Geologic Implications of New Anisotropic Pre-Stack Depth Imaging for Salt Tectonics and Early Reservoir Sand Distribution, Western Mississippi Canyon Area, Northern Gulf of Mexico
Petroleum discoveries in the salt minibasins in western Mississippi Canyon (MC) including Mars (up to 900 MMBO and 1 TCF), Ursa (up to 400 MMBO and 700 BCF) and now Thunder Horse (up to 1.6 BBO and 1.4 TCF) demonstrate the potential richness of the petroleum systems involved. Current exploration activity focuses on Miocene sediments in the remaining large untested structures under salt canopies and complex overhangs that were not clearly imaged in earlier generations of seismic data. New anisotropic Kirchoff pre-stack depth migrated 3d seismic data have vastly improved the imaging of salt overhangs, salt flank tapers and flares, steep flanks and secondary salt and fault welds. These new images allow more confident interpretation of depositional rates and hence reservoir sand probabilities from the salt sediment interface geometries at deposition. The dynamic equilibrium between sediment aggradation rate and salt flow rate controls the geometries of the salt flanks: salt bodies taper inward during more rapid deposition and flare out upward during slower deposition (Talbot, 1995). We show several examples from wells drilled near salt-sand interfaces that support this concept. The new imaging also provides clearer understanding of secondary weld geometry and development in MC and better discrimination between welds and salt walls. These factors are critical in assessing prospect seal potential. Primary salt welds are often in part fault welds with significant relative lateral motion of depocenters toward each other but individual minibasins show little internal shortening. The new data clearly show a basement event that is remarkably flat and continuous wherever it is well-imaged even under tens of thousands of feet of solid salt. We suggest that the primary minibasins formed in the initial Louann salt basin were not rigidly fixed to an extremely extended faulted basement in MC and that basement features have had relatively little influence on subsequent development despite a rich literature of claims to the contrary. Rather, the minibasins formed a dynamic thin-skinned complex with mobile salt that was effectively insulated from a basement, presumably the floor of the primary Louann salt basin, that is remarkably smooth. The primary minibasins in MC apparently moved downslope over and with their ductile salt substrate in a manner very analogous to the much younger minibasins in the modern Louisiana continental slope.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009