Print this pageThe Effects of Salt-Related Deformation from Depositional Profiles: The Example of the Wilcox in the Deepwater Subsalt Province of the Northern Gulf of Mexico
Seafloor topography, whether regional or local, significantly influences the type and distribution of deepwater sand fairways on any part of a continental margin depositional profile. For successful exploration, it is critical to determine whether a given area is in a slope or basin-floor setting. If possible, it is also important to distinguish between the upper, mid, or lower slope and the proximal, medial, or distal basin floor. Within these sub-domains, unique sedimentary architectures for reservoir sand bodies define subtle, but significant, differences for risking reservoir presence, distribution and quality.
Many continental margins are structurally complex due to gravitational failure above salt detachments and consequent salt withdrawal and diapirism. The resulting seafloor topography typically perturbs and complicates the standard depositional profile, resulting in significant variations in reservoir architecture, which is highly dependent on whether the trap is located at minibasin entry points, exit points, or lateral margins.
Interpreting depositional environments, including something as fundamental as the toe of slope, is particularly challenging for the subsalt Wilcox in the deepwater northern Gulf of Mexico. First, subsalt seismic data are typically inadequate for imaging sand fairways. Second, there is a gap of over 300 km between onshore wells located near the Wilcox shelf edge and wells in the distal deepwater province. Third, results from the distal subsalt tests can be ambiguous because local slopes on basin-floor minibasin flanks can be confused with regional slope across the margin.
Structural analysis can be used where well and seismic facies analysis are insufficient. There is a prominent boundary between a distal province of vertical diapirs and associated turtle structures and a more proximal province of inclined feeders and expulsion rollovers. Because vertical, symmetrical structures likely form on the basin floor and asymmetric, basinward-vergent structures probably form on the slope, the province boundary is interpreted as the Wilcox toe of slope. Moreover, mapping of different salt-withdrawal and diapir geometries can be used to predict local seafloor topography. The results suggest that most wells in Walker Ridge and Keathley Canyon are located on the Wilcox basin floor, but that the two most proximal are at the entry points to lower-slope minibasins.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil