Geochemistry of pore waters from Shell Oil Company drill holes on the continental slope of the northern Gulf of Mexico

Pore waters were analyzed from 6 holes drilled from M.V. “Eureka” as a part of the Shell Oil Co. deeper offshore study. The holes were drilled in water depths of 600–3,000 ft. (approximately 180–550 m) and penetrated up to 1,000 ft. (300 m) of Pliocene-Recent clayey sediments. Salt and anhydrite caprock was encountered in one diapiric structure on the continental slope.

Samples from holes drilled near diapiric structures showed systematic increases of pore-water salinity with depth, suggestive of salt diffusion from underlying salt plugs. Anomalous concentrations of K and Br indicate that at least one plug contains late-stage evaporite minerals. Salinities approaching halite saturation were observed.

Samples from holes away from diapiric structures showed little change in pore-water chemistry, except for loss of SO₄ and other variations attributable to early-stage diagenetic reactions with enclosing sediments. Thus, increased salt concentrations in even shallow sediments from this part of the Gulf appear to provide an indicator of salt masses at depth.