied to a subsea development in the vicinity of the proposed Princess Field in the Mississippi Canyon Protraction Area, northeastern Gulf of Mexico. Results of the work were used help select the location of development infrastructure within a narrow swath close to the seafloor faults, thereby eliminating additonal drill-ing costs for extended-reach wells.
The Greater Mars-Ursa Area (GMUA) is located in the south-central part of the Mississippi Canyon (MC) Protraction Area in the northeastern Gulf of Mexico (Figure 1). This area is named for the two major producing assets in the basin, the Mars and Ursa fields. The GMUA is a salt-controlled mini-basin on the mid slope, which covers by at least 22 outer con-tinental shelf (OCS) blocks or an area of 550 sq mi (Figure 2). The GMUA has been one of the most prolific deepwater ba-sins for the production of oil and condensate. Princess Field is a subsea development and the newest field in the basin, with first production coming online in 2002. In addition to the nu-merous challenges facing drilling of deep objective-level tar-gets in the basin, near-surface geohazards also pose a substan-tial risk for most of the wells in the GMUA (e.g., Ostermeier et al., 2000; 2001; 2002). The potential seafloor geohazards in the GMUA relate primarily to seafloor stability. The cause of this instability is related directly to depositional and structural processes that create steep topography on the seafloor such as mass transport deposits (MTDs - in the sense of Piper et al., 1997), fault scarps, and fluid expulsion features. In the near-surface interval (0–5000 ft below the mudline (BML)), typical geohazards encountered in the GMUA are:
occurrence of shallow water flow (SWF) in overpressured, wet sands;
presence of small accumulations of shallow gas, particularly in the deeper near-surface section (>4000 ft BML);
shallow- and deep-seated faults that could form fluid migration path-ways;
fluid expulsion features with their associated gas chimneys, which can cause subsurface fluid mobility; and
buried MTDs at various scales (100's ft to 10's mi) with vari-able lithologies. Historically, the GMUA is best known for its problems with SWF (Byrd et al., 1996; Alberty et al.; 1997; Eaton, 1999; Ostermeier et al., 2000; 2001; 2002; Winker and Shipp, 2002). However, the focus of this study is a complex fault network on the eastern basin margin that substantially constrained the subsea development layout at Princess Field centered in MC 809 (Figures 1 and 3).