--> ABSTRACT: Thermohaline Pore Water Trends of Southeastern Louisiana Natural, Anthropogenic, or Unjustified Environmental Fear?, by Don Marlin, William H. Schramm; #91020 (1995).

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Thermohaline Pore Water Trends of Southeastern Louisiana Natural, Anthropogenic, or Unjustified Environmental Fear?

Don Marlin, William H. Schramm

A significant number of research articles dealing with thermohaline movement of subsurface pore water fluids have presented the hypothesis of dissolution of salt diapirs and the formation of dense, saline brines at shallow depths that are concurrent with large scale fluid migration. A critical foundation of these studies is the determination of salinity from the spontaneous potential (SP) log, and the ability to drive large volumes of fluid at high flow rates vertically through the sediment. The SP measures the electrical contrast between formation fluid and mud filtrate properties. Derivation of salinity from SP response and subsequent averaging of salinity values over several hundred feet of non-homogeneous sediment, and disregard for the presence of thick, impermeable arbonate sediments in the southeastern Louisiana Frio Formation cloud these findings.

The calculation of salinity is based on water resistivity (Rw) variations and the geological constraints on deriviation of this variable. Regional and local variations in Rw or salinity are a function of depth, temperature, mud filtrate invasion, and increases in mud filtrate salinity with duration of contact with the formation. The use of the SP log to calculate such trends illustrates a decrease in salinity below geopressure with depth regardless of proximity to salt domes. Maximum salinity is found well above geopressure where clay de-watering and ion transfer occurs. Only accurate sampling of actual pore fluids or corrected SP calculations should be used to indicate thermohaline trends before environmental assessment is presented.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995