Evidence for Large-Scale Vertical and Lateral Migration of Formation Waters, Dissolved Salt, and Crude Oil in the Louisiana Gulf Coast

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Abstract
Spatial variations in properties of both inorganic and organic components of Louisiana Gulf Coast fluids provide insight into migration pathways of formation waters and hydrocarbons. The salinity variation of subsurface formation waters, for example, helps delineate the regional permeability field of a sedimentary basin and pathways of long-range solute transport. Variations in the carbon isotopic composition of crude oils help to establish sources of hydrocarbons and pathways of migration. In the Louisiana coastal parishes, aqueous fluid flow directions in most of the deep, geopressured sediments are nearly vertically up. Contrary to the presumed membrane behavior of the shale-rich sediments of the geopressured zone, however, pore water salinities also increase upward. It is possible, therefore, that fluids are being channeled through fractures and faults, rather than through matrix porosity. Geochemical characteristics of crude oils in thermally immature Tertiary reservoirs of the shallow hydropressured zone suggest that they have been emplaced by vertical migration from deeper and older geopressured source rocks. The 2 to 3 km-thick, sand-rich hydropressured zone which overlies the geopressured sediments in south Louisiana is far more dynamic hydrologically than previously recognized. Dissolution of salt diapirs generates brines at shallow depth and helps to produce pore-fluid density inversions sufficient in magnitude to drive large-scale vertical and lateral flow. Vertical mixing of oxygenated meteoric and formation waters may explain the presence of many biodegraded crude oils in south Louisiana. Calculated fluid flow directions in the geopressured zone north of the coastal parishes have a pronounced lateral component to the north. Regional variations in pore water salinities suggest that fluids from the overpressured zone are channeled laterally updip through regionally continuous sand sequences of the Wilcox Group where they mix with brines formed by the dissolution of salt domes in the Northern Louisiana and Mississippi salt basins 200 km to the north. Geochemical characterization of crude oils in Wilcox reservoirs in this area suggests that long-range lateral migration from deeply buried Lower Tertiary source rocks to the south has occurred. Some of these crude oils have been altered by water washing, suggesting a zone of significant fluid flow exists along the axis of the Mississippi Embayment.