Chemical Evidence for Migration of Deep Formation Fluids into Shallow Aquifers in South Louisiana

Ronald K. Stoessell and Lesley Prochaska
Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70148 USA;
[email protected]; [email protected]

Local accumulations of brackish groundwater occur across south Louisiana within shallow, southward-dipping Pleistocene, Pliocene, and Upper Miocene aquifers. Low Br/Cl and K/Cl concentration ratios, high Na/Cl concentration ratios, trace Mg concentrations, and low ⁸⁷Sr/⁸⁶Sr ratios in the brackish waters from these aquifers are inconsistent with the saline-water sources being in situ marine formation fluids of similar age. The ratios are consistent with the saline-water sources being Lower Miocene or Paleogene marine formation fluids which dissolved recrystallized halite. The region overlies the Jurassic Louann salt and is cut by a series of east-west trending gravity faults with southward-dipping fault planes. Deep formation fluids have dissolved halite in salt diapirs and moved up fault planes to enter shallow aquifers and mix with the in situ groundwaters.

AAPG Search and Discovery Article #90080©2005 GCAGS 55th Annual Convention, New Orleans, Louisiana