Spatial variations in the salinity of pore waters in northern deep water Gulf of Mexico sediments: implications for pathways and mechanisms of solute transport

Abstract

Geofluids (2010) 10, 83–93AbstractSpatial variations in the salinity of pore waters in sedimentary basins can provide important insight into basin‐scale hydrogeologic processes. Although there have been numerous studies of brine seeps in the deep water Gulf of Mexico, much less is known about porewater salinities in the vast areas between seeps. A study has been made of spatial variation in pore water salinities in sediments in an approximately 500 km by 200 km area of the northern deep water (water depth >500 m) Gulf of Mexico sedimentary basin (GOM) to provide insight into pathways and mechanisms of solute transport in this portion of the basin. A second objective was to document salinities in the upper 500 m of the sedimentary section, the approximate depth to which methane hydrates, a potential future energy resource, may be stable. Elevated salinities would reduce the P–T stability range of hydrates. Salinities were calculated from borehole logs using a dual electrical conductivity model. Even though much of the northern GOM is underlain by allochthonous salt most of the undisturbed shallow sedimentary section has not been permeated by hypersaline waters. These waters are limited to areas near brine seeps. Hypersaline waters having salinities in excess of 100 g l−1 become more common at subseafloor depths of 2 km and greater. A field study at Green Canyon 65 and published numerical simulations of fluid flow above tabular salt bodies suggest that brines produced by salt dissolution migrate laterally and pond above salt and/or within minibasins and that the dominant mechanism of vertical solute transport is a combination of compaction‐driven advection and diffusion, not large‐scale thermohaline overturn. Superimposed on this diffuse upward flux of dissolved salt is the more focused and localized expulsion of saline fluids up faults.