ABSTRACT

Previous research has suggested that dissolution of salt diapirs and the formation of dense, saline brines at shallow depths 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 fluid vertically through the sediment. Derivation of salinity using the perfect shale membrane model and contouring iso-salinity values over intervals of Lower Miocene and Upper Oligocene sediments that contain thick, impermeable carbonate deposits cloud these findings.

The calculation of salinity is based on water resistivity (Rw) variations and the geological constraints on derivation of this variable. Application of the imperfect shale membrane model to determine Rw from the SP log provided a closer approximation to Rw from produced water samples over St. Gabriel Field in Ascension and Iberville Parishes, Louisiana than past SP models. Further analyses of temperature, pressure, salinity, and freshwater hydraulic head trends of Lower Miocene and Upper Oligocene deposits over the field and surrounding area suggest that dissolution of salt occurred prior to hydrocarbon generation and large scale fluid migration is not dynamic at present. Important controls that should be used in future studies of thermohaline fluid movement are the identification of local structure, stratigraphic variation, shale membrane efficiency, and time of salt diapirism.