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

R. Brent Bray, Jeffrey S. Hanor

Fluvial-deltaic deposition, growth faulting, and salt diapirism in the Tertiary section of southeast Louisiana have combined to produce a structurally complex geologic framework that 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, and density to a depth of 10,000 ft 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 in the region.

Three regional hydrologic flow regimes can be defined in the area as a function of depth: (1) a shallow, hydropressured, meteoric flow regime that is driven by differences in topographic elevation; (2) an intermediate-depth, hydropressured, thermohaline regime driven 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 their deposition, the entire interval between 2000 and 10,000 ft is now largely filled with hypersaline waters. These brines have been derived from the dissolution of salt domes, which act as local sources of both saline waters and heat. Some domes have salinity plumes that extend thousands of feet above the top f salt and many miles laterally. Two areas in which overpressured saline brines are apparently bleeding 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. Significant temperature, pressure, and salinity gradients have made it hydrologically dynamic. Of economic interest is the possible relation of hydrocarbon migration in the region to the migration of water and dissolved salt.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990