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NIKIEL, AMANDA M., Marathon Oil Company, Lafayette, LA; and JEFFREYS. HANOR, Louisiana State University, Baton Rouge, LA

Abstract: Spatial Variations in Formation Water Salinities, South Pelto and South Timbalier Areas, Eastern Louisiana Continental Shelf

Spatial variations in pore fluid salinity, pressure, and temperature were determined from borehole logs along a 100-km long transect from the Louisiana coastline to the shelf edge in the South Pelto and South Timbalier outer continental shelf areas to help identify driving forces and pathways of regional fluid flow and solute transport. This information is also useful in identifying pathways of hydrocarbon migration. The Pleistocene to Upper Miocene sediments were deposited in fluvial, deltaic, and open marine environments and consequently contained waters of fresh to normal marine salinities (35 g/L) at the time of their deposition. Most of these sediments, however, now contain hypersaline fluids having salinities of up to 125 to 150 g/L. Exceptions are shallow Upper Pleistocene sediments that contain fluids less saline than seawater. These shallow brackish fluids probably represent the remnant of a regional freshwater lens formed during the last low-stand of sea level.

A regional plume of water having salinities in excess of 100 g/L dips gulfward to the shelf edge at depths of 3 to 5 km below the seafloor. The locus of this plume is within the sandier Pliocene section. Potential sources of dissolved salt include updip shallow salt structures and the underlying remnants of allochthonous salt. Regional faults in the area may have acted as vertical conduits for upward transport of brines. Down-dip fluid flow was presumably in part gravity-driven. The distal end of this regional salinity plume near the shelf edge is now highly overpressured and fluid flow should be in the opposite direction, i.e., updip. Hence, this part of the regional salinity structure was probably established prior to over pressuring. Further work is now needed to quantify the magnitude of the driving forces for and rates of fluid migration. 

AAPG Search and Discovery Article #90924©1999 GCAGS Annual Meeting Lafayette, Louisiana