Groundwater upwelling, near Bay St. Elaine salt dome in southeastern Louisiana, as inferred from fluid property variations

Fluid pressures, temperatures, and salinities derived from 62 well logs were mapped in three vertical cross sections across the Bay St. Elaine salt dome in southeastern Louisiana to qualitatively determine groundwater flow patterns around the dome. Near-hydrostatic conditions prevail in the sand-rich shallow section from depths of 0 to 2.5 km. However, fully enclosed within the hydropressured section are lenses of slightly overpressured sediments, which slope up toward the dome. Hydraulic heads may be as much as 500 m above ground level within these enclosed lenses. In some strongly overpressured sediments deeper than 2.5 km, hydraulic heads are as much as 2500 m above ground level. Pore-water salinities increase with depth within the hydrostatically pressured section, fr m 0 wt.% at the surface to maximum values of 10-12 wt.% at the top of the overpressured section. Within the overpressured section, salinities decrease to as little as 6 wt.% with increasing depth. Immediately adjacent to the northeastern flank of the dome is a steeply dipping lobe of relatively fresh waters (6 wt.% sodium chloride) that separates the flank of the dome from saltier (8-10 wt.%) waters away from the dome. This low-salinity water mass appears to be connected to similar low-salinity waters in the overpressured section and is postulated to be due to pore-water upwelling from a depth of 4 km to a depth of about 1.5 km.