LARGE-SCALE SUBSURFACE FLUID FLOW IN OFFSHORE LOUISIANA DRIVEN BY THE LOCALIZED DISSOLUTION OF SALT

The physical hydrogeology of the northern Gulf of Mexico sedimentary basin (GOM) has been described in terms of the interplay between topographically-driven groundwater flow and deeper overpressured fluid systems. However, the hydrogeology of the GOM is much more complicated than this. The presence of salt and the formation of brines by dissolution at shallow depth play a key role in the development of density-driven fluid circulation that effectively decouples the topographic and overpressured fluid regimes. An excellent example of this is found at the giant Bay Marchand salt dome, Louisiana continental shelf. Data on fluid pressures, temperatures, and salinities derived from wireline logs from a 24 square km area on the southeast flank of the dome demonstrate the role that the localized dissolution of salt has had in generating kilometer-scale subsurface fluid flow within this region. Three distinct hydrogeologic regimes exist. The shallowest is a Recent to Pleistocene-hosted hydropressured regime having fluids of normal marine salinity. The deepest regime is characterized by overpressured Miocene sediments having marine salinities and less. The middle regime, hosted by Pliocene and upper Miocene sediments, is characterized by hypersaline waters derived by localized salt dissolution at a solution cavity near the top of the dome. At least two salinity plumes having a cumulative thickness of a kilometer or more are migrating to the south and southeast downdip within sandy intervals. The depression in isotherms on the flank of the dome suggests rapid downward advection of these dense, cooler waters. Discontinuities in salinity variations in the southernmost part of the area appear to reflect fluid compartmentalization by faults. Excess dissolved salt presently within the study area corresponds to the dissolution of at least 0.3 cubic km of the dome, but much more of the dome than this has undoubtedly been destroyed over time.