Spatial Variations in Subsurface Pore Fluid Properties in a Portion of Southeast Louisiana: Implications for Regional Fluid Flow and Solute Transport: ABSTRACT

Abstract

ABSTRACT Fluvial-deltaic deposition, growth faulting, and salt diapirism in the Tertiary section of southeast Louisiana have combined to produce a structurally complex geologic framework which contains potential pathways for large-scale lateral and vertical fluid migration. A study has been made of spatial variations in pore water salinity, temperature, pressure, density, and viscosity to a depth of 10,000 ft (3 km) in a five-parish area situated across the northern margin of the South Louisiana salt basin to help identify driving forces and pathways for subsurface fluid migration and solute transport. Three regional hydrologic flow regimes can be defined in the area as a function of depth: 1) a shallow, hydropressured, meteoric flow regime which is driven by differences in topographic elevation; 2) an intermediate-depth, hydropressured, thermohaline regime apparently driven in part by variations in pore water density; and 3) a deep, geopressured regime driven by high pore fluid pressure gradients. Although the sediments in the area originally contained fresh to marine waters at the time of deposition, the interval between 2000 and 10,000 ft is now largely filled with hypersaline waters. These brines are derived from the dissolution of salt domes, which act as local sources for both saline waters and heat. Some domes have salinity plumes that extend thousands of feet above the top of salt and many miles laterally. Two areas in which overpressured saline brines are apparently migrating upward along radial faults into the overlying hydropressured zone appear to be related to deep-seated salt structures. Faults and lack of lateral continuity of interbedded shales have made the sand-dominated section to a depth of 10,000-ft hydrologically open on a sub-regional scale. Significant temperature, pressure, and salinity gradients have made it hydrologically dynamic. Of economic interest is the possible relation of hydrocarbon migration to the migration of pore water and dissolved salt.